, " Wy R e Ty Wy . ! e doa k. it i National Academy of Sciences | -.__ ‘ -;lational Research Council NUCLEAR SCIENCE SERIES The Radiochemistry of the Rare Gases _— - ' o —— — —— T Atomic il iifl IR RY Commission I L COMMITTEE ON NUCLEAR SCIENCE L. F. CURTIBSS, Chairman National Bureau of Standards ROBLEY D. EVANB, Vice Chairman Massachusetts Institute of Technology Jd. A. DaJUREN, Secrelary Wenstinghouse Eleotric Corporation C. J. BORKOWBKI Oak Ridge Natonal Laboratory ROBERT G. COCHRAN Texas Agricultural and Mechanical College SAMUEL EPSTEIN California Institute of Technology U. FANO Natonal Bureau of Standards HERBERT GOLDSTEIN Nuolear Development Corporation of Amerioca J. W, IRVINE, JR. Magsachusetts Institute of Teohnology E. D. KLEMA Northwestern University W. WAYNE MEINKE University of Michigan J. J. NICKBEON Memorial Hospital, New York ROBERT L. PLATZMAN . Laboratoire de Chimie Physique D. M. VAN PATTER Bartol Research Foundation LIAISON MEMBERS PAUL C. AEBERSOLD Atomic Energy Commission J. HOWARD MoMITL1LEN National Science Foundation CHARLES K. REED U. 8. Air Force WILLIAM E. WRIGHT Office of Naval Research SUBCOMMITTEE ON RADIOCHEMISTRY W. WAYNE MEINKE, Chairman University of Michigan GREGORY R. CHOPPIN Florida State University GEORQGE A. COWAN Los Alamoa Bclentific Laboratory ARTHUR W. FAIRHALL University of Washington JEROME HUDIS Brookhaven National Laboratory EARL HYDE University of California (Berkeley) HAROLD KIRBY Mound Laboratory GEORGE LEDDICOTTE Osk Ridge National Laboratory JULIAN NIELSEN Hanford Laboratories ELLIS P. STEINBERG Argonne National Laboratory PETER C. STEVENSON University of California (Livermors) LEO YAFFE McGill University CONSULTANTS NATHAN BALLOU JAMES DeVOE Centre d'Etude de 1"Energle Nucleaire University of Michigan Mol-Donk, Belglum WILLIAM MARLOW National Bureau of Standards S M133r ¢ .| ' The Radiochemistry of the Rare Gfises By FLOYD F. MOMYER, JR. Lasorence Radiation Laboratory Universily of California Livermore, California October 1960 LOBS ALAMOS SCIENTIFIC LABORATORY APR - 6 1881 3 LIBRARIZS — PROPERTY = =3 — i=s i . Subcommittee on Radiochemistry National Academy of Bciences —National Research Council Printed in USA. Price $0.75. Available from the Office of Techmical Ssrvices, Department of Commeroe, Washingtion 26, D. C. FOREWORD The Subcommittee on Radlochemlsgtry 1s one of a number of subcommlttees workilng under the Committee on Nuclear Scilence wlthin the Natlonal Academy of Sclences - Natlonal Research Councll. 1Its members represent government, lndustrlial, and unlverslty laboratorles 1n the areas of nuclear chemistry and analytical chemistry : The Subcommittee has concerned itself with those areas of nuclear sclence whilch involve the chemlst, such as the collec- tion and distributlon of radlochemlcal procedures, the estab- lishment of specificatlons for radiochemlcally pure reagents, avallabillity of cyclotron time for service irradlations, the place of radiochemlstry in the undergraduate college program, etc. This serles of monographs has grown out of the need for up-to-date compllations of radlochemlcal informetion and pro- - cedures. The Subcommlttee has endeavored to present a serles which will be of maximum use to the working sclentist and which contalns the latest avallable Information. ‘Each mono- graph collects 1n one volume the pertinent informatlon required for radlochemical work with an individual element or a group of closely related elemente. An expert In the radlochemlstry of the particular element has written the monograph, following a standard format developed by the Subcommlttee. The Atomlc ‘Energy Commission has sponsored the printing of the serles. The Subcommittee 1ls confldent these publlcatlons willl be useful not only to the radlochemist but also to the research worker 1n other flelds such as physics, blochemlstry or mediclne who wishes to use radlochemical technlques to solve a specifilc problem. W. Wayne Melnke, Chalrman Subcommittee on Radlochemlstry iii INTRODUCTION This report dealing with the radiochemistry of the rare gases was pre- pared at the request of the Subcormmittee on Radiochemistry of the Committee on Nuclear Science of the National Research Council a8 one of a series of monographs on the radiochemistry of all the elements. The early sections of this monograph are devoted to general reviews of rare gas properties of interest to the radiochemist and to some general dis- cussion of separation techniques for rare gases, The last three 6hapters are respectively a discussion of the removal of rare gases from targets, a dis- cuseion of techniques used for counting radioactive rare gases, and a collec- tion of radiochemical prdcedures for rare gases. The author would appreciate receiving from readers any additional in- formation, published or unpublished, which should be included in such a report on the radiochemistry of the rare gases. | _ The author takes this opportunity to acknowledge the able assiatance of Mr. R. A. daRoza in the preparation of this monograph. iv CONTENTS Foreword Introduction I. General References Pertinent to Rare Gas Radiochemistry II. Table of Isotopes of He, Ne, A, Kr, Xe, and Rn ) III. Review of Features of Interest in Rare Gas Radiochemistry IV. Sample Solution and Exchange with Carriers VY. Counting Rare Gas Activities | VI, Collection of Ra.di-ochemic'al Procedures for the Rare Gases Procedure 1 — Removal of Kr and Xe from Air and Their Subsequent Separation Procedure 1 A — Removal and Separation of Kr and Xe Fiassion Products from U235 Targets Procedure 2 — The Extraction, Purification and Industrial Uses of Krss " Procedure 3 — Rapid Removal of Fiesion Product Kr from U Foil Procedure 4 — Recovery of Fission Product Xe from U Metal . Procedure 5 — Rapid Removal of Fission Product Xe | | from UO, or UO,(NO,), - 6H,O Targets Procedure 6 — Separation of Long-lived Fission Gases Procedure 7 — Removal of Rn from Th Targets and its Collection on the Cathode of a Discharge Tube Procedure 8§ — Determination of Active Gas Half-lives by the Charged Wire Technique (II) iii iv 27 . 29 34 34 40 43 46 46 48 48 49 51 The Radiochemistry of the Rare Gases FLOYD F. MOMYER, JR. Lawrence Radiation Laboratory University of California Livermore, California GENERAL REFERENCES PERTINENT TO RARE GAS RADIOCHEMISTRY "H. Remy, Treatise on Inorganic Chemistry, Vol..I (translated by J. S. Anderson), Elsevier Publishing Co., Few York (1956). S. Dushman, Vacuum Technique, John Wiley and Sons, New York, 1949. R. T. Sanderson, Vacuum Manipulation of Volatile Compounds, John Wiley and Sons Inc., New York (1948). R. E. Dodd and P. L. Robinson, Experimental Inorganic Chemistry Chap. 2, Elsevier Publishing Co., New York (1954). S. Brunauer, The Adsorption of Gases and Vapors, Vql. I, "Physical Adsorption'", Princeton Universeity Press, Princeton, N.J. (1942), A. I, M, Keulemans, Gas Chromatograzhx, Reinhold Publishing Co., New York, 1959, 2nd edition. Lawrence A. Weller, '""The Adsorption of Krypton and Xenon on Activated Charcoal -- A Bibliography," Mound Laboratory i'eport MLM-1092, Miamisburg, Ohio, 1959. A. Guthrie and R. K. 'Wakerling, editors, Vacuum Equipment a.fid Tech- niques, National Nuclear Energy Series, Div. I, Vol. 1, McGraw-Hill Book Co., Inc,, New York, 1949. C. D. Coryell and N. Sugarmé.n, editors, Radiochemical Studies: The Fission Products, National Nuclear Energy Series, Div. IV, Vol, 9, Booke 2 and 3, McGraw-Hill Book Co., Inc., New York, 1951, Papers 64-70, 139, 141, 145-50, 154, and 311-17, II. TABLE OF ISOTOPES OF HELIUM, NEON, ARGON KRYPTON, XENON AND RADON. " Isotope Half-life Type of Decay Method of Preparation He3 Stable (abundance 1,3 X 10_4% atmos.) Natural (abundance 1.7 X10™°% wells) He4 Stable (abundance ~1007) Natural He® ~0. 8 sec B Beg(n,a) Ne 18 1.6 sec fl+ F_lg(P. 2n) Nelg ~18 sec fi+ Flg(p,n) Nezo Stable (abundance 90.92%) Natural Nez'1 Stable (abundance 0.257%) Natural Nez'z Stable {abundance 8. 82%) Natural 23 - 22 . 22 Ne ~40 sec B Ne "{n,y); Ne"" (4, p); 23 26 Na""(n,p); Mg~ (n,a) Ne24 3.38 min g Nezz(t, p) A35 ~1.8 sec fi+ S3z(u.,n); C135(p,n) a3 Stable {abundance 0. 337%) Natural 37 . 34 37, .. A -_35 day EC S™ "(a,n); C1™ (d, 2n}; c*’(p, n); X* (4, a); Ca40(n. a) A38 Stable (abundance 0. 063%) Natural 39 - 38 39 A ~265 yr B A7 (n,vy); K™ (n, p) A0 Stable (abundance 99. 600%) Natural 41 . - 40 40 A ~110 min B A7(d,p); A" (n,¥)s 41 . K" (n,p) A42 >3.5yr B A41(n,y); parent K42 Kr 0 9.7 hr EC - v8%(p, apall) 77 ; + T4 Kr ~1.2 hr EC~20%, B ~80% Se'“(a,n) Kr'?8 Stable {(abundance 0, 3549%}) Natural 79 ' + 76 79 Kr 34,5 hr EC 95%, B 5% Se “(a,n); Br (4, 2n); Br' ’(p,n); Kr ' (d,p); Kr?B(n.v) KrBO Stable {abundance 2,27%) Natural x Listed are thoee nuclides having an "A'" or "B'" classification in '"Table of Isotopes'', D. Strominger, J. M., Hollander and G. T. Seaborg: Revs. Mod. Phys. 30, No. 2, Part II, April 1958, the original literature may be found in said article, Further information and references to TABLE OF ISOTOPES OF He, Ne, A, Kr, Xe, and Rn,. {Cont'd) ‘Half-life Isotope Type of Decay Method of Preparation Kr81 ~10 sec IT Brsl(p,n); daughter Rbs-l Krsl 2, l><105 yr EC Kreo(n,y) K82 Stable (abundance 11.569%) Natural | K™ 114 min IT 5e%%a,n); Kr®%(a,p); Kraz(n.y)§ X-rays on . Kr83-; fission U, daughter Br®3; daughter RBS> Kr83 Stable (abundance 11, 55%) Natural; fission U Kr84 Stable (abundancé 56. 90%) Natural; fiseion U Kr3®™ 4 36 hr p 78%, IT 22% se®?(a, n); Kr®4(4, p); Kr84(n,y); Rb85(n,p);' Sr88(n, a); fission U, daughter Br35 I:(r85 10.3 yr B K:r84(n,y); fission U Kr86 Stable (2bundance 17.37%) Natural; fission U ®Kr®7 78 min | 'y Kr28(a,v); kKe384, p); Rb87(n, p): fission U K98 ‘2.8 hr B Fission U, Th Kr? 3 min 8 Fission U, Pu Kz’ 33 gec - g Fission U, Pu Krg1 9.8 sec B Fission U, Pu Kr % 3.0 sec p Fission U, Pu, Th K> 2.0 sec B Fiassion U, Pu K:94 1.4 sec B Fission U Kr?! l sec B Fission U, Pu XelZI 40 min [3+ 112.7(p, n) xel?2 20 hr EC 1127(p, 6n) Xe123 2 hr EC, 'fi+ 1127(p,5n) . Xe124 Stable {abundance 0, 096%) ‘Natural Xe 2™ 55 gec IT (7) 1127(a, 6n)Cs?>; daugh- ter Cs]'25 xe!?3 18,0 hr EC Tel?2(q, n) Xe 24 (n, v) Xe126 Stable (abundance 0, 090%) Natural ' xel?7T 75 sec IT 1127(a, 4n)Cs '%7; daugh- ter CSIZT ' (Table Continues on page 4) TABLE OF ISOTOPES OF He, Ne, A, Kr, Xe, and Rn, (Cont'd) Isotope Half-life Type of Decay Method of Preparation xel?T 36.41 day EC Te'?%(a,n); 1'%7(q, 2n) 127(P.m); Xe 2 (r, y) xe 128 ~ Stable {abundance 1.919%) Natural ' Xel?9™M g 0day IT xe'%8(n, y) Xe129 Stable (abundance 26, 44%) Natural Xe130 Stable (abundance 4. 08%) ' Natura.ll xe!3!™ 12 0 day IT Xe'3l(n,n'); fission U Xe131 Stable (abundance 21. 18%) Natural; fission U Xe132 Stable (abundance 26. 89%) Natural; fission U xe!33m 5 2 day IT , Xel3?(a,v); fission U xe'3® 5,270 day B Xe132(n,v); Xe 1324, p); .. xe 340, 20): Te 3%, n); cs133 (. p); Be136(n a): fission U xe 134 Stable (abundance 10.44%) ‘Natural; fission U Xe 113%™ 15 min IT - %e138(n, 2n); Xe!34m,v); _ Ba138(n, a); figgsion U xe!®® 9,13 hr 8 xe 3% n, v); xe 3¥(4, p); 36(n, 2n); Ba138(n, a); fission U Xe136 Stable {abundance B, 87%) Naturél' fission U Xe137 3.9 min p 136(n v); fission U Xe 138 17 min B Fission U xe !9 41 sec B Fission U, Th xe!? -10 sec 8 Fission U, Th xe 41 ~2 sec g Fission U xe 143 1.0 sec B ' Fission U xe 44 ~1 sec B Fission U Rn?%® <6 min ' a 65%, EC 35% At (%, sn) Rn?%7 <10 min EC 96%, a 4% Aat T4, an) Rn?%® 22 min . EC~809%, a-20% Spall Th; Pb(Clz, apall) Rn?%? 30 min EC 83%, a17% Spall Th; Pb(C'%, spall) ’a?'% 2.7h: ' a-96%, EC~4% Spall Th; Pb(C %, spall) RnZ!! 16 hr EC 74%, a26% Spall Th; Ph(C'%, spall) Rn?!'? 23 min a Spall Th; Pb(C ', epall) Rn% 1> ~10705ec {est.) a U??7chain from Th?32 (2, 9n) TABLE OF ISOTOPES OF He, Ne, A, Kr, Xe, 'and Rn, (Cont'd) Isotope Half-life Type of Decay Method of Preparation Rn’ 16 ~10"%sec (est. ) a ~ u?28 hain from Th232 | (a.8n) Rn? 17 1072 sec a - U%%%nain from Th?3? . (a,7n) Rn218 0.019 sec a U230chain from Th232 | (a, 6n) ' R.nz'19 3.92 sec a Member U2'35.deca.y | chain anzo 51.5 sec a Member Th232' decay | chain Rn?%! 25 min B ~80%, a~20% Th%>%(p, spall) anzz 3.8229 day a | Member Uz'38 decay "chain III. REVIEW OF FEATURES OF INTEREST IN RARE GAS RADIOCHEMISTRY The rare gases, Group O of the periodic table, are helium, neon, argon, —'kry-pton, xenon, and radon. Helium and neonl possess no radioactive isotopes of half-life long enough to permit radiochemical studies in the ordinary sense. One can conceive situations in which the separation of the remaining four rare gases from contaminants and from one another might be necessary. However, the most common problem by far is the separation of krypton and xenon re- sulting from fission of heavy elements from other fission producte and from each other. The literature on rare gas radiochemistry is \largely concerned with this problem, and most detailed discussion in this monograph will like- wige center around krypton and xenon. As rare gas separations generally depend on some property which varies greatly and in a regular manner as one proceeds through the group from helium to radon, the further applicati'on of the procedure to include other rare gasea will often be simple if required. Reviews of the '""chemical properties' of the rare gases will be found in most texts on inorganic chemistry. . In a practical sense, the rare gases are chemically inert. They will remain chemically unaltered in any reactions chosen to quantitatively remove impurities other than rare gases from them. . However, rare gas atoms do interact with other atoms,. molecules, or ions in their neighborhood. Whether these interactions are properly considered chemical or "van der Waal's' in nature is of no concern here, The existence in discharges of species such as HeH" and He2+ has been established. Hydrates for the four heavier members of the group are known to exist, and solubilities of rare ga.ae.s in a number of solvents are relatively high, Solubility in a given solvent generally increases with atomic number of the rare gas._ A number of studies of rare gas solubility dependence on sol- vent, temperature, and partial pressure of the rare gas (Henry's law is ap- plicable over wide ré.nges) have appeared in the literature. One of the more comprehensive studies also outlines a proposed system for recovering krypton and xenon from gas streams by their absorption in a counter-current stream of organic liquid (kerosene). 2 In handling rare gases, especially tracer amounts, one does well to remember that co_nta.?:t with liquids or solids (including system walls) or condensation from the gas phase of other liquids or solids may result in re- moval of rare gases from the gas phase through solution, adaorption or phys- ical occlusion. Thus in quantitative work rare gas carriers are usually added for essentiallylthe same reasons that carriers are added for species to be separated from solutions, . . A series of substances known as clathrates has received considerable 3,4 Clathrates of argon, krypton and xenon have been prepared by study. crystallizing quinol under an atmosphere of the rare gas at high preasure. Rare gas atoms are contained in "cages' within the reéulting crysfal, the number of these cages setting an upper limit on the amount of rare gas in the crystal (one rare gas atom for every three quinol molecules). The actual concentration of rare gas in the crystal depends markedly on the conditions of crystallization and is usually considerably less than the theoretical limit. ' Clathrates of fission-product krypton have recently been prepared for use as sources of Kr85 activity. > The preparation described in the reference re- sults in krypton concentrations equal to 25% of the theoretical limit, This correEPOnds to the concentration in the gas phase at about 25 atmosphéres pressure and results in 3 curies of Kr85' per gram of clathrate (using fission product krypton. which is about 5% Kr85). The material may be ground to a powder with no appreciable loss of activity and the loss on standing is only a few parts per million. per day if the material is protected from water and other subsfancea which dissolve quinol. ' The chemical operations involved in separating and purifying rare gases are performed on elements or compounds other than rare gases. The Bepara- tion of the rare gases from one another must be accomplished on the basis of differences in some physical property (usually vapor pressure or extent of 6 adsorption). It may often happen that this physical separation will also sepa- rate some or all of the impurities other than rare gases from a particular rare gas. Thus it ie often neither necessary nor most convenient to purify the rare gases chemically as a group as the first step or steps in a radiochem- ical procedure. Depending upon the conta.ml:.nati.ng species and their amounts it may be possible that no chernical operations whatever will be necessary. Substances which may contaminate rare gases are limited to those which may exist in appreciable concentratione in the gae phase at the temperature of the experiment. At said temperature thie will include: 1) gases, the term being used here to denote appreciable amounte of substances above their critical temperature or having vapor pressures greater than 1 atmosphere; 2) unsaturated vapors, substances whose vapor presa-urés are less than one atmosphere but which are present in amounts appreciable but small enough - that their partial pressures are less than their vapor pressures; and 3) satu- rated vapors in equilibrium with solids or liquids having appreciable vapor pressures. The word "appreciable in the foregoing must of course be de- fined in the context of the experiment. Although the above is a large category, the number of contaminants usuélly encountered is quite small. The rare gas'es must often be purified from the constituents of air, from volatile species involved in target dissolution such as hydrogen, hydrogen halides and oxides of nitrogen, and occasionally from small amounts of hydrocarbons and trace amounts of elemental halogens. Methods of removal are listed below for each of these. The list is by no means complete as regards methods which have been or could be used, but it is hoped that it is representative enough to be useful. 