【放射化学系列】砹的放射化学

Atomic I&iergy G,ommisE&orl ~: 1 r.. Radiocbem@&y CM Astatine k:. ! INationalAcademyofSciences “v National Research Council B NUCLEAR SCIENCE SERIES 3_—.–– The Radiochemistry of Astatine COMMITTEE ON NUCLEAR SCIENCE L. F.CURTIS43,Chai?vnatz ROBLEY D.EVANS, ViceChz@zan NationalBureauofStandards MassachusettsInstituteofTechnology J.A.I)eJUREN,Secretary WestinghouseElectricCorporation H.J.CURTIS BrookhavenNationalLaboratory SAMUEL EPSTEIN CaliforniaInstituteofTechnology HERBERT GOLDSTEIN NuclearDevelopmentCorporationof America H.J.GOMBERG UniversityofMichigan E.D.KLEMA NorthwesternUniversity G.G.MANOV Tracerlab,Inc. W. WAYNE MEINKE UniversityofMichigan A. H.SNELL Oak RidgeNationalLaboratory E.A.UEHLING UniversityofWashington D.M. VAN PATTER BartolResearchFoundation ROBERT L.PLATZMAN ArgonneNationalLaboratory LIAISON MEMBERS PAUL C.AEBERSOLD W. D. URRY AtomicEnergyCommission U.S.AirForce J.HOWARD McMILLEN . WILLIAM E.WRIGHT NationalScienceFoundation OfficeofNavalResearch SUBCOMMITTEE ON RAD!OCHEMISTRY W. WAYNE MEINKE, Chairman EARL HYDE UniversityofMichigan UniversityofCalifornia(Berkeley) NATHAN BALLOU HAROLD KIRBY NavyRadiologicalDefenseLaboratory Mound Laboratory GREGORY R.CHOPPIN GEORGE LEDDICOTTE FloridaStateUniversity Oak RidgeNationalLaboratory GEORGE A.COWAN ELLIS P.STEINBERG Los AlsmosScientificLaboratory ArgonneNationalLaboratory ARTHUR W. FAIRHALL PETER C.STEVENSON Universityof.Washington UniversityofCalifornia(Livermore) HARMON FINSTON LEO YAFFE BrookhavenNational‘Laboratory McGillUniversity Radiochemistry of Astatine By EVAN H. APPELMAN Argo?vw National Laboratory Lemont, lllinoi-s March 1960 , Subcommittee on Radtochemist~ NationalAcademy ofSciences—National Research Council Printedh USA. Price$0.60 Availablefrom theOfficeofTechoice.1 Servlcea,DepartmentofCommerce. Waabhgton 26,D. C. FOREWORD The Subcommittee on Radiochemiat~ is one of a number of sub- cammlttees working under the Committee on Nuclear Science ulthln the IiatlonalAcademFof Sciences-NationalResearchCouncil. Itsmmdxxs representgovernment,i.ndustfial=d universitylabo=toriesin the areas of nuclear chemistry and analytical chemistry. The Subcommittee has concerned itself with those areas of nuclear science which involve the chemist, such as the collection and distribution of mdiochemical procedures, the estabMshment of specifications for mdio - chemicaIly pure reagents, the problems of stockplllng uncontaminated materials, the availability of cyclotron time for service irradiations, the placeof radlochemistry in the undergmduate college program, etc. This series of monogmphs has grown out of the need for up-to- date compilations of mdiochemlcal tifonnation and procedures. The 9ub- conmittee has endeavored to present a series which till be of maximum use to the working scientist and which contains the latest available hl%mm- tion. Each monograph collects h one volume the pertinent tifonnation required for ratiochemical work with an individual element or a gxoup of closely related elaents. An expert h written the mmograph, committee. The Atomic the series. the radiochemlstry of the particular element has folloulng a standard format developed by the Sub- Energy Comlssion has sponsored the prtithg of The Subcommittee is confident these publications will be useful not ODJ.Yto the radlochemlst but also to the research worker in other fields such as physics, biochemistry or medicine who wlahes ta use radlochemical techniques to solve a specific problem. W. Wayne Mdnke, Chairman Subcouauittee on Radiochemiatry iii CONTENTS I. 11. III . rv. v. VI. VII. General Reviews of Astatlne Chemi9try and Radlochemlstry Isotopes of Astatlne Hazards Involved in Handling Astatlne Summary of the Chemical Propert.tesof Astatlne preparation of Astatlne Techniques for Counting Astatlne Samples Collection of Detailed Procedures for Isolation and Purlflcatlon of Astatlne A. Isolation of Astatlhe from Targets B. Miscellaneous Radlochemical Methods 1 1 2 2 7 8 9 9 21 28References v INTRODUCTION This volume astatine is one of which deals with the radiochemistry