1) Nitrogen: Mole amounta may be qfia.ntitatively removed by reacting with titanium sponge, 14-20 mesh at 1000-1100°C. Use of lower temperatures " (ca. 850°C) has been reported. 6 Oxygen is also removed quantitatively, Quartz or ceramic furnaces are necessary as these temperatures are above the sbftening point of Pyrexl glasses,. The reaction is quite exothermic and so the temperature must be monitored and flow rate of the gas controlled to prevent destruction of the furnace. Calcium has often been used aa.tisfa.cltorily to remove nitrogen (400-500°C). Calcium does, however, tend to become passive through formation of surface filme, For small amounts of impurities this problem is sometimes circumvented by conducting the reaction in the gas phase with Ca vapor. 7 Clean uranium turnings at 800°C will aleo react with nitrogen (and decompose hydrocarbOnsB). 2) Oxygen: Will be removed with nitrogen in the above reactions, Oxy- gen also reacts rapidly with copper turnings above 350°C to give CuO. 3) Hydrogen, carbon monoxide, light paraffin hydrocarbons: Paesage 9 of the gas stream over CuO at 500°C will oxidize H, and CO to CO, rapidly. Hydrocarbons will be oxidized in like manner to HZQ and COZ but a to HZO temperature of 900°C is necessary to achieve rapid reaction. Subsequent re- moval of oxygen may be necessary (and is easily accomplished), if high enough temperatures are used that the dissociation pressure of oxygen over CuQO becomes appreciable. : 4) Water: In addition to condensation of water in cold traps (a method often not specific enough), water may be removed by passage through any one of a number of desiccants. PZOS' Mg(ClO4)Z,' and Ca.Clz are representative, with equilibrium partial pressures of water over them at room temperature -5 of 2 X 10 resent the extremes of efficiency in common desiccants. Magnesium per- , 5%x10°%, and 0.2 mm Hg respectively. P,O; and CaCl, rep- chlorate has achieved wide use as it is efficient enough for practically all pur- poses and is easily regenerated by distilling the water off in vacuo at 220°C. Where 'la.rge amounts of water are involved, gases are often driéd first with C‘.=1C12 to remove most of the water and then with Mg(GlO4)2 to remove re- maining traces of moisture, 5} Carbon dioxide: May be removed by passing the gas through a trap . containing Ascarite, a granular commercial preparation which is esé_entia.lly asbestos impregnated with sodium hydroxide. Sofnolite, or soda-lime, a mixture of NaOH and Ca0 may also be used. Sweeping the gas through a solution of alkali metal hydroxide is' also effective. Solutions are sometimes less convenient in vacuum systems than solid materials, ‘however. 6) Hydrogen halides: May be removed in the same manner as carbon dioxide. | | 7) Oxides of nitrogen: Dissolution of targets in nitric acid will result in evolution of NZO' NO and NO2 ir varying proportions, NZO usually occurs in relatively small amounts. Its removal may be accomplished by catalytic reduction with hydrogen or by oxidation to NO by scrubbing the gas with.a solution of strong oxidizing agént such as permanganate. NO and NO, may be removed by sweeping the gas stream through a solution of sodium hydrox- ide. Performing the solution under reflux will in fact wash a good portion of the higher oxides back into the dissolver flask. If alternate methods of solu- tion are available it is often most convenient to avoid the us e of nitric acid. | 8)- Halogens: Free halogens are all relatively volatile,. and active halo- gens may often be present in trace amounts in tai-gets from which rare gases are to be remdved. If the solution process leaves them partly or entirely in the zero oxidation state, the gas stream will be contaminated.- Sweeping the gas through a solution of sodium hydroxide will rapidly convert all the free halogens to non-volatile species (halide and hypohalite). 10 The rapid ex- change of the halogens with their corresponding halides in solution is another 8 useful means of decontamination from tracer halogens, 1,12 Iodine is quan- titatively removed by contact with silver (silvered Raschig rings have been used as a trap packing for this purpose).- Iodine is reduced to iodide in 1’\1’a.HSO3 solution. _ Where reaction rates are slow enough that purification may not be com- pleted in one pass, it may be necessary to circulate fhe offgases from target dissolution through a purification train. The classic, and still most generally useful, device for circulation (or transfer) of gases is the Toepler pump, with which rates of about a liter per minute may be obtained. Other circulat- ing pumps which may be useful in particular instances have been designed. These consist essentially of a chamber designed to permit only unidirectional flow of gas and a means of producing periodic pressure changes within the chamber. Alternate heating and cooling of the gas in the chambet has been used to achieve flow rates of 0,5 liters per hour. 3 An iron piston enclosed in glass and electromagnetically operated has produced rates of a few liters per minute ,1 Pressure differentials de.vel.dped were. not noted in the reference but are probably small. Another achieves aimilarly. high flow rates by pulsing the chamber pressure by means of a fnyla.r diaphragm driven by high preasure gas. 13 . Ultimately the radiochemist must concern himself with the physical properties of the rare gases. Table 1 summarizes some data which may be of interest, Table 1 He Ne A.r Kr Xe Rn Atmospheric abundance 5.25 1,82 0.934 1.14 8.7 6x107'% (Volume %) x107* x1073 x10"% x107° Boiling Point (°C) -269 -246 -186 -153 -107 -65 Melting Point (°C) -272 -249 -189 -157 -112 -71 ' (25 atm.) Atomic diameter in 3.57 3.20 3,82 3.94 4.36 _——- crystal (angstroms) Distillation and adsorption techniques are those which first come to mind for separation of the rare gases. The pure rare gases have of course been successefully produced commercially by the fractional distillation of air, with the exceptions of helium which is extracted from certain Texas natural gases and radon which is obtained as 2 member of the 4n+2 natural radioactive decay séries. Fractional distillation is discussed in most texts on physical chem- istry and the ‘fractionation of rare gases from air is discussed in numerous places. 17-20 Glueckauf used 2 procedure combifiing fractional distillation of air and adsorption techniques in the detg_rmination-of krypton and xenon con- tents of the atmosphere. 21 It is very unlikely that the radiochemist will have need to use such low temperature fractionation columns in the laboratory so there will be no d'iscussion. of the method here. On the other hand, simple distillation and condensation processes will be used in practically every experiment in transferring gases or effecting rough separations. Thus a plot ( for low temperatures) of vapor pressures as a_fu.nction of temperature for rare gases and other volatile species commonly encountered will prove very handy to the radiochemist. The reader is refer- red to the generally ava.fl.able Handbook .of Chemistry and Physuca22 for data necessary for such a plot. Dushma.nZ (page 781 et seq.) also tabulates vapor preasuré data for a number of substances at low temperatures. In order of decreasing volatility (decreasing vapor pressure at a given temperature below the critical temperature) the common gases are NZ' A and 02. Kr, Xe, Rn and COZ-. and HZO' Gases listed in pairs have vapor pressure curves at low temperatures which are very similar, Efficiencies of processes such as the transfer of a gaseous species con-’ tained in a system to a cold trap attached to the system are determined by the relation of the initial pa.rt-ia.l. pressure of the species in the system to the vapor pressure of its solid or liquid after condensation in the cold trap, As an ex- ample, the vapor pressure of solid Kr at -195°C ie 2-3 'mm Hg, so roughly 99%' of the Kr in a system at 20 cm Hg pressure may be collected in an attached small cold trap at -195°C. As liquid nitroger is generally the coldest con- veniently available refrigerant, Kr is often manipulated by condensation at this temperature. The resultant losses decrease with increasing initial krypton pressure and may be minimized by using- the emallest systeme and the [a.rgest amounts of krypton carrier practicable -- usually they may be made small ‘enough that they are acceptable for the sake of epeed and convenience. If one must of necessity collect krypton at pressures comparable to 2-3 mm Hg from a system, he must resort to lower temperature refrigerants, refrigerated adsorbents such as charcoal, or Toepler pumps. Presence of small amounts of noncondensible gases may seriously lower the rates of transfer procegses | such as the above krypton condensation, due to the relative slowness of gas- eous -interdif_fusio_n processes. Thus one wishes a vacuum system with a base pressure orders of magnitude lower than the lowest vapor pressures involved in nganipulazions ‘he may wish to perform. Systems with base pressures of 10 to 10 © mm Hg are easily constructed and will suffice for the'manipula- 10 tions involved in most rare gas radiochemistry (assuming that macro rather than tracer amounte are involved). | Removal of only the least volatile species from a gas stream is feasible when a temperature can be found at which its vapor pressure ie sufficiently less than its partial pressure in the gas stream that condensation is essen- tially complete, while the vapor preseuree of other constituents are higher than their partial pressures over the t:rap'. Assuming equilibrium conditions and insolubility of the other epecies in the condensed phase, passage of the g3s stream through the cold trap effects the desired separation quite simply. Iflc;ondensa.t'ion occurs under equilibrium conditiona, the fraction of species lost (passing through the trap with the gas stream) is the ratio of its vapor pressure at trap temperature to its partial pressure in the influent gases. To ' improve the approach to equilibrium conditions, flow rates are limited and cold traps packed with some material to provide a large contact area and to prevent mechanical blow-through of condensed materials. Glass wool, beads or ringe are materials commonly used for this purpoae. Stainless steel bails (3/16 in.) combined with suitable glase wool plugs have aleo proved very sat- isfactory. Such a packing has the advantage that the steel may be warmed by the induction of eddy currents in it to achieve rapid and efficient removal of condensed materials. Time for thermal equilibrium to be reached in traps must always be allowed before initiating gae flow. As cooling of trap packings under vacuum may be quite slow, temporary introduction of some easily re- movable gas such as helium to improve heat transfer is often useful. In like fashion several condensed materials of widely differi.izlg volatil- ities may often be separated by warming to a suitable tempéra.ture and distilling the more volatile components into another cold trap. It is often necessary to melt and refreeze the condensate remaifii.ng behind several times, each time removing the volatile material evolved to the other cold trap, to effect com- plete removal of occluded or dissolved traces of the volatile materials (e.g.. the distillation of CO, from CO, The subject of trap refrigerants should be touched on briefly, Liquid -ice condensate at dry-ice temperature), nitrogen (-195.8°C}, dry ice (-78. 5°C), and ice (0°C) are commonly available in most laboratories, -Fortuna.teljr temperatures lower than that of liquid nitrogen will usually not be required in handling rare gases other than helium and neon, It might be noted that pumping on liquid nitrogen will cool it by evaporation to its triple point (-210, 9°C, 96.4 mm Hg). Témpera.tu.rea inter - mediate to those of liquid nitrogen, dry ice, and ice are often necessary and are commonly obtained by cooling a suitable liquid with liquid nitrogen or dry ice to obtain a partially frozen '"slueh'" at the melting point. Slush or liquid refrigerants provide better heat transfer from the trap than solid refrigerants, 11 thus dry ice is usually mixed with acetone to provide a slush refrigerant at -78.5°C. Discussion of cold baths may be found in most texts where vacuum 24,25 The author has found the following baths par- techniques are discussed. ticularly useful in working with rare gases: acetone, -95°C, n-pentane, -130°C, and isopentane, -160°C. It is usually most convenient to refrigerate traps through which large ~ quantities of gas will flow with liquid nitrogen, dry ice, or ice since such traps are easily replenished in place on the line. '"Slush-cooled' traps at other temperatures may usually be reserved for 6pe1-a.tions where they need take up only small amounts of heat. Adsorption and Adsorption Techniques By far the majority of separations of rare gases to be found in the litera- ture have employed adsorption techniques. Of the large number of adsorbent rnaterials available, a.ctivated- charcoal has most often been us.ed because it has the largest surface area (largest adsorptive capacity) per unit mass and because the rare gases adsorb on charcoal to widely differing extents. For a given adsorbate gas and adsorbent material the amount adsorbed per unit mass of adsorbent, called v and usually expressed as cc STP (Standard Temperature and Pressure,i.e., 0°C and 760 mm Hg) of gas per gram adsorbent, is a function of temperature of the adsorbent and partial pressure of the gas over the adsorbent. Three functional relationships among these variables are of interest: 1) Isotherms showing the dependence of v on P at constant temperature; 2) Isosteres relating P and T at constant v; and 3) Isobars relating v and T at constant P. In experimental studies of adsorption the isotherms are most usually determined at a series of tempera- tures. From these data the other relationships may be obtained if desired. The Clausius-Clapeyron equation also relates heats of adsorption and the pres- sure change with temperature (shape of the isostere), A number of equations have been used to exp-resa adsorption isotherms, most of which are empirica.l in nature. Langmuir first derived the hyperbolic isotherm for monolayer adsorption from theoretical considerations. 26 Brunauer, Emmett, and Teller27 later .derived equations for multilayer ad- sorption which reduce to Langmuir's equation in the case of a monolayer. The hyperbolic isotherm may be written v = vsbP/(l +bP), P = v/b(vs—v) , or P/v = l/b\a"3 + P/vs » where Vg is the cc STP of adsorbate requiret_:l to form a monolayer per gram of adsorbent, and b is a constant dependent on temperature and the nature of the adsorbate and adsorbent. The last form is particularly useful for the extension of data by interpolation since the plot of P/v versus P is linear. At low pressures, or perhaps more definitiw;rely low 12 v , it is found that adsorption obeys Henry's law (v = kP), and interpolation .on a linear plot of P- versus v is possible. It will be noted that for P much less than 1/b, or v much less than Vg the hyperbolic isotherm reduces to Henry's law with k equal to ‘m_rB . Freundlich's parabolic equation (semi-empirical in nature) is often useful for linear interpolation on a log-log plot in cases where it is obeyed: v = KP /n . Peters and Weil28 studied the adsorption of argon, krytpon and xenon on charcoal and calculated the constants in the Freundlich equation from their data at several temperatures, These are given in Table 2 for handy reference. Adsorption data for a number of gases on charcoal are tabulated in Chapter 8 of Dushman. 23 The adsorption process releases heat and as expected from LeChatelier's principle adsorption decreases with in~ creasing temperature, other factors remaining constant. Adsorption increas- es with increasing pressure. Adsorption of different gases tends to increase with decreasing volatility (extent of adsorption tends to increase in the same order as the boiling points of the gases), Table 2. Adsorption constants for argon, krypton and xenon: v = KPl/n T°C K 1/n Argon -80 0.500 0.950 -18 .0.0764 1.0 0 0.0581 1.0 Krypton -80 : 2.927 0.711 -18 - 0.497 . 0.885 ) 0.340 1.0 Xenon _ -80 15.99 0.574 ' -18 2.458 0.692 0 1.583 0.77 Theoretical treatments discuss adsorption in terms of: 1) formation of a monolayer of adsorbate on the surface, 2) formation of multilayers on plane surfaces, and 3) capillary condensation in small pores. Adsorption of a gas is considered the result of van der Waal's forces of the same type involved in its condensation. The magnitude of the interactions resulting in adsorption is indicated by the fact that heats of adsorption are generally comparable to the heat of condensation for a given gas, Experimental isotherme may conform over wide ranges to theoretically derived equatione (as the hyperbolic iso- therm) for suitable values of the constants in the equation. However, values of the constants thus obtained are often not easily related to the physical models used in the derivations. For discussion of adsorption in general and a key to the literature on the subject the reader is referred to Chapters 7 and 29 8 in Dushman’> or to §. Brunaues's book on physical adsorption. 13 Some comments on the nature of activated charcoal may be worth while. 30,31 There are numer- These are mostly from two recent review articles. ous materials from which activated charcoal may be prepared and many meth- ods of preparation, 32 For example, one might start with coconut shells and heat in the absence of air to remove the greater part of the elements other. than carbon as volatile materials. The resuiting material is then activated with steam at 700-1000°C. The large specific surface area of charcoal (of the order of 1000 square meters per gram) is due to its high porosity, as opposed to the nonporous adsorbents such as l.amp. black whose large specific surface area is the result of extremely small particle size. Thus the surface per gram of lamp black is a mmarked function of the average particle size, whereas the specific surface of activated charcoal is almost independent of particle size. Furthermore, most of the surface area in charcoal apparently Tesides in pores with diameters comparable to molecular dimensions. It follows that one might expect principally monolayer adsorptidn on charcoal. Thus adsorption on charcoal might be expected to obey the hyperbolic iso- therm, and does so over wide ranges. The potential pore structure of 2 char- coal is dependent on both the starting material and the conditions of pyrolysis. Charcoale from hardwoods,; nutshells and certain grades of coal generally give the highest porosity. The material after pyrolysis (though porous) still has low adsorptive capacity and the process of activation consists essentially of burning out more material in order to either open new pores or to enlarge existing pores to diameters which will a.c.cor.nmpdate adsorbate molecules (perhaps both processes occur), Steam is widely used for activation as the reaction is endothermic and better temperature control is possible. One ex- pects then that the propérties of the material he receives in the laboratory depend not only on ‘materials used in preparation but also very markedly on the history of its pyrolysis and activation. Excessive activation can in fact - remove more surface area than it creates, the adsorptive capacity going through a maximum as the activation proceeds. Since the distribution of pores as to size includes m#ny with diameters comparable to molecular dimensions, a "molecular sieve' effect may be expected to some extent, Thus some pores will accommodate a given molecule but are too small for a slightly larger molecule. During activation the adsorptive capacity for various species may therefore not be increasing in the same ratio. Charcoal has in the past been referred to as an allbtro;pe of carbon, but elemental analyses indicate atom percents of carbon always less than 100% and sometimes as low as 75%. The other elements present are principally hydrogen and oxygen, but appreciable amounts of nitrogen, sulfur, chlorine, and inorganic ash may also Be present, It appears likely that the surfaces on which adsorption occurs are not carbon surfaces but hydrocarbon surfaces modified by the presence of oxygen and 1k traces of other elemente. This implies that all adsorptiofi sites are very likely not identical. | | _ Activated charcoal is thus a class of substances whose chemical and - physical properties may vary within rather wide limits. Let us hasten to add that this does not really impair ite usefulness as an adsorbent. The adsorp- tive properties of charcoals in separation systerns are usually not so critical that even variations like factors of two or more will result in loss of sépara- tion, though such variations certainly affect the procedure in detail. The intent is to point out that the experimenter sheuld not be surprised if_the ad- sorptive properties of his charcoal differ from those he sees quoted in the literature. In one study of the adeorption of xenon on seven charcoals at room temperature, the amount per gram of charcoal adsorbed at a g.iven pressure varied as much as a factor of five, 3.3 These charcoals were of course chosen on the basis of differences in materials and preparations expected to prbduce a range of adsorption characteristics. In order that early experience may be used to predict with fair accuracy the course of separations in systems built at later dates, it is probably wise to originally purchase a quantity of char- coal sufficient for many years' needs. Before use and between successive uses it is necessary to outgas char- coal, both to prevent contamination of samples with small amounts of residual gases from previous runs and to remove adsorbed materiale (such as water) whose accumulation fna.y_ adversely affect the adsorptive properties of the charcoal. This process should not be s0 e:lctrern_e as to change the material through further activation. A routine which has proved satisfactory is out- gassing in vacuo at 350-400°C. At Lawrence ‘Radiation Laboratory the out- gaseing is routinely continued until the pressure is 10_4 mm Hg or less as judged by a thermocouple vacuum gauge in the vacuum system manifold. One may operate in various ways to achieve separations of.gases through differences in the adeorption on charcoal (or any other adsorbent). It may prove useful to point out that such separations are basically similar to sep- arations of species in solution using ion exchange resins, The many.radio— chemists who are familiar with separations using such resins will note close analogies even in the various techniques used. | Separation procedures must be designed with kinetic cofisiderationa in mind -- adsorption from a stream of gas ie not an instantaneous process, It was stated earlier that the adsorptive capacity of charcoals does not depend on particle size, but it is expected that rates of adsorption will be so depend- ent. A reduction in the average particle diameter increases the exterior surface (per gram adsorbent) presented to the gas phase and shortens paths through cracks and pores along which adsorbate molecules must diffuse in order to reach adsorption sites in the particle interior. Approach to equi- 15 librium conditions should thus improve with smaller particle size and in- creased contact time with the adsorbent (decreased flow rate), A compromise is necessary as too small particle size will result in high resistance to gas flow. In flow systems low temperature adsorbent beds may not be maintained in thermal equilibrium with the refrigerant if the gas enters the bed at ele- vated tempera.tufe or if the rate of heat release from the adsorption processes occurring ie high., Flow rates and conditions affecting the rate of heat trans- fer are thus important in this respect. Although it is certainly of great inter- est and long-range usefulness to understand in detail the kinetics of these processes, the more practical and faster approach to design is empirical. Deeign factors are chosen to improve the kinetics insofar as possible and the experiment is tried, If the experiment succeeds, factors affecting kinetics .are usually nof varied to deterrnine the extreme conditions under which the procedure is satisfactory. Thus in plé.ces such as the Detailed Procedures where flow rates and charcoal particle sizes may be quoted it should be under- stood that these are sufficient for the separation, but may or may not be necessary should other considerations make changes desirable, _ Most rare gas separations will commence with total adsorption of all the gases of interest in an adsorbent trap. In thie manner one may minimize pfbblenis arising from non-uniform introduction of sample, temperature variations, and adsorption kinetics by ueing lower temperatures and more adsorbent than should really be necessary. By treating all adsorption steps in this manner one tends to reduce losses, since in subsequent desorption steps slow processes may affect the history of the separation in detail but will not result in actual loes of sample. In their early work on rare gases, Peters and Weilz8 showed that argon, krypton and xenon could be separated by total é._ds orption followed by fractional desorption in vacuum from activated charcoal. They adsorbed the mixture on charcoal at -190°C (roughly 10cc STP of each gas was used on 38 grams of charcoal) and then raised the trap tb a suitable temperature and desorbed the gases using a mercury diffusion pump backed by a Toepler pump to trans- fer them to a gas buret for measurement. Temperatures at which each gas may be desorbed '"pure" of the others are somewhat a compromise between speed and separation factor -- removal of argon at -93°C, krypton at -80°C and xenon at 0°C would be typilcal. " From knowledge of the isotherms, amounts of adsorbent and of each adsorbate initially, and the volume with which the adsorbent bed is equilibrated before each lift of the Toepler pump one could calculate in a straightforward but laborious fashion the composi;‘.ion of the gas in the adsorbed and gaseous phases after each stroke, assuming uniform distribution of the gases on'the adsorbent. Even cursory examination of the 16 isotherms suggests that while separations performed in this manner may pro- duce gases which are around 99% pure, separation factors of about 100 are the limit obtainable and would not be éatisfa.ctory in most radiochemistry. It might be noted that Glueckau.f34 analyzed mathematically the 0peratiofi of a system in which rare gases are successively adsorbed and desorbed on a series of charcoal traps and used such a system for separating helium and neon as the final step in the determination of their abundance in 2ir. A simi- lar system was later used35 for the separation of argon, krypton and xenon in an experiment to determine the relative yields of krypton and xenon isotopes from uranium fission by volume measurements on the separated gases and later mass spectrographic analysis. In these procedures a series of many traps was required. A sidelight in the work of Peters and Weil28 is worth mentioning to point out a matter of safety. They performed an experiment in which it wae shown that anzz adsorbed quantitatively from liquid air onto silica gel. As they point out, silica gel was used because of the explosive potentialities of charcoal and liquid oxygen. Because of this hazard it seems advisable to use liquid nitrogen, rather than liquid air or oxygen, to refrigerate charcoal traps as a safeguard in case of breakage, and to operate at reduced pressure when introducing air to a charcoal trap cooled in liquid nitrogen to avoid con- densation of liquid air in the trap itself. The techniques of Peters and Weil and of Glueckauf mentioned above have today been replaced by what may be generally termed chromatographic techniques. These 'u.su.a.lly involve adsorption of a .sa'mple on one end of an adsorbent column, and subsequent passage of some eluent gas through the column to achieve separation. I.n fact, Gl.uecka.u.f21 in his later determination of the Kr and Xe contents of the atmosphere separated the rare gases by elu- tion from a charcoal column ra.fher than by the method used in the He and Ne determination already mentioned. Keulemans' recent book36 will serve as "~ an introduction to the subject of gas chromatography and as an up-to-date source of literature references. Only a few remarks will be made here and some journal references cited. Chromatographic seeparations involve a sta- tionary bed of liquid or solid adsorbent through which a stream of liquid or gas moves. Rare gas separations on charcoal involve a stream of gas moving through a packed column of solid and thus would be classified as gas-solid chromatography. Methods of operating chromatographic columns will gener- ally fall into one of three categories: 1) Elution development in which a sample of the gases to be separated is adsorbed at one end of the column and a flow of slightly adesorbed carrier gas (eluent) set up through the column. 2) Frontal analysis in which a stream of gas containing the sample mixture 17 is continuously passed through the colufnn. 3) Displacement development in - which the sample mixture is adsorbed at one end of the colfimn and a contin- uous flow initiated of gas more strongly adsorbed than a.fiy of the gases in the sample. Elution development results in the eventual emergence from the column of bands or "peaks'' of the sample components- (diluted of course with eluent) in the order of their increasing adsorption. If the components differ enough in their adsorption it ise possible under pfoper conditions to obtain each component in pure form as a peak separated by pure eluent from adja- cent peaks. If the stream of gas consists of an ""unadsorbed' carrier .gas in addition to the mixture of sample gases, frontal analysis results first in the emérgence of pure carrier ‘'and then of the least adsorbed aampie component, diluted with carrier. Eventually the component which is next to the least ad- sorbed also breaks through, and so on until the column is completely saturat- ed with all components and the influent and effluent gases have the same com- position. Only a sample of the least adsorbed component may be 6bta.ined pure of other sample components. In displacement developmefit, bands of the components separated by zones containing a mixture of the species in adjacent bands emei-ge in order of increasing adsorption. Eventually the pure dieplacer emerges. Partial recovery of fairly pure componenta is possible. Theoretical predictions as to the details of chromatogi—aphic separations vary as the adsorption isotherm is linear or nonlinear, and the process 'ideal" (thermodynamically reversible) or '"non-ideal'". Linear-ideal and nonlinear ideal chromatography were treated by ‘l!.’ilson31r and by de Vault. 38 Linear ' 39-46 non-ideal chromatography has been treated by a number of investigators, and nonlinear non-ideal chromatography by Klinkenberg and Sjenitzer. 