of a series of monographs on radiochemistry of the elements. There is included a review of the nuclear and chemical features of particular interest to the radio- chemist, a discussion of problems of dissolution of a sample and counting techniques, and finally, a collection of ra&io- chemical procedures for the element as found in the literature. The series of monographs will cover all elements for which rad.iochemicalprocedures are pertinent. Plans include revision of the monograph periodically as new techniques and procedures warrant. The reader is therefore encouraged to call to the attention of the author any published or unpub- lished material on the radiochemistry of astatine which might be included in a revised version of the monographs. vi The Radiochemistry of Astatine* EVAN H. APPELMAN Argonne National Laboratory Lemont, Illinois March 1960 I. General Reviews of Astatine Chemistry and Radiochemistry Edward Anders, Ann. Rev. Nucl. Sci.~ 203 (1959). Earl K. Hyde, J. Chem. Ed. ~ 15 (1959). Earl K. Hyde, J. Phys. Chem. ~, 21 (1954) II. Isotopes of Astatinel Mass No. Half-life Principal modes of decay < 202 43 sec. electron capture, a <203 1.7 min. e. c. 203 7 mime e. c., a 204 25 min. e. c. 205 25 min. e. c., a 206 2.9 hr. e. c. 207 1..8hr. 9@e. c., l~a 208 6.3 hr. electron capture *This monograph Subcommittee on Nuclear Science was prepared at the request of the Radtochemistry of the Committee on of the National Research Council. 1 80 l mlml@ aafdeciw 208 I 1.6 hr. e. c.+o.5~a+7 209 5.5 hr. 95* e. c., 5% a,~ 210 8.3 hr. electron capture + 0m2~ a + y 211 7.2 hr. 5$ e. c., 41$ a 212 0.22 sec. a 213 ?“ a 214 2X1O -6 ~~sec. a“” 215 10-4 Bee. a 216 3 x 10-4 Eec., a“ 217 i).018sec. a 218 2 sec. a + 0.1$4@ 2.19.. 0.9 min. 97$ a, 3$!3 111. Hazards Involved In Handling Astatine All the precautions customsxy in the handling of highly radioactive substances must be observed .Inwork with astatlne. The tendency of astatine to concentrate In the thyroid makes It particularly dangerous,2 and lte”volatility makes it necessaryto provide adequate ventilation during all operations. *t210 Is addition- ally hazardous because of its hard gamma ray and Its 140-day, alpha-emlttlng P021Q daughter. IV. SunInaryof the Chemical Properties of’Astatlne3-7 Since astatlne has no long-lived Isotopes, chemical studies of It must be conducted at very low concentra- tions--usually of the order of 10-15 ~. Thls”uAkes the astatine concentration comparable to that of the least of the impurities In the experimental system. The re- 2 action of the astatlne with such impurities often leads to Irreproducible and unlntenpretable--not to say exce&l- Ingly frustrating--results which lend considerable un- certainty to our knowledge of the chemistry,of this element. We might anticipate that astatlne, as the heaviest halogen, would have properties roughly similar to those of Its lighter brethren. However, a close examination of the chemistry of the halogens reveals marked differ- ences among them, and the radlochemist must at all times be acutely aw?xreof those properties which distinguish astatine from the other halogens. At least four oxidation states of astatine have been identified in aqueous solutlon: Astatlde, At-, 1s f’ormedby reduction of higher states with S02, zinc, As(III) at PH > 5, or ferrocyanlde at PH > 3 and Ionic strength < 0.1. It Is characterized by nearly complete coprecipitation (>9C@) with AgI, TII, or Pb12. The so-called “At(O)” Is the form In which astatine is usually found when left to Its own devices in acidic solution. In the absence of macro quantities of other halogens, At(0) Is characterized by high volatility, a tendency to be adsorbed on metal or glass surfaces, and by ready, but quantitatively unpredictable extractability from acidic aqueous solutions into organic solvents.” !l?hus In a single extraction CC14, benzene, toluene, ene, n-heptane, or Isopropyl ether will remove ~ of the astatine froman equal volume of an solution. cyclohex- from 70 to aqueous Vsrlous workers have reported degrees of coprectplta- tlon of At(0) with insoluble iod~des and iodates ranging 3 fran O to 90j%. The astatine may be nearly quantitatively precipitated with elemental tellurium formed in situ In .