47 If very large amounts of gas are involved, adsorption isotherms will usually be nonlinear. Gas-solid chromatography is also likely to be non-ideal as the attainment of equilibrium between the phases offen cannot be considered in- stantaneous, among other things. Thus the radiochemist may well be con- cerned in his separations with the nonlinear non-ideal case, the least acces- . Bible to accurate theoretical treatment. The théory of the simplest case of linear-ideal chromatography for a single adsorbate provides useful qualitative insight into the processes occurring, however, Keulemans3 treats this case on page 112 et seq. of his book. A brief account of the reasoning will be given here. _ Consider an adsorbent uniformly packed into a cylindrical column of constant diameter and at constant temperature throughout. If an adsorbate is present in some small cylindrical element of the column taken in cross- section, it will dietribute itself so that a fraction g of the total ie in the gas phase and a fraction (1 --¢) on the adsorbent, As a fraction g of the adsorbate 18 is in the gas phase, a given adsorbate molecule resides in and thus moves with the gas phage g of the time on the average. It then follows that the rate of movement of adsorbate through the element is just ¢ times the linear flow rate F of eluent gas. Assuming the perfect gas laws, one may calculate g and F, Let us call Ge the volume available to the moving gas and Ae the weight of adsorbent in the element e. Pa. is the partial pressure of adeorbate and v the amount adsorbed in cc STP per gram of adsorbent. The factor for - converting gas volumes in cc-mm of Hg at column temperature to standard conditions may be called C = 273 "K/(T.,K X 760 mm Hg). The fraction of adsorbate in the gas phase will then be ¢ = CP_G_/(CP, G, +vA)=(l+va_/ CPa. Ge)-1 . As the column is uniformly packed the ratio of Ae/Ge will be the same in any element of column volume and equal to the ratio of total weight A of adsorbent to total void space G in the column. Furthermore, v/Pa = k will be constant for all v if the isotherm is linear, in which case k may. be referred to as the Henry'e law constant. In this case, ¢ = (1 + k.A/C(I‘:)-1 and is the .sa.me at constant temperature everywhere in the column regardless of adsorbate concentration. The linear flow rate of eluent will be F = qL/C GP, . length in centimeters, G is the cc of total column void space, Ptot is total gas pressure in mmm Hg, and C is the consetant defined above, The rate of movement of adsorbate is then OF = (1 + xA/C G)_1 {(gL/C GPtot) = qL/Ptot (CG + kA). In most instances ¢ << 1, or (kA/C G) >> 1, so that this expres- sion simplifies to OF = qL/kA P, In either case the rate of adsorbate » where q is the gas flow in cc STP per minute, L is the column movement is constant everywhere along the colurhn regardless of the concen- _tration of adsorbate. It follows that the rate of movement of all points in a band is the same, and the band shape does not change during its travel along the column. In ideal chromatography, frontal analysis results in the gradual eiten- sion of a band of adsorbate at.consté.nt concentration down the column, assum- ing constant column temperature and constant partial pressure of adsorbate in the gas entering the column. By equating total adsorbate introduced to the column to the amount present in the gas phase plus adsorbent phase in a band of length I on the column, one arrives at £ =({(q L/Pfot\) (CG + vA/Pa)_lt , where t is total time of flow in minutes, In the case of a linear isotherm this is just !a = 0Ft, as one intuitively expects. Even if the isotherm is nonlinear, (v/Pa) is constant everywhere in the band, for Pa. constant, This formula is thus a useful approximation in this case if P_ is known and con- stant in the gas entering the column, and the isotherm is known at column temperature. All the above discussion is strictly for a single adsorbate, as appreciable concentrations of a second adsorbate may affect the adsorption of the first. 19 In many experiments, the sa.rriple introduction is accomplished in a man- ner corr’espofiding to frontal analysis, with subsequent separation of the com- ponents by elution development. Detailed Procedure | (see last chapter) in which air containing Kr and Xe is admitted to a charcoal column over a long period is an example. After the admission of the sample is complete, the Kr and Xe are separated and recovered by elution into a suitable. receptacle with helium at suitable column temperatures. In such ‘cases a minimal requirement for satisfactory operation is that the length of the band of least adsorbed sub- stance be less at the end of sample introduction than the column length. Thus in Procedure 1, the final length of the band of krypton must be less than the column length. In the subsequent separation by elution the rates of movement for two adsorbates are & F and csz, go that times of emergence can be “1 calculated. For separation to be achieved the rear of the first band must overtake the front of the second band. Thus where T is the time for emer- gence of the rear of the first band, T o F =To,F + 1, =L, where [, is the band width of the more highly adsorbed component. Thus L = Izkz/(kz _kl) is the minimum column length in which separation of two components may be achieved, in the linear-idedl case, If, as is usually the case with charcoal, the isotherm is hyperbolic, ~then V/Pa = b(vs—v). -Thusl_l as v increases toward Vg (that is, the fraction of the surface covered with adsorbate becomes appreciablezs), V/Pa decreas - es. It will be seen that ¢ thus incre-éses as v increases, and the more con- centrated regions in a band move faster. Assuming ideal conditions the re- sult is an emergent band with a very sharp front and a2 long tail. In a prac- tical sense, if one has used a value of k obtained from the linear low pressure region of the isotherm to estimate the emergence time, the peak frofit will emerge earlier than calculated. In the latter portions of the peak tail where concentrations lie in the linear region of the isotherm, rate of movement will approximate that calculated. ' Departures from '"ideality'" such ae those due to appreciable rates of longitudinal diffusion in the gas or non-instantaneous mass transfer between phases result in broadening of bands during their travel, usually in a nearly symmetrical fashion. Superposition of these effects can give useful qualita- tive pictures.of the bands and their motion in the various cases. Thus non- linear, non-ideal conditions lead to asymmetric bands emerging earlier than under linear -ideal conditions. As to peak shape, one might ekpect the emer- gence first of a region in which adsorbate concentration increases slowly, then a eteep main peak front, a less steep main peak back, and finally a long tail in which adsorbate concentration slowly approaches zero. There are two general approaches to more rigorous theoretical treat- ment of chromatography. 20 The "'plate' theory treats the column ase a series of fuxtaposed "equiv- alent theoretical plates! in each of fivhich a separation factor between two species corresponding to the ratio of their distribution coefficients is achieved. The distribution coefficient is a ratio of concentrations in the two phases for a substance,. The ""rate'" theory takes into account the physical processes such as convection and diffusion, and séts up partial differential equations to des¢ribe them. The solution of these equations (in the adsorbate concentrations and their derivatives, with time and axial distance along the column as independent variables) is then essayed. Both approaches involve various simpl.-i.fy-lng assumptions. The journal references to theory given earlier include accounts of both treatments. It might be apecifically noted that one article by (_Trlueclcau.f48 deals with the effects of heating on movement and shapes of peaks when the adsorbate is a highly radioactive gas such as'Kr85. Peaks are sharpened and separations improved. One might say qualitatively that as the latter portions of the band move through regione warmed by passage of the active gas, ¢ is larger there, The tail would thus tend to move faster and ''catch up" with the main peak, In the event that the radiochemist has need to use charcoal after exposure to high radiation fielda, iiL might be noted that the adsorption of krypton and xenon on charcoal was found to be unaffected by previous irradiation with 1- Mev electrons (doses up to 1350 watt-hr per gram). The "rate" theory predicts the nature of the dependence of column effi- ciency (i.e., the '"height of an equivalent theoretical plate' ) on various fac- tors such as eluent flow rate, adsorbent particle size, column inlet and outlet pressure, nature of the eluent gas, etc. Although the development of the theory becomes mathematically fairly complicated, application of the results to practice is often relatively simple. The radiochemist who plane to make much use of gaé chromatography will certainly find -study of the theory inter- esting and profitable. However, one actually needs only a bare acquaintance with the theory to build a system to separate rare gases. The following paragraphe are written for the radiochemist who meets a problem involving rare gas separations, and who wishes to solve it with a2 minimum expenditure of time and effort. For this purpose the equations given earlier for the linear-ideal case may prove useful. The assumption of linear-ideal conditions is of course a coneiderable departure from reality in most cases, and the treatment recorded earlier is overly simplified in some respects even for these conditions. Unless one actually determines the adsorptive properties of his charcoal, he must also keep in mind that these may vary by factors of two or more from quoted values 21 in the literature. The author is in fact unaware of adsorption data in the lit- erature for Kr and Xe at temperatures less than -80°C. Isotherms at lower temperatures (such as -195°C) must thus be estimated by rather dubious ex- trapolations. ‘The distribution of rare gases will generally be heavily in favor of the adsorbent phase in separation systems so that rate of band movement may be written OF = q L/kA%o_b. Not all of these quantities may be varied at will with- out impairing column efficiency. Thus theory predicts an optimum value for the linear flow rate of gas: th:rough the column, Linear :Eldw rates of the order of 10 em/sec might be considered typical, though this parameter is not par- ticularly critical and may be varied by factors of two or three without serious effect. Column efficiency di'ops less rapidly near the optimum value with increased linear flow than with decreased linear flow, Efficiency is also impaired at very large column diameters, Column diameters are generally around 1 cm or less, Again, efficiency is not a rapidly varying function of diameter, and columnse of several centimeters diameter have been success- fully used (e.g., Detailed Procedure 1, given in last chapter). As the bulk density of charcoal generally liee in the range 0,5 = 0,2 gm/cms. it follows that extreme values of L/A, column length per gram of charcoal, found in various satisfactory columns will differ by only about a factor of 10. 'In addition, L/A is not conveniently varied once one has constructed his system, Columne are most often operated with an inlet pressure of eluent sufficiently above atmospheric pressure to sustain the desired flow, and with atmospheric pressure at the outlet. Thus %otis-nearly always close to 760 mm Hg. One wishes the linear flow rate of eluent to be as nearly constant as possible along the length of the column, and it ie thus desirable that the ratio of inlet to outlet pressure be as near unity as possible. As a certain difference between inlet out outlet pressure is required to maintain the desired flow in a given column, roughly speaking, it follows that higher operating pfessures are preferable in this respect. Band velocities for a given adsorbate a.fe most conveniently varied over wide ranges by adjusting the column temper- ature, i.e., by changing the ratio v/Pa = k which in the case of a linear - isotherm is a constant. Adsorption generally increases rapidly with decreas- ing témperature. and band velocities will often vary several orders of magnitude over accessible temperature ranges. Thus for Kr at 1 mm Hg partial pressure over charcoal, the values of v in cc STP per gram are about 0.1 at 25°C, 3 at -80°C and 100 ( a rough guéss) at -195°C. Thus for a2 typical chromatographic column one niay generally find some temperature at which the band velocity of an adsorbate of interest is auch as to give a suitable retention time. In addition to adjusting band velocity, one may adjust column length to achieve initial sample retention. Increases in column length wil} usually improve separations of several components also, since retention times are proportional to L, the column length, while band broadening increases roughly as Ll/z. A few examples of prior calculations which one might perform in es- timating the p'erformance of a proposed chromatographic column are given below. If the calculations indicate that the proposed operation is unsatisfac- tory or borderline, then various column parameters and/or operating con- ditions may be adjusted in the indicated directions before construction of the system. If the operation appears to be satisfactory with a suitable margin of safety, particularly as regards initial retention of sample. in a band of width small with respect to column length, the system might be tried as i:roposéd. In the latter case, further changes which may prove neceassary can probably be made on the basis of a very few trial runs. The length of an adsorbate band was given earlier as ‘Ea = (th/Ptot) (CG + vA/Pa)_l, and for the usual case where CG << vA/Pa this may be re- ' written to a close approximation as £, =@qP_t/P,_ NvA/L)-1. In this form it is obvious that the band length is simply the total cc STP of adsorbate in- troduced divided by the cc STP on the adsorbent per unit column length. If the partial pressure of adsorbate is constant during sample introduction, v may be determined if the isotherm is known at column temperature. Values of other quantities required for the calculation of band length are e:k-perimen— tally accessible, Assume that 100 cc STP of Kr in air is introduced to a char- coal column at -195°C, and that the pa.rtié.l pressure of Kr in the influent gas is constant at 0.02 mm Hg. The column containe 5 grams of charcoal per centimeter length. If the adsorption of Kr under these conditions is 10 cc STP er gram of charcoal, one arrives at f = - 100 cc STP Kr ' per g ' ' Kr (10 cc STP Kr/gm)(5 gm/cm) 2 cm at the end of sample introduction. The value of v used above is strictly an estimate as the adsorption isotherm is not experimentally known at this.tem- perature. The foregoing corresponds to a typical sample introduction in Detailed Procedure 1, however, and using a p-y survey meter on bands of radioactive Kr, widths of a very few centimeters are noted. The actual col- umn length used in Detailad Procedure 1 is about 25 cm, a considerable margin of safety thus being allowed for uncertainties in the calculations and variations from run to run in such factors as the partial pressure of Kr in the gas entering the column. Le.t us assume that the above sample also con- taine 100 cc STP of Xe. Under the same conditions of temperature and rare gas partial pressure, values of v for neighboring rare gases are typically about a factor of 10 greater for the heavier of the two. Thus it ie safe to say 23 that Xe will be initially retained‘ in a band narrower than the band of Kr, Beyond this, no statements as to the actual disposition of Kr and Xe on the column will be attempted. The values of v being discussed here are compar- able to v, the cc STP required to form a monolayer, which is of the order of 100 cc STP per gram of charcoal. The portions of the isotherms involved are unknown, but definitely nonlinear. A more serious complication is the fact that at these concentrations on the adsorbent the adsorption of each rare gas will certaifily be affected by the preeence of the other, and by the adsorption on the column of large quantities of air, Having retained the Kr and Xe in suitably small portions of the column, their separation by elution development may be attempted. As the adsorptivi- ties, and thus the band velocities, differ by about an order of magnitude under most conditions, sufficient separations are easily obtained, even allowing for band '"tailing''. However, because of this tailing the purity of the Kr with respect to Xe may be expected to be better in general than the purity of the Xe with respect to Kr. The purity of both fractions will usually be sufficient for most purposes. Although the equation given earlier for mifiim_um column length to achieve separation, L = 12'k2/(k2—kl) where ''2'" refers to the more and "1" to the less adsorbed component, may be useful in other con- nections, it is of little use with rare gases in the mode of operation under diecussion. If kl/kl is gi'ea.ter than 2, the ca.lcul.ated minimum length will be less than the band width of the less adsorbed gas at end of sample intro- duction. That is, in the linear-ideal case, krypton could be eluted off a col- umn only a few percent longer than the band width of xenon, leaving pure xenon, However, all but a few percent of the krypton would have left the column be- fore the end of sample introduction. Elution of Kr from a column would take an unnecessarily long time at -195°C, so one raises the column temperature in order to bring the Kr off in a few minutes. A sample calculation of eati- mated retention time during the elution developrflent appears in the second example below, - : Adams, Browning, and Ackley50 recently reported on a éystem for re- moving Kr and Xe fission products from homogeneous-reactor off-gases and retaining thern until radioactive species have decayed sufficiently to permit release to the atmosphere. It was of course necessary in this case ‘that the system be designed as accurately as possible before trial. Some discussion - of the theory used in designing the system is given in the article and a refer- ence to a more detailed treatment of the theory is noted. 5.1 While the linear- ideal approximation would not be satisfactory for use in design in this case, it nevertheless provides a useful illustration of the calculations and a comparison of the rough predictions with experiment. Fission gases contained in a stream 2 of oxygen flowing at a rate of 2 liters per minute (at 1 atmosphere preasufe and 25°C) were passed through two parallel series of i:ipes containing charcoal. Each series of pipes contained a total of 520 lb of activated charcoal and con- sisted of 40 ft of 0. 5-in. pipe, 40 ft of 1-in. pipe, and 60 ft of 6-in. i.d. pipe. The charcoal was at 25°C. From the total internal volume of the pipes and the weight of charcoal one arrives at an-average bulk density of the charcoal equal to 0.69 grn/cm3. Assuming uniform packing the ratio L/A in each section of pipe is 1/(bulk density)(cross-section in cmz). Values of L/A in cmn/gm are thus 1.14 for 0,5-in, pipe, 0,285 for 1-in. pipe, and 0.00792 for 6-in. pipe. In the 0.5-in. pipe oF = qL/k-APtot _ 1000 em3 /min. X I, 14 cm/gm _ 760 mm Hg X k 1.50/k'in cm/min. Similarly OF = 0.375/k in the l-in. pipe and 0.0104/k in the 6-in. pipe. As there is roughly 1 cm3 of void space per gram of char- coal in the column, one may calculate that only a few percent of the adsorbate in the column resides in the gas phase. Use of the simpl.er formula above for 8F is thus justified. Retention time in each section equals the length of pipe o . . . . _ {1220 1220 . divided by band velocity, Total retention time is Fhus Ty = 150 t 5375t fi% k = 1.80 X 105 X k(min.) = 125 X k (days). The experimental values of k given in the article are about 1.3 for Xe and 0.08B for Kr (in cc STP of adsorbate per gram charcoal per mm Hg adsorbate pressure). The calculated retention times are thue 10 days for Kr and 163 days for Xe. Observed re- tention times were 10 days and 60 days respectively. This article also con- tains interesting experimental data on variations of Henry's law constant with temperature, rmoisture content of the adsorbent, nature of carrier, partial pressure of krypton, and the adsorbent used. | Koch and Gra.ndyEz Tecently reported studies of Kr and Xe separations on charcoal using tracer Kr85 and }(e133. The tracer rare gases were de- posited at one end of a column of charcoal and eluted off using columns of ebout l-cm diameter and lengths of 5, 16 or 43 cm, helium eluent flow of 0.6 or 1.6 liters /min., and operating temperatures of 25°C, 0°C and -80°C. Emer- gent peaks were observed by measuring 'ra.dioactivity in the exit stream, and times for breakthrough, peak maximum and 1% of peak activity in the tail recorded. Satisfactory separations were obtained in all the trials, ‘To cal- culate the linear-ideal approximations for retention times in the case of 1.6 liter/min. helium flow, 43-cm.column, and 25°C column temperature, we may write Tp = L/gF = kAZ%D_b/q- In the article the effective (bulk) density of the 40 to 50 mesh charcoal is estimated at about 1 gm/crn3. Column vol- ume is estimated geometrically to .be 43 cm3. Thus TR = W = 20.4 k (in min,). Using k as 0.08 for Kr and 1.3 for Xe from the last example, the calculated Kr retention time is 1. 6 min. and the Xe retention time 26 min, Observed times for emergence of the peak max- ima were 2.1 and 20 min., respectively. 25 The emergence of various components from a column may be detected by means of the corresponding change in some property of the effluent gas. Keulemans discusses various methods of detection. The most widely used technique involves a thermal conductivity cell (ké.thérometer) which detects changes in the thermal conductivity of the effluent gases passing through it.53’ >4 For radioactive gases another method is tb pass the gas leaving the column through an ionization chamber or near a counter of some sort55 to detect the activities of the various componeéents as they leave the column. In separations by charcoal chromatography impurities other than rare - gases will interfere in certain cases. Thus while argon, oxygen and nitrogen adsorb to slightly different extents, the separation of argon from large amounts of oxygen and nitrogen is best accomplished chemically. If only krypton and xenon are of interest it is not necessary to remove air before the chromatographic separation. Methane is very difficult to separate from krypton by adsorption on charcoal or by distillation, thus light hydrocarbons must usually be separ- ated chemically from krypton. Certain materials should always be removed from the gas before its introduction to charcoal since they are difficult to remove from charcoal and their accumulation tends to 'poison' it (e.g., water, 50 oxides of nitrogen, HCl). Most impurities may either be removed from the mixture of rare gases before their adsorption or from the separated rare gas fractions after their elution. This choice may be made as a matter of convenience in the particular experiment. Though charcoal is the most generally useful adsorbent it is by no means the only possible one. "Adsorption is a very general property of surfaces. Silica gel has specific areas less than but of the same order of magnitude as those of activated charcoal (its use in the adsorption of radon has already been mentioned). 28 There is in partiéular_ one other class of adeorbents which should be mentioned. These are the so-called "molecular sieve'' ma- 56,57 now commercially available from Linde Air Products Company, terials, Tonawanda, New York. Actually these are zeolites and as such their adsorp- tion properties are discussed to some extent by Dushman. 23 Thus, the 5A Linde _.Molecular Sieve 18 CaO. A1203.SiO'2. and the 4A Sieve is a sodium alumino-silicate. Their peculiar property is that removal of the original water of hydration (activation) occurs with little change in the lattice structure, Thus a highly. porous medium results in which the pores are of remarkably uniform (and molecular) size. The 4A and 5A materials have pores of roughly 4 and 5 angatréms diameter, respectively. Specific surface areas as great as those of many activated charcoals have been reported. The extent of ad- "sorption for a given gas is strongly dependent on the degree of dehydration of the a.cisorbefit and in fact goes through a maximum at nearly complete dehydra- tion. Because the pores in which adsorption occurse are of molecular dimen- 26 sions, the size of the adsorbate molecule strongly influences the extent of adsorption. Thus the adéorptiOna of two species may be quite different rela- tive to one another from the adsorptions on charcoal, and separations impos- gible on charcoal may be feasible. A recent article gives an example which will illustrate the point, 58 The mixture studied was nitrogen-krypton -meth- ane., Omn a charcoal column (20°C), the nitrogen came off first and the krypton and methane considerably later but together. On 5A Molecular Sieve (2% water by weight) at 20°C, the nitrogen and kryptofi came oif together, followed considerably later by the methane. It was then found that the separation of all three gases could be achieved by using a mixed adsorbent of "molecular sieve" and charcoal. It was further found that the relative positions of the peaks were quite close to those calculated by using the weight percents of the two adsorb- ents. in the mixture to obtain a weighted average of the peak positions usi:ng the pure adsorbent. The possibility that "‘molecular sieves" or mixed adsorb- ents could be usefu.l in the separation of rare gases from samples is worth keeping in mind. The author is unaware of any such radiochemical procedures in the literature to date. IvV. SAMPI.E SOLUTION AND EXCHANGE WITH CARRIERS The radiochemist may have need to recover rare gases from a variety of target materials, though most usually from heavy elements in which fission “has occurred. Any method suitable for bringing the target material into a clear solution may be used in principle. Since the recovery of the rare gases must be effected from the volatile products of solution, solution procedures which produce the more tractable gdseous products are to be desired. Meth- ods reported in the literature for the solution of spent fuel elements and re- covery of the rare gases therefrom55 certainly exemplify the most extreme problems likely to be met as regards volatile products of dissolution. With the emall targets that the radiocheémist usually encountere the feasibility of complete initial containment of the evolved gases will often simplify matters. Solution of metal targets is usually done with acid. If the choice is available the products from solution with hydrochloric acid are simpler to handle than those with nitric acid. Thus uranium is easily dissolved in con- centrated HCIl with a few drops of 3079 HZOZ added and the HCl furnes simply removed in a trap containing caustic. Solution of uranium in warm 92% H3P04 has also been found satis;‘fa.cf:orys.9 and hydrogen is the only gaseous product. Aluminum dissolves nicely in 6N NaOH, again with no gaseous prod- ucts by hydrogen., This method of solution may be useful for aluminum catch- er foils or as a preliminary to the solution of small targets clad in relatively - large amounts of aluminum, 27 Solution must be carried out in a closed system or a system in which the rare gases are removed from the effluent gas stream prior to 'venting.. The former is preferable in quantitative work as introduction of carriers and. their exchange with the active rare gases may be effected before any chemical or adsorption operations are performed on the gas phase, With the rare gases one has no problems with rates of exchange between chemical states. The problem is simply one of achieving complete physical mixing of the carrier and active gases., As diffusion processes are relatively slow, thorough ex- change in the gas phase is usually insured by convective mixing (as by alter- nate cooling and heating of parts of the system). One must also consider the complete removal of the tracer active gases from the target solution, or at least the complete exchange of dissolved gases with carrier. Removal -;f rare. gases from solutions by bubbling through inert carrier gases such as helium has been studied theoretically (starting from Fick's la.w of diffusion) and ex- perimentally. 60 It was found as expected theoretica.ll.-jr that the depletion of tracer xenon in solution is exponential with time, and half-times for the re- moval are of the order of a few minutes. Thus gaseous -specieé with half- lives of more than a few minutes are often swept out of solution with helium, Boiling the clear solutions is also effective. Efficiencies of various other methods for removing rare gases from targets (specifically xenon from uran- ium) have been studied. 60 Loss of xenon from uranium on the formation of the hydride at 300°C was found to be 1% or less. Thus uranium metal may be converted firat to the hydridé and the xenon then removed from the hydride by some method. Amalgamation of uranium from freshly decomposed uranium hydride and boiling, or solution of uranium hydride in aqueous AgC lO4 and boiling are satisfactory. Uranium dissolves in saturated potassium cupri- chloride with the evolution of a small amount of hydrogen. The diffusion of rare gases from metals at low temperatures is gener- ally negligible. Thus dissolver systems may generally be evacuated with the target in place before solution is comrnenced-if thie is desirable. From uran- ium metal containing éaseous fission products there is no detectable loss at lesr than 1000°C, 2 At higher temperafures evolution of rare gases begins, accompanied by swelling and cracking of the sample. Negligible diffusion . from most metals might be expected at room temperature as the atomic di- ameters of the heavier rare gases are often larger than interatomic distances in metal lattices, Movement of rare gases must thus be mainly through "microcracks" in the lattice. A particularly neat method for removing rare gases from targets has been reported recently. It is based on the fact that long-chain fatty acid- salts of heavy metals emanate rare gases quantitatively and very rapidly {more rapidly than gases can be swept fromn solutions). Removal of more 28 than 999 of the 3. 9-sec an 19 Btearate targets and barium stearate catchers were used in studies of inde- was achieved from barium stearate. Uranyl pendent yields in fission chains containing rare gases. There are perhaps a few comments ta be made concerning the addition of carriers. Amounts of carriers added for rare ga.sés of interest are usually determined by consideration of how much gas one will need to perform sub- sequent counting operations accurately and conveniently. Actual percentage yields are of no primary interest assuming complete interchange of carrier and activity. Rather, ones needs are expressed in terms of so many cc STP yield of gas. The yields required may vary considerably from experiment to experiment and one point in particular éhdu,ld be kept in mind when one finds it desirable to reduce the amount of carrier to something less than amounts previously used with success. It often happens that most of the losses in a procedure are of the type which may be expreased in terms of a certain resid- ual pressure left behind in a given vol-u.me in a transefer by condensation, or a certain partial pressure of a gas in the exit stream from a trap in which the gas is condensed, These are losses in terms of cc STP of gas and bear no relation to the amount of carrier gas added. Thuse if one obtains 90% yield in an experiment with 100 cc STP of carrier, and in a later experiment wish- es to increase his specific activity by using only 20 cc STP of carrier he may find that his yield is only.50% (10 rather than 18 cc STP). There ie essentially a lower limit to the amount of carrier one may use without modification of the procedure to reduce losses of the type mentioned (where these are the prin- cipal losses), V. COUNTING RARE GAS ACTIVITIES Methods which have been used ta count rare gas activities may be con- veniently divided into several categories. The choice of method will be de- termined by the usual considerations of kind and energy of radiation, level of activity, half-life of activity, and type of information desired, 1) Incorporation of the rare gas to be counted in the {filling of a Geiger- Muller or proportional counter: The use of this technique involves one in all the problems of counter construction and filling. The experimenter faced for the first time with these problems in detail must be referred to standard texta on the subject. 64,65 Only a few comments will be made here. This is of course the most efficient means of counting gaseous activities as a result of favorable geometry and absence of absorption losges,. It is thus desirable and often necessary in the assay of low levels of activity or of active species which emit radiations of short range (soft beta~emitters, or alpha-emitters such as radon isotopes). Both Geiger and proportional counting have been 29 used, each having advantages and disadvantages. The electronic equipment associated with Geiger counters is gimpler and less expensive, However, proportional counteres do have a g-r.eater range of usefulness if one may some- times wish to count fairly high levels of activity, since dead-times and the resulting coincidence losses are smaller. Since performing counts in this manner is relatively slow in any case, time s'avings are important if very many samples must be run. The counter fillings containing the rare gas activ- ity will of course vary in composition and some method must be found for ex- tablishing reproducible counting conditions if the best accuracy is to be obtain- ed. Counting conditions can be reproduced at least as accurately by using the pulse height observed with some suitable monoenergetic radiation (as Fe x-rays) to set high voltages and discriminator levels as by running more time consuming plateaus. This method is applicable to both Geiger and proportion- al countera, but is more accurate with the latter as gas: rnu;tiplications vary more rapidly with voltage in the proportional région than do pulse heights with voltage in the Geiger region. As usual it is much easier to calibrate this type of counter with samples whose assay is known and to reproduce the same counting conditions in assaying unknowns, than to attempt absolute counting. If absolute counting must be attempted, corrections are necessary for end effects (electric field distortion at the tube extremities) and for wal] effects depending on the-specific ionization of the radiation in the counter filling and the minimurmn number of ion pairs required for detection, - Counters may be constructed with active volumes over which the field is radial by installation of field tubes concentric with the wire and held at the proper voltage. 