— acid solutions, and will.partially precipitate with in- soluble sulfides and hydroxides. The extractability of At(0) Into hydrocarbons or CC14 decreases markedly with the addition of halide ions, While the extractability into ethers is not greatly altered. The,At(0) becomes unextractable into all organic solvents when an acid aqueous solution Is rendered alkaline. The extractability Is usually largely restored if the solu- tion is reacldlfied wlthln a short time. From the alkallne solution the astatlne Is completely copreclpitated with TII or with AgI, the latter precipitated from an NH40H solution. It has usually been assumed that the astatlne species present in “At(0)” solutions is At2. The ef’feetof halide ions might thenbe explained In terms of the formation of such complexee a~ At21-, which would not extract Into CC14 or.hydrocarbons, but might extract Into ethers as HAt21. ,. The bexvlor in alkaline solution Is,explicable In terms of reversible hydrolysis to At- and HOAt. However, as we have already noted, astatlne is subject to reaction with impurities. At(0) should be especially vulnerable, Since not only is At2 expected to be extremely labile In Its reactions, but any reaction which tends to break up the At2 molecule becomes thermodynamically favored at these low astatlne concentrations. To make matters worse, most of the experiments involving At(0) have been carried out without adequate control of the oxidation po- tentials of the solutions. Thus in addition to At2 these solutions may contain assorted compounds of astatlne with 4 whatever organic impurities happen to be around, the exact species present varying from one solutlon to the next. Tt Is small wonder that irreproducible behavior has been observed. These complications may be largely avoided If another halogen and halide Ion are present at macroconcentratlons. Now not only does the macro X- --~ couple control the oxidation potential of the system, .but the.astatine is in the form of a known Interhalogen compound, since there- actions At2 + X2 = 2AtX should be rapid and quantitative. Further, the maaro halogen will react preferentially with many impurities which might otherwise react”with the astatlne. In the presence of iodine and Iodide the moderately extractable species AtI smd the unextractable complex ion At$- appear to be formed.’ At 21°C. the distrtbutilon between aqueous solutions and CC14 is represetitedW ‘ D = organic astatine/aqueoua astatine = 5.5/1+2000(1-) From fluchsolutions the astatine is not coprecipitated with AgI.or Pb(103)2. Addition of Tl+ to these solutions pre- cipitates TII-~ removing the Iodide, most of the ~, and all of the astatine from solution.. The 12 and astatiinemay readily be removed from the precipitate by washln~:l”twith acetone. Pb12 does not coprecipltate astatine from’these solutLons If the stolchiometric 12 concentration is low; when it is high, both 12 and astatlne are partial.’lyad- sorbed by the precipitate but may be removed by ‘acetone. In a system containing ~, =, and Br-, the astatlne is largely unextractable Into CC14, being present prlmar- Ily as the slightly extractable AtBr and the unextractable AtBr2-. 5 The intermediate positive astatlne state or states, which we may designate At(X), have been identified prlmsr- ily on the basis of what they do not do. At(X) is com- pletely unextractable Into CC14 or benzene, though It may extract Into ethers from several molar HC1 solutions, and it does not coprecipltate with Insoluble Iodi.desor Iodates. At(X) is formed by oxidation of At(0) with Br2 or C12 (but see followlng discus~ion of At03-) or by photochemical oxidation with a VOH -- V02+ mixture or with Fe- at low FeU concentrations. These photochemical oxidations are reversed In the dark, returning the astatlne to the extractable At(0) state. The radlochemlst must alway6 oonsider the possibility of Interference from such photo- chemical reactions. Likely