66, 67 General discussion of end effects and wall effects, including an approxima.te theoretical derivation of wall effect corrections may be found in Rossi and Staub65 (page 234, Appendix 9). End effects are generally determined empir- ically by counting aliquots of the same sample in two tubes of identical con- struction except for different lengths, 68 and wall effecta by a similar tech- nique using tubes of different diameters (remémbering that the average ion- ization produced by the radiation per unit path length, and thus the wall effect, depends on pressure and composition of the filling). Counter fillings may.be rmade using any of the rare gases and a suitable quench. Positions and shapes of plateaus depend on so many factors as to preclude brief diecussion here. Some examples appear in the literature, but the researcher will probably find some experimentation necessary as op- timum fillings will be determined by his electronics as well as by his counter construction (particularly in proportional counting). Purity of materials, especially from highly electronegative species such as oxygen which tend to form negative ions, is important, A large variety of polyatomic gaseous quench materials may be used. Light saturated hydrocarbons are among the .30 most common. It might be noted that krypton and xenon purified according to Detailed Procedure 1 are used in proportional counter fillings with no further purification, At Lawrence Radiation Laboratory tech.fiical-gra.de is opefitane (roughly 20 cm Hg pressure) is used as quench after drying with sodium wire. Argon, krypton, and xenon pressures up to 20 crmn Hg have been used in con- junction with this quench. Plateaus are geflerally better at the smaller pro- portions of rare gas, but several cm Hg pressure is desirable to allow accu- rate and convenient measurement of the aliquot counted. ) When low levels of Kr85 are to be counted it should be borne in mind that the atmospheric level of Kr85, is now appreciable, In the Paris region this level in 1954 was 1200 disintegrations /min. and in 1958 4200 d/m per liter of krypton removed from the atmosphere. 69 One should assay Kr in carriers used and correct for this background where necessary. In very low level work it may be necessary to use for carrier krypton which was re- moved from the atmosphere. at an early date. | 2) External counting in fixed or reproducible geometry: For higher levels of activity of gamma-emitting or energetic beta-emitting rare gases, the gas may be counted in 2 thin-walled container whose geometry with respect to a2 thin-walled counter is accurately reproducible. 70 Practically such con- tainers are limited to fairly small volumes, and overall counting efficiencies are generally lower than with the '"fixed geometry' tubes to be described next. As will be seen below the main advantage of this method over the use of "fixed geometry'" tubes is that a series of samples may be counted in rapid succes- sion on one counter (8ay by constructing containers to fit the shelves in a counter ordinarily used for plated solid samples). 71 The effort which rn@st be expended in calibration is thus considerably reduced. _ For best accuracy and counting efficiency the container for the gas may be affixed permanently to 2 thin-walled counting tube, as in commercially available thin-walled counting tubes with concentric glass jackets (R.a.dia.tioh Counter Laboratories, Skokie, Illinois). Re.ynolda']r2 used this type of tube for counting i(rss. Combined geometry and absorption factors resulted in an estimated counting efficiency of 5-209% with jacket volumes of 3-10 cc and 30 rng/cm2 counter wall. Thus Kr85 samples with specific activities as low as 100 to 200 disintegrations /minute per cc STP could be accurately assayed. These counters were used in the Geiger region in the aforementioned work. The counter was sealed permanently into a vacuum system and provisions for reducing the aliquot of the sample being counted by a series of expansions and dilutions with inert carrier are described for cases where the activity '1.5 in- itially too great, At Lawrence Radiation Laboratory these same tubes are used for count- ing and following the decay of a variety of rare gas activities. They are suit- 31 ably filled and used as proportional counters to reduce coincidence losses and permit decay curves to be taken from initially higher activities. With the particular electronics used, filling with 10 cm Hg pressure of a mixture con- taining equal volumes of argon and isopentane gives satisfactory counter char- acteristics,. .Absorptior_x losses vary with the energy of the beta radiation be- ing counted, but overall efficiencies for various activities are still in the range quoted by Reynolds. Each tube is fitted with a base which may be plugged into the counting circuitry, and the connection to the jacket is fitted with a stop- cock and ground-glass ball joint to allow easy attachment to and removal from vacuum systems. To facilitate the counting of many samples in ré.pid succes- sion, a panel into which a number of tubes may be plugged is provided, and a switching arrangement allows rapid selection of the tube to be counted. As regolution of fission product 2. 80-hour Kr'88 from 4.35-hour Kr85m is diffi- cult and tedious, a few tubes were purchased with special 200 mg/cmz glass walls to remove the softer Kr85m betas by absorption and permit simpler accurate determination of KrBB. This brings up one special point concerning: the counting of KrBB. It was noticed that decay curves of Kr88 exhibited scat- tering definitely beyond statistical expectation. As the scattering became roughly statistical when the decay reached the KrBSrn tail, and Xe133 and Xe135 decay curves behaved statistically in the same counter it was assumed that the behavior of the Kr88 decay curves was associated with the 18, O-min. R.b88 daughter, As the Rb88 is expected to be ionized immediately after its 73,74 formation it was hypothesized that accumulation of local charges on the glass walls of the jacket containing the gas was resulting.in irregular depoegi- tion of the Rl_)a,8 on the walls. It was found that silvering the internal walls of the jacket to provide a conducting surface encloeing the gas sample resulted in decay curves for Krss conforming to statistics. The matter was pursued no farther but conducfing walls are always provided for containers in which Kr88 is to be counted in high geometry. It was statéd above that the efficiency of these counters was of the order of 10%, but it should be noted that no effort is ordinarily made to determine the actual efficiency accura.te.ly. The countere are calibrated for each nuclide of interest in some suitable manner and the same counting conditions are re- produced in subsequent counting of unknown samples. Such counting proce- . dures are undoubtedly familiar to most radiochemists in other contexts and are essentially no different for rare gaéea._ Thus, if one counts the KrB8 from a U235 target in which a known number of fissions has been induced by_therm.al neutrons, he can later determine the number of such fissions which occurred in a U235 target irradiated with thermal neutrons by counting the KrBB (using the same tube and counting conditions). The calculation of the calibration fac- tor k in this case involves the number of fissions f, the activity A°, of Kr88 32 extrapolated back to a suitable zero time(the midpoint of the irradiation for sufficiently ehort irradiations), and a suitable aliquot factor, Thus, f = kl x A° x SCSTP carzier _ 4y 4° X ¢c STP carrier/P —TP._R'—WTG— ooc.' The actual volume 0°C _ . V of the counter jacket provides no useful information in most cases and its inclusion in the counter calibration factor without actual determination saves considerable work. POOC is the measured préssure of the carrier in the counter corrected to 0°C from the temperature of measurement. It is often possible to determine the number of fissions f by assaying the target solution for some fission product for which counters have already been calibrated 99 (e.g., Mo’ ’). Ultimately, of course, experiments involving a direct deter- mination of fiseions in targets must have been performed. While one can achieve accuracy sufficient for practically all purposes, the use of these count- ers has the obvious drawback that each counter must be separately calibrated and that counting a number of samples in a short time requires a tube for each sample due to the time required for accurate ma.nii:ula.tion of gases. 3) Depositionl of rare gases on the cathode in a glow disecharge: Trace amounts of rare gases may be deposited on wires or foils used as the cathode in a gldw discharge chamber, Tracer rare gas activities are collected in a glow discharge tube along with a few hundred microns pressure of some gas to support the discharge (nitrogen, air, helium). A few hundred volts dc potential is applied to cause a glow discharge and the discharge maintained - for a few minutes. Deposition and retention of the gaseous activities on the cathode is surprisingly high (in the percent range for RnZZZ)-. Heating re- moves the activity, but it is quantitatively retained for days at room temper- ature, The tecfi.nique has proved useful in studies of the half-lives and radia- tions of rare gas activities, 76,77 4) Indirect study of rare gas activities through their daughters: Although not a method of counting rare gases themselves, cognizance should be taken of the feasibility of studying rare gases through counting their daugh- ter activities.. Thus some early experiments were performed in which rare gas fission product daughters were collected on a negatively charged wire . coaxial with 2 moving gas stream!: 78,79 Rough half-lives for the rare gases could be obtained from flow rates and the daughter activity as a function of distance along the wire. Fractional chain yields have also been studied through the relative amounts of daughters of rare gas activities found in heavy metal ' stearates (from which rare gases emanate very rapidly) and on container walls where the decay products of rare gases leaving the stearate 'depoéit. 63 Such. techniques may be useful where gas counting is inconvenient or where the rare gases themselves are too short-lived to count, but the immediate or subsequent decay pfoducts are of reasonable half-life for radiochemical assay. 33 5) Rare gas scintillation counting: It is interesting to note that the rare 80, 81, 82 The gases have been found to be satisfactory scintillants in counters. _ main usefulness of such scintillants promises to be in the counting of heavy particles such as fission fragments. They have the highly desirable property that light output is dependent only on total energy expended {and not on ioniza- tion density). 6) The feasibility in specific instances of counting rare gas activities. which have grown into solid samples of their parent activities, plated for counting, should be borne in mind. For example Xe135 and Xe133 were studied in this manner. 83, 84 Fission product iodine was separated and plated out for counting as Ag I or Pdi,. Radioxenon growing into the plated eamples was retained quantitatively for weeks, even in vacuo. VI. COLLECTION OF RADIOCHEMICAL PROCEDURES FOR THE RARE GASES ' The largeet collection of papers dealing with rare gaa radiochemistry of which the author is aware ia found in the last one of the nine General Ref- erences given at the beginning of this monograph. The following procedures were chosefi for description with a view to calling attention to as many useful techniques and operations as possible. The author would appreciate information concerning other procedures which should be included in such a collection. PROCEDURE 1 — REMOVAL OF Kr AND Xe FROM AIR AND THEIR SUBSEQUENT SEPARATION -Source — F. F. Momyer, Jr., et al. unpublished work at Lawrence Radiation Laboratory, Livermore, California Figure 1 shows a photograph of the system used in the separation. A standard pumping arrangement consisting of a trap refrigerated in liquid nitrogen, a mercury diffusion pump, and a mechanical forepump is used to- ® 0 107% mm Hg. Proceeding from left to right obtain a base pressure of 10~ along the separation train let us designate and label for purposes of later discuseion the following: 1) a flowmeter; 2) T,, a trap packed with 3/16 in. stainless steel balls and suitable glass wool plugs to retain fine particles of condenpates and preceded by a glass coil; 3) Cl’ a t.ra.p similar to T, but containing 100 grams of activated charcoal; 4) CZ’ C3. and C4, three small- 34 in Procedures 1 and 1A ion system used 1 —Separat ig. F 35 PROCEDURE ‘1 (Continued) er traps each containing 10 grams of activated charcoal; 5) T,, an unpacked trap similar to the preceding three; 6) a 200-cc bulb; 7) an outlet for sample removal; and 8) a mercury manometer. Helium ie admitted to the line through the charcoal trap at the lower left. This trap is cooled in liquid nitro- gen to remove trace impurities from the helium. Partially obscured by this charcoal trap is another manometer between T1 and C1 which may be cloesed " off from the system with 2 stopcock, It is certainly not obvious from the photograph but connections and stopcocks have been so arranged that helium flow may be directed through any charcoal trap, through 2n ionization chamber (fi.n’seen behind the drape), into any subsequent charcoal trap, and thence to the atmosphere. The current through the ionization chamber is amplified by a vibrating reed electrometer whose output drives the recorder partially seen at the right of the bench. Thus a continuous record of activity in the ionization chamber during a separation is obtained. To the left of the recorder is the thermocouple vacuum gauge povirer supply and meter. The large traps a;re' constructed of 41-mm and the small traps of 22-mm o.d. glass tubing, and their respective lengths are about 25 and 20 cm. The charcoal is Columbia Grade-G activated charcoal (6-14 mesh), ground in a ball mill and sieved to vield 20-50 mesh material. The system'is used for the separation of rare gases from various types of samples, and the procedure described below admittedly does not treat a problem often encountered. It was chosen to represent the extremes of sample size, flow rates and partial pressures of rare gases for which separa- tion has been successfully achieved with the system, and to afford a frame- work within which the lfirgest number of operationa could be described. The sample is 100 cubic feet (STP) of air compressed into.a bottle con- taining 100 cc STP of active krypton and 75 cc STP of active xenon. Average yields are about 90% for krypton and 95% for xenon. The time required for the separation is about 8 hours of which 3 hours are spent in bleeding the sample into the system. Before the run all charcoal traps have been heated to 350°C and pumped on until the thermocouple vacuum gauge in the ma.nifo.ld. is '"pinned" (probably less than 1074 mm Hg). 1. Gallon dewars of liquid nitrogen are pla.ced on T1 and C 1° including the glass coils. The refrigerant level should be above the connection from coil to trap. 2. Through a pressure gauge and reducing valve the sample is connect- ed to the line below the flowmeter. Sample is bled into T, and C1 until equi- librium pressure over C, is about 35 cm Hg. Wait a few minutes until ther- 36 PROCEDURE 1 (Continued) mal equilibrium is reached as evidenced by steady pressure and decreased boiling of the liquid nitrogen. 3. Usinga mechanical pump on the exit from C1 and suitable valving, establish sample flow through T, and G, of 15 liters STP per minute at about 35 cm Hg pressure. Reduced prlessure is necessary to avoid condensation of liquid air in the system, An NRC-4S5S Rotary Gas Ballast Pump* (4 cfm) is used to handle the necessary large amounts of air over prolonged periods. Keeping the liquid nitrogen replenished, bleed the sample in at the above rate until the pressure in the bottle is about one atmosphere. Pumping for a few minutes with all valves open completely will reduce the pressure in the bottle to about 10 cm Hg. This represents suificiently quantitative removal of the rare gases from the bottle. 4, Close the manometer by C, and the stopcock at the exit side of Tl’ place liquid nitrogen on CZ' C3 and C4, and introduce helium at 1-2 psi gauge to all the charcoal traps. Thermal and pressure equilibrium in the traps will be evidenced by cessation of helium flow into the system. 5. Set up helium flow of 1250 cc STP per minute through C,, the ion- ization chamber, C2 in liquid nitrogen, and thence to the air. When steady" flow ig established remove the liquid nitrogen from C 1+ Allow C, to warm elowly in air for 5-10 minutes. (This avoids high pressures in the system from too _rapi& desorption of the roughly 30 liters STP of air adsorbed on Cl). After 10 minutes immerse C,in 10-20°C water. Krypton activity will be ob- served in the ionization chamber in about 5 minutes, the activity rising rapidly to a miximum and then dropping off more slowly (full-width at half-maximum about 2 minutes). When the activity is lees than 19% of the peak value (10-15 minutes), the elution is halted. 6. Set up helium flow of 750 cc STP per minute through C, in liquid nitrogen, into the ionization chamber, and thence to the air., Remove the liquid nitrogen from G, and after 2 couple of minutes immerse C, in dry ice- acetone slush (-78,5°C). Elute approximately ten minutes to remove air from CZ' If activity appears in the ionization chamber before this period is up, stop the elution immediately and proceed with the next step. 7. Direct the helium flow (750 cc/min) so that the sequence is C, in * ) National Research Corporation EqQuipment Division, Newton Highlands 61, Massachusetts. 37 PROCEDURE 1 (Continued) dry ice-acetone, the ionization chamber, C3 in liquid nitrogen, and out to the air. Remove the dry ice-acetone from Cz and replace with 10-20°C water. .The krypton activity will appear in 1-2 minutes in the ionization chamber, Full-width at half-maximum is about 30 seconds. Elute until activity in the ionization chamber is less than 1% of the peak value (should take less than ten minutes), and stop the elution at that point. 8. Repeat Step 7 to elute krypton from (_‘,3 to C4; 9. With C4 still in liquid nitrogen, pump until the pressure on the ther- mocouple vacuum gauge in the manifold is less than one micron. This should require less than five minutes and results in the removal of helium from C4 with no loss of krypton. If the trap doee not pump down to one micron in less than five minutes, discontinue pumping and proceed to the next step. 10. Place T, in liquid nitrogen. At this point C4 containe the krypton and some residual air. The removal of air is essentially accomplished by warming C4 elowly, passing the evolved gases through T2 in liquid nitrogen to condense any krypton evolved, and every time the pressure reaches about 1/2 atmosphere in the éystern beyond T-2 pumping the gas off briefly by rotat- ing the stopcock to the vacuum manifold through the open position. It should be borne in mind that once krypton is condensed in Tz the partial pressure of krypton in the system beyond is 2-3 mm Hg (the vapor pressure of krypton at liquid nitrogen temperature) and that one thus wishes to keep the total time which one pumpe on T2 to a minimum to reduce losses. Remove the liquid nitrogen from C4 and allow the trap to warm in air a few minutes until evolu- tion of gas begins, use the pump in the manner described above, and each time -evolution of gas slows down speed the procese by heating the trap to successively higher temperatures (dry ice—é.cetone. room témperature water, and finally warm water -- about 40°C). When the trap is at 40°C and the pressure in the system is steady, pump off the last vestiges of air, leaving ' 2-3 mm Hg pressure of kryptan in the sysfern. (The distillation of krypton from C4 to T2 will be seriOuély slowed by any observable pressure of residual air}). Allow 5-10 minutes for the complete distillation of krypton. If the vol- ‘ume of the systermn has been measured beforehand, the stopcock between C4 and T, may be closed, T, warmed to room temperatfire, and the pressure of 2 2 krypton measured to get a rough determination of the yield. Yield should be about 909%. Distill the krypton into an evacuated bulb attached to the system outlet and cooled in liquid nitrogen, and remove from the line. 11, Place boiling water on G, and C4 and pump down to base pressure 38 PROCEDURE ) {(Continued) to remove traces of krypton. Cool the traps to room temperature, then place in liquid nitrogen, and admit helium to speed the cooling. 12, Set up helium flow of 1250 cc STP per minute through G, at room temperature, the ionization chamber, C2 in liquid nitrogen and thence to the air. The xenon will be eluted to C, fri:.u'n'C]L in boiling water in about an hour. The time may be reduced to about 30 min. by heating C1 more rapidly in a furnace set to reach an eventual temperature of 350°C., Elute to less than 1% of peak activity, 13, Set up helium flow of 750 cc STP per minute through C2 in liquid nitrogen, the ionization chamber,. C4 in liquid nitrogen, and thence to air. Warm C, to about 40°C and throw away any amall krypton peak which appears (less than five minutes). Elute xenon to C4 using either boiling water or a furnace as in Step 12 (15 to 30 min. ). 14, TZ is cooled in .liquid nitrogen, C4 warmed to 100°C, residual air pumped off if any is present, and the xenon distilled to T2 in 10-15 minutes. Xenon losses are slight in this process as the vapor pressure at liquid nitro- gen temperature is only about 50 microns. Measure the xenonl yield (should be 95% or greater) and distill to an evacuated bulb for removal from the line. Comments l. For smaller samples in which the rare gas partial pressures are higher over T1 the poseibility that a large fraction of the xenon may originally be condensed in T, at -195 °C should be noted. (It seldom happens that this occurs with krypton since its vapor pressure is higher (2-3 mm Hg) at -195°C, but partial preesure of krypton over the trap should also be considered with each sample). If this occurs and one wighes quantitative recovery of the xenon one could distill xenon to ('J1 before starting the elution from C,. Itis usually more convenient to recover the xenon from Tl and C, separately however, The fastest way to accomplish the recovery from T1 is as follows: Cool the ' glase coil before T. in liquid nitrogen. Use an induction coil to produce eddy currents in the ateell balls and thus heat them rapidly to well above room tem- perature.. Pump off residual air which may have been occluded in the conden- sate through the glass coil and then distill the xenon, carbon dioxide, and . water into the coil. Distill xenon and carbon dioxide from the coil at -78.5°C into a bulb attached to the outlet at the upper left of the rack and cooled in liquid nitrogen. Thaw and refreeze the wat.er several times until occluded or dissolved gases are removed. On another rack remove the CO, from the ~ xenon by passage through Ascarite or by distilling the xenon from a bath at 39 PROCEDURE 1 (Continued) abour -150°C into liquid nitrogen. The vapor pressure of xenon at i1sopentane “slush" remperature (-160°C) is only a few mm Hg so liquid isopentane at a few °C higher temperature 18 usually used to speed the distillanon. One may of course use dry ice-acetone on 1'1 10 remove water without condensing Kr and Xe. Most of the carbon dioxide will then pass onto Cl' Z. Cross-contaminanon of the krypton and xenon fractions has never been observed in terms of their radioactivity. In one run with inactive krypton and active xenon, the krypron was distilled directly off of C, afrer the elution from C:l and no xenon activity was observed in th3e krypton. - A lower limat for the separation facror in this one elunion of 5 X 10° was calculated. It thus appears quite likely that the two subsequent elutions could be eliminated i1n many experiments, 3. The inclusion of a thermal conducrtivity cell in series with the iomza- tion chamber to allow detecton of nonradioactive species eluted from the char- coal has proved to be a valuable aid in many operanons. As an example, the elution of air from the charcoal traps contaiming Kr can be performed more efficiently if one can observe the emergence of the air "peak'. Also, with rare gas samples not active enough to register in the ionization chamber, one can observe the elution peaks of the carriers in the thermal conductivity cell rather than operating purely on the basis of previoualy observed retention imes, 4. In a recent modificarion which promises 10 1mprove the versatility of the system, the second small charcoal trap, Cy, was replaced with 60 grams of 30-60 mesh 5A Linde Molecular Sieve in a coiled glass column about 1 cm in diameter and 120 cm long. Separations of methane-krypton mixtures (typ- ically abour 50 cc STP of each) have been performed very satisfacrtorily on this column. Eluring the mixture from C, to the top of this column at -195°C, and then raising the temperature 10 -36°C (ethylene dichloride *'slush-), the following elution data with He ilow of 600 cc STP per minute were found in several runs: Kr, peak rerention rime 18 mun., full-width at half-maximum 3 min, ; CH4, peak rerention time 35 min,, full-widrh at half-maximum 8 min. PROCEDURE 1A — REMOVAL AND SEPARATION OF Kr AND Xe FISSION PRODUGTS FROM U23° TARGETS This procedure is used for U235 targets of the order of 100 mg 1n weight and clad in a few hundred milligrams of 25 aluminum, Even though only one of the rare gases is of interesr it 18 the pracrice to Lo PROCEDURE 1A (Continued) add carriers for both to circumvent the vagaries of tracer behavior. The amount of carrier added is determined by the usual considerations concerning the counting operations to be performed later, but 100 cc STP each of krypton and xenon would be typical, The system as shown in Figure | may be used for the separation. Safety precautions such as shielding and containers to catch active solutions in case of breakage must of course be provided. The dissolver flask é.t the extreme left is a 250-ml round bottom flask fitted with a 34/45 inner-tapered ground- glaés joint, fitting an outer 34/45 joint into the top of which a separatory funnel has been sealed. The outlet for evolved gases is through an arm at the top fitted with an 18/9 ground-glass ball joint. The trap to the right of the flask is packed with Ascarite. 1. A bulb containing the carriers is attached at the upper left of the rack., The target is placed in the flask and the tapered joints greased and fitted together. Air is evacuated from the entire dissolver eyatem. This is permissible with most targets as the diffusion of the rare gases from uranium at room temperature is negligible. The other alternative is to pull the air in the s_y'stern onto Cl at -195°C. T1 is cooled in dry ice-acetone and C1 in liquid nitrogen. 2. With the 5t0pcolck béfore Cll closeci. open the carrier bulb to allow carriers to expand through the rest of the system. Start the solution by drip- ping in concentrated HCl with a few drops of 30% HZOZ added. Rate of reac- tion may be controlled by warming or cooling the flask. A wet towel contain- ing ice may be wrapped around the neck of the flask to reflux most of the water and acid fumes back into the solution. Inclusion of a reflux condenser cooled with ice water ébove the dissolver is more esthetically pleasing and probably more efficient. If hydrogen pressure approaches atmospheric it may be nec- essary to transfer some to C. during the solution. (See Note 4 below). Dilute 1 the final few cc of solution to 20-30 cc with cold water and cool in an ice bath, 3. Close the carrier bulb to retain some carrier for flushing. Puyll other gases onto C, to a residual few mm Hg of hydi'ogen. The solution should boil -- warm slightly if neces_aa.fy. It was originally cooled to avoid too vig- orous boiling and splashing during this operation. 4. Close the stopcock to C1 and open the carrier bulb. Again pull all gases onto C 1 5. Close the stopcock to C, and admit a few cm Hg of air to the system 1 through the separatory funnel. Pull the air onto CI' {Provisions for the use of helium for thie final flush of the system would be desirable). 41 PROCEDURE 1A (Continued) 6. Krypton, xenon, hydrogenla.nd air are now on C1 at liquid nitrogen temperature, and one follows the previous procedure starting from Step 4. The final steps will. be simpler as the sample contains very little air, and the hydrogen is nearly quantitatively removed by the elution from dry ice-acetone. 7. Radiochemical analyses necessary for the purpose of the experiment are performed on the target solution, As an example, in calibrating Krsa 99 counters one might asgay Mo’ ’ to determine the number of fissions in the target, assuming calibration factors for this fiesion product to have been pre- ‘viously determined. Notes 1. The system has given results in calibration runs which are indistin- guishable (the same to within 2-3%) from those obtained with an earlier sys- tem in which a portion of the carriers was admitted to the system prior to ‘solution, the rest bubbled through the clear solution, and the entire system finally fluehed out through C, with helium bubbled in through the solution. 1 2. In the absence of air the system has been used to separate argon, krypton and xenon. The argon is adsorbed along with the krypton and xenon at liquid nitrogen temperature, and eluted off the charcoal at -78.5°C. . The ar- gon will not be separated from nitrogen and oxygen if air is present; and in this case éhemical means are used to pui:ify the argon. 3. In experiments involving Xe135 separations within a few hours of 135 ‘irradiation, no evidence of I reaching the charcoal has ever been observed. Resuits of the experiments performed were generally not very sensitive to 1135 contamination, however. In experiments where results would be greatly affected by slight iodine contamination of the xenon fraction, this point should be checked more closely. Detailed Procedures 4 and 5 contain examples of other effective means of iodine removal. 4... In building a system for the solution of varioug types and sizes of targets, the author would recommend in’clusiofi ahead of T1 of a trap packed- with CuO and heated to 500°C for conversion of hydrogen to water, followed by a trap filled with a desiccant such as magnesium perchlorate. The size target which may be dissolved in a systerfi of convenient volume is then not limited by the volume of hydrogen evolved, and the fact that the system contains only the rare gases and a small amount of unburned hydrogen after dissolution is complete simplifies the total interchange of carrier and activity as well as the ultimé.te quantitative transfer of the carriers to C'l' Such a system. has been used successfully for the diesolution of targete weighing up to a few grams. 4o PROCEDURE 2 — THE EXTRACTION, PURIFICATION 85 AND INDUSTRIAL USES OF Kr Source — E. J. Wilson, GC. Evane, J. Chadwick, J. Eakins, K. J. Taylor, report AERE I/R 2216 (1957). The procedure described in this paper is for the recovery of fission product krypton from kilogram amounts of irradiated uranium- metal. A brief description of th_e operations involved is given below to point out methods which may be applicable to emaller scale experiments. 1, Three 2.3-kilogram uranium slugs containing fission product krypton are dissolved in a 70-liter stainless steel container, Dissolution is accom-- plished with roughly 12 liters of 10N I-INO3 ried out "furhelessly' with respect to oxides of nitrogen bjr admitting oxygen at 104°C. The dissolution is car- over the solution (See Note 1). Approximately 6 liters of water are used to recombine acid fumes and wash them back into the vessel during solution, Solution is complete in about 20 hours and the final result is a solution of uranyl nitrate in approximately 3N H__N03. A slight excess of oxygen is used to main- tain flow to the purification train during dissolution and the dissolver system is finally flushed with helium to effect complete removal of fission gases to the pui'ification train. 2. Gases leaving the dissolver are circulated around an apparatus in which they receive the following treatment; 1) rough drying with Ca._ClZ. 2) carbon dioxide removed with Sofnolite {an NaOH-CaO mixture), 3) final dry- ing with magnesium perchlorate, 4) removal of oxygen by reaction with cop- per turnings at 600°C, 5) any water from oxidation of hydrogen or hydrocar- bons removed with Sofnolite, "6) traces of hydrocarbons and gaseous compounds of nitrogen removed by reaction with clea.n’uranium turnings at about 800°C, and 7) rare gases adsorbed on charcoal at liquid nitrogen temperature. From the charcoal trap, remaining gases (mainly helium) return to the point of entry from the dissolver. The system is inltially filldwith one atmosphere of helium. A loosely inflated balloon attached to the system maintaine the pres- sure at one atmosphere durling sudden bursts of evolved gases. Circulation of about one liter per hour is maintained by two identical parallel pumps consist- ing of a chamber permitting unidirectional flow within which the pressure is varied by periodic heating and cooling. 3. Upon completion of the dissolving and flushing the dissolver with helium the charcoal trap will contain all the fission product krypton and xenon. . Argon present as an impurity in the oxygen ueed for dissolving will also be adsorbed on the charcoal and will in fact constitute the bulk of the adsorbed 43 PROCEDURE 2 (Continued) gases (several hundred cc STP). The charcoal trap is removed from the line and the rare gases desorbed by warming the charcoal in boiling water and ~ evacuating with a Toepler pump. The desorbed gases may be transferred di- rectly to the final purification apparatus with the Toepler pump_, or if the total volume of the rare gas fraction is very large, it may be most convenient to transfer the gases first to a series of storage bulbs. The smaller aliquots of rare gases contained in these bulbs may then be purified singly as convenient. 4. The final purification apparatus consists of a train of five charcoal traps. All are heated and outgassed to a vacuum before the run and then all but the second are cooled in liquid nitrogen. The second trap is the chromato- graphic column. It contains 10 grams of charcoal in a column about 20 cm long and is cooled to -50°C with a dry ice-chlorobenzene mixture. The sample of rare gases to be separated is transferred to the first charcoal trap cooled in liquid nitrogen. Helium is introduced to saturate all charcoals at atmos- pheric pressure and a flow of 3.5 liters per hour of helium set up through the train of charcoal traps. Gradual lowering of the dewar around the first trap transfers first the bulk of the argon to the chromatographic column, and finally upon complete removal of the dewar the remaining gases deposit in diffuse bands at the head of the column. 5. Radioactivity in the gas leaving the column is monitored with a GM counter suitably placed near the exit line from the charcoal. Until activity is noted in the effluefit gas the flow is directed from the column to the last char- coal trap (a precaution in case of an error in judgment). As soon as activity is noted the flow is changed to pass through all three charcoal traps subsequent to the column. Activity appears in about an hour and approaches background in 2-3 more hours. The elution is stopfed at this point with most of the kryp- ton removed from the column and all the xenon still adsorbed. Helium is pumped off the charcoal trape while atill cooled in liquid nitrogen. 6. The krypton may at this point contain a few volume percent of hydro- carbons previously undecomposed by uranium and unseparated from krypton by charcoal chromatography. These are removed by warming the krypton- containing charcoals in boiling water and circulating the evolved gasés over uranium turnings at 800°C to decompose hydrocarbons, and then over pyro- phoric uranium at room terfiperatqre to remove any hydrogefi resulting. Puri- fication is considered complete when the preessure in the system reaches a steady state. 7. Krypton is transferred to suitable storage bulbs with 2 Toepler pump, 3y ! PROCEDURE 2 (Continued) or by adsorption on charcoal contained in the storage bulbs and cooled to liquid nitrogen temperature, Notes 1. The dissolution of uranium in nitric acid has been studied in t:lei:a.il.85 The dissolution in 11.7M HNO3 proceeds according to the following equation: U +4.5 HNO, —~ UOZ(NOE;)Z +1.57 NO +0.84 NO, +0.0005 N,O +0.043 N, +2.25 H, In the presence of oxygen, however, the equation for the reaction is U + 2 I-INO3 + 0. '3/2 OZ. - UOZ(NO‘;})2 + HZO’ so that oxides of nitrogen are not present in the products. 2. Xenon could of course be recovered from the chromatographic column at 100°C if desired. In this particular procedure the uranium is dissolved after xenon activities have largely decayed to avoid working with high levels of radioactivity. There is a report describing the early recovery of multi-curie amounts of Xel33 from uranium. 8 Aside from problems of shielding and remote control introduced by the high levels of radioactivity, the procedure is . very similar. The same dissolving and preliminary purification apparatus is in fact used. As xenon is 8o much more strongly adsorbed than argon, the argon is removed by pumping on the charcoal containing the rare gases with the trap at -90°C, rather than by elution from a charcoal column. The final xenon fraction contains some argon and most of the fission product krypton "present in the sample. As the final step in the procedure (after all fhe steps in the Kr =~ purification) trace impuritiés other than rare gases are removed with a calcium getter. 7 The apparatus for this step consista esaentially of a stainless steel crucible containing calcium chips and heated to 550-600°C. A "mirror'" of calcium is deposited on an adjacent cold pyrex glass surface. Cleanup is followed by taking periodic pressure measurements with a McLeod gauge and is considered complete when no further pressure decrease occurs. To obtain smaller amounts of Xe 133 (of the order of 50 millicuries), solution of 10-20 gram amounts of irradiated uranium was carried out with nitric acid and oxygen in a smaller system. ‘Effluent gases were passed through a frap packed with glass rings and cooled to -80°C to remove water vapor and NOZ' Complete removal of xenon was again effected by sweeping the solution with helium, and the helium stream was passed through a charcoal trap at -50°C to adsorb the xenon. Subsequent purifica.tidn of the xenon follow- ed the same pattérn except that 0.03 cc STP of inactive xenon carrier was added. L5 PROCEDURE 3 — RAPID REMOVAL OF FISSION PRODUCT Kr FROM U FOIL Source — G. D. O'Kelley, N. H. Léza.r, and E. Eichler, Phys. Rev, 102, 223-227 (1956) This procedure was used to obtain samples of the 15-min, Rb'89 for study and the operations are briefly described below as given for this purpose. The technique for removal of krypton from the target may prove useful in other contexts. The target is a uranium film (0.3 rng/cmz) deposited on.a 1. 6-cm-diam- . eter Pt disk. It is mounted at one end of a vacuum-tight recoil chamber with ' a3.12 mg/cmz' Al foil between the uranium and the recoil chamber. This amount of aluminum absorber should stop Xe fission fragments but permit passage of the krypton fission fragments into the recoil chamber. A rubber serum bottle cap is used to cover and seal the exit line at the opposite end of the recoil chamber from the ta.rget.' ' 1.- Target and recoil chamber are placed in a pilé and irradiated with neutrons for 3 minutes. (The half-life of Kr89 is 2.6 min. ) 2. The tar-get is removed rapidly from the pile via a pneumatic tube and krypton isotopes shorter-lived than Kr89 allowed to decay out for 3 min. 3. The rubber cap sealing the recoil chamber is punctured with a double-walled hypodermic needle, Air is flushed down the center tube of the hypo, and air containing the krypton in the recoil chamber exits through the coaxial outer annulus of the needle. The effluent gases are passed through a trap packed with glas®s wool and cooled in dry ice-acetone and thence into an evacuated cell. The Rbag daughter of ZEC:'89 charged strip of Al foil for 3 min. The foil is removed and Rb is collected on a negatively 89 counted. Due to straggling some xenon recoils penetrate to the recoil chamber, and a few percent by activity of Cs 13.8 is found on the foil, PROCEDURE 4 — RECOVERY OF FISSION PRODUCT Xe FROM U METAL Source — E. J. Hoagland and N. Sugarman, Paper 147: Radiochemical Studies: The Fission Products, ‘National Nuclear Energy Series, Div. IV, Vol. 9, Book 2, McGraw-Hill, New York (1?51) pp. 1035, This procedure effects rapid dissolution of the target and separation of iodine and xenon fission products. It was designed for studies of the indepen- dent fission yield of Xe135. PROCEDURE 4 (Continued) 1. The uranium target is placed in the dissolver flask, and the flask attached to the vacuum line in which the separation is to be performed. Air is evacuated from the entire apparatus and heliumn is then admitted to about one half atmosphere pressure. 2. The target is dissolved in concentrated HCl containing a small amount of I for '"holdback'' carrier., After the metal is dissolved any resid- ual oxide or carbide may be dissclved by the addition of a small amount of conc, HNOs. It was found that iodide ion was not oxidized during diesolution if present originally in low concentration, 3. The liberated gases are passed in succession through a cold trap at -85°C a conc. NaOH solution, an NaHSO, an activated charcoal trap at -85°C, and an activated charcoal trap at -195°C. solution, two cold traps at -85°C, When dissolution is complete the solution is swept with about 300 cc of helium at 10 cmn Hg over a period of 3-4 min. (See Note 1, below). The procedure to this point can be completed. within 10 min. after the end of an irradiation. The dissolver flask is immediately closed off. 4. All the xenon and eome of the krypton are now on the charcoal trap at -85 °C. The remainder of the krypton is on the second charcoal trap at -195°C. In this procedure the first charcoal trap is heated to 250°C and the desorbed gases (containing all the xenon) transferred By’ a Toepler pump to an evacuated storage bulb. Aliquots of xenon for counting are té.ken by filling thin-window gas-tight containers from the storage bulb. 5. The amount of Xe135 growing into the target solution from 1135 may be determined by repeating the helium sweeping at a2 suitable known later time after irradiation and counting an aliquot of the xenon fraction so obtained. Early experiments indicated no I135 reached the firét cold trap, NaOH solu- tion, or NaHSO, solution. Thue the helium sweep containing the xenon activ- ity may be transferred directly to the storage bulb, and aliquots for counting taken. Note 1. The double-ended dissolver flask used in this procedure is illustrat- ed in the article. The closed end ie used for solution of the target and storage. For sweeping with helium the flask may be rotated through 180° about the | horizontal axis of the sidearm qpnnecting it to the system through a tapered glass joint, The solution then rests on a sintered glass disk through which helium may be introduced. A emall bulb between two stopcocks below the b7 PROCEDURE 4 (Continued) sintered glass disk is used to measure out aliquots of helium for sweeping. Efficiency of xenon removal by the helium sweep wasa found to be about 95%. PROCEDURE 5 — RAPID REMOVAL OF FISSION PRODUCT Xe FROM UO, OR UO,(NO,), - 6H,0 TARGETS Source — F. Brown and L. Yaffe, Can. J. Chem. él, 242 (1953). This procedure was also designed for studies of the independent yield of Xe135. The targets used were 200-300 mg of U0, or U0, (NO,), - 6H20 enclosed in thin-walled aluminum capsules. _ The target is placed in a flask containing about 50 mg of KI as holdback carrier, attached to the vacuum system, and the apparatus evacuated. H2 is started bubbling through the solution and hot 1:1 sulfuric acid admitted to dissolvej the capsule and contents rapidly. The solution is boiled while hydro- gen is bubbled through. Effluent gases pase through a reflux water condenser for rough drying, a trap cooled to dry-ice temperature, a trap containing NaQH pellets, and two charcoal traps at dry-ice temperature in successioén. ) traps are evacuated to less than 5-microns pressure and sealed off with cool- bubbling is continued until 30 min. after the end of irradiation. Charcoal ant still in place. Kr and Xe activities are on the first trap, and Kr but no Xe activity on the second. The xenon activity.is coufited by placing the char- coal trap in a well-type ionization chamber. Later samples of xenon may be obtained for determination of the xenon activity growing into the solution by repeating the hydrogen sw'eep. PROCEDURE 6 — SEPARATION OF LONG-LIVED FISSION GASES Source —J. A. Ayres and I. B. Johns, Paper 311: Radiochernical Studies: The Fission Products, National Nuclear Energy Series, Div. IV, Vol.9, Book 3, McGraw-Hill, New York (1951), pp. 1763. This procedure was used in studies of the efficiencies of various methods of xefion removal from urapium metal, and gave results reproducible to 2-3%, The uranium sample containing fission product xenon is placed in a dis- tilling flagk, and the flask connected to the purification train. Air is removed by flushing the entire apparatus with COZ' The uranium is then diseolved by running in 92% H,PO,, the rate of reaction being controlled by warming or 48 PROCEDURE 6 (Continued) cooling the flask as retju.ired. Dissolution is very fast at the boiling point of the acid. Xenon, 'hy"drog.e-n. and some fission product iodine and tellurium enter the pufification train ‘Bubbling the gases through a saturated NaH'SO3 solution removes I and Te Pa.saa.ge over hot CuO ox.ldlzes the H, to HZO which is c0ndensed in a subsequent flask, The active xenon and small amounts of other gases are then collected in a eudiometer over KOH. After the uran- ium is dissolved, any xenon remaining behind is swept from the solution into the eud:Lometer with CO After the CO2 is removed by reactlon with KOH, air may be swept in if neceesary to prov1de an easily handled volume of gas. The sample is withdrawn to an evacuated bulb connected to a counting cha.mber into which an Al finger-type GM counter is sealed. The aliquot of the xenon being countéd may be calculated from knowledge of the volume of the counting chamber and of the rest of the system containing the sample, PROCEDURE 7 — REMOVAL OF Rn FROM Th TARGETS AND ITS COLLECTION ON THE CATHODE OF A DISCHARGE TUBE Source —F. F. Momyer and E. K. Hyde," Journal of Inorganic and Nuclear Chemistry, 1, 274-95 (1955). The overall yield of this procedure is no more than a few percent, It has proved useful, however, for the collection and retention of rare gases on wires or foils in studies of decay characteristice of rare gas n_L_lclide_s._. . The thorium metal target, irradiated with 340-Mev prpto.ns_ to fn-oduc_e . radon isotopes by spallation, is placed in the disSQIQer flagk, the eystern evacuated, and hot co'ncentrated HCI! containing ammqnium__ fl_qoe_ilicate ca.taLyst dropped in to dissolve the thorium. Evolved gaees er_e bfil_:bl_e‘d_ through sodium hydroxide solution, passed through a trep_copled in dry i,ce/a.cet_one, and t_h_en- paseed through 3 successive trape cooled in liqnid nitrogen. After soluticn is complete the dissolver and sodl.urn hydroxide solution are closed off and the 4 cold traps pumped down to 10 - 10 -5 mm Hg. The traps a.:_.-e_shu__t off from the pump and gases condensed in the traps are distilled to the last trap by . keeping it immersed in liquid nitrogen and warzning the others. The contents of this trap are finally distilled into a tip on the glow discharge tube which is cooled in liquid nitrogen. The discharge tube is then closed off, the tip warmed, and a glow diecharge run for a few minutes to collect radon on the cathode, This technique has been discussed earlier in this monograi:h under counting procedures. Usually, small amounts of condensed impurities pro- 49 PROCEDURE 7 (Continued) vide a few hundred micflrons pressure to support the 'discha..r.ge. If this is not the case, one may add some gas such as air, nitrogen or helium to achieve the necessary pressui-e in the discharge tube. Any radon not deposited on the cathode may be frozen back into the last trap at liquid nitrogen temperature for use in later experiments. Notes 1, Stoner and Hyde76 studied isotopes of radon produced by bombard- ment of gold with nitrogen ions in similar fashion., The gold was dissolved in mercury while being heated by induction to remove the radon from the target. 2., Mathur and Hyde77 by bombardment of KI with 100-Mev protons using a similar procedure. The studied neutron deficient xenon isotopes produced KI was dissolved with water in a closed system and the xenon removed from solution by pumping through the same series of traps used in the radon pro- cedure. The NaOH scrubbing was omitted. 3. The observed retention of trace amounts of rare gases in traps at -195°C should be commented upon, It was also observed in the radon experi- ments that fission product rare gases and their interfering P-y activity could be largely removed from radon by warming the trap containing all the gases briefly in air and distilling the fiasion products.into another trap at -195°C. Distillation was monitored with a B-y survey meter and continued until the activity in the trap ceased to drop rapidly (about 30 sec). Well over 90% of the rare gas fission product activity in the radon could be removed thus., The vapor pressures of the rare gasea at -195°C certainly exceed the partial pres- sures of the tracer amounts of rare gases in the above experiments. Retention in the cold traps is thus not due to condensation of the pure solids. Speculation as to the exact mechanism involved seems'pointlesé without further experimen- tal data. It seems worth calling attention to the fact that the retention is not a result of processes involving the rare gases alone (that is, édsorption on or condensation with other substances must be involved), so that circumstances under which trace xenon, for example, might not be held up inla portion of a system at -195°C are poesible. PROCEDURE 8 —- DETERMINATION OF ACTIVE GAS HALF -LIVES BY THE CHARGED WIRE TECHNIQUE (O) Source — C. R, Dillard, R. M. Adams, H. Finston, and A. Turkevich, Paper 68: Radiochemical Studies: The Fission Products, National Nuclear Energy Series, Div. IV, Vol.9, Book 2, McGraw-Hill, New York (1951}, pp. 624. A number of the earlier experiments to determine the half-lives of short-lived rare gases were performed using this interesting technique. A solution of uranyl nitrate is contained in a small polystyrene vessel - over a porous graphite disk through which nitrogen may be bubbled. The vessel is connected through a baffle system, to prevent spray carry-over, to a 10-ft aluminum tube of 1-in, inner diameter. A fine copper wire down the center of the tube is maintained at -1000 volte to effect collection of the daugh- ters of gaseous activities passing through the tube. The entire apparatus is placed in a reactor and the solution irradiated with neutrons while maintaining a constant flow of nitrogen through the system, From experimental data the linear flow rate of gas through the aluminum tube may be calculated. 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