posslbilltles for At(X) are HOAt and HAt02, with the ether-extractable species being the corresponding polyhalo-acids HAtC12 and HAtC14. However organoastatlne compounds cannot be excluded from consideration. Astatate, At03-, has been identified as an unextract- able qpecles completely copreclpitated with Ag103, Ba(103)2, or Pb(103)2. It Is formed by oxidation of lower astatine states with Ce+4, hot persulfate, orhotperlodicacld. + is added to an 12It Is also formed when Ag --1- solution containing AtI, presumably In accordance with the reaction AtI + 212 + 31-1# + 5Ag+ = At03- + 5AgI + 6H+. Although the product of C12 oxidation of At(0) Is prlmarlly At(X), when no chloride is present In solution other than that formed by hydrolysis of the chlorlne, partial copreclpi- tation of the astatlne with Pb(IO=)a is observed, and this >= may indicate partial oxidation of the No evidence has been found for a At(X) to At03-. peraBtatate. 6 The following pdential diagram, referred to 0.1 M. acid, summarizes the oxidation-reduction behavior of asta- tine In acid solution. unknown At(o) At- -- -0.3 -- AtI -- -1.0 -- At(X) AtRr At(X) -- -1.5 -- At03- -- < -1.6 --.H5At06 v. Preparation of Astatlne Astatine for chemical and mkdical studies and for tracer use Is prepsred by bombardment of metallic bismuth or bismuth oxide with alpha particles of energy exceeding 20 Mev, according to the reactions Bi209(u,xn) At213-x. The reactions with x = 2, 3, and 4 have threshold energies of’20, 28, and 34 Mev, respectively.8,9 “’Metallic bismuth, the more common target material, Is customarily fused or vaporized onto aluminum or gold ~ck- ing plates. Sihce astatlne may be volatilized from molten bismuth it is necessay to cool the target carefuliy. Bismuth is a poor thermal conductor, and the cooling prob- lem Increases with the thickness of the bismuth layer. The back of the target Is “generally water-cooled. The face is most effectively cooled by a flow of helium, though a“s“tatic helium atmosphere is often used. An 0.5 to 1 mil stainless steel or copper cover foil pressed tightly to the surface of the bismuth helps to dissipate the heat evolved and also prevents astatine from escaping from the target. When bismuth oxide is used, it is generally pressed into small holes drilled in the face of a thick alumlnum plate and thereafter treated in the same manner as the metallic targets. Melting of the target material Is much less likely in this case. 7 In all focussed aB the target. cases the beam of alpha particles should be de- much as possible to avoid local hot spots on tion of a Various astatlne isotopes are also formed by spalla- reactions brought shut by high energy bombardment variety of elements. VI. Techniques for Counting Astatlne Samples Astatlne211 may be assayed by counting either its alpha particles or the x-rays accompanying its electron capture. The alpha counting may be csrried out in any conventional alpha counters, suoh as gas-flow ionization chambertior proportional counters, or zinc sulfide scin- tillation counters, Reproducible and adherent astatine samples may be obtained by evaporating astatine solutions in about 2 ~HCl to dryness on silver or platinum foils under an infrared lamp. Under nmst other conditions such evaporations show erratic losses of astatine. The requirement of tirtually weightless samples to avoid self-absorption severely restricts alpha counting as a means of assaying astatine. In Coprecipitation experi- ments one may circumvent this difficulty by counting In. finitely thick samples of homogeneous precipitates, i.e., samples so thick that no alpha particles from the bottom of the sample are counted. However, the absorption prob- lem can be almoBt completely eliminated by the use of x- ray counting methods, which permit the direct assay of solutions and of bulky and Inhomogeneous preoipitates. Al- though the x-rays may be counted with a Geiger counter, much greater effic~ency is obtained with a sodium-iodide 0 6clntlllator. It Is advantageous to reduce the relative- ly high background of *he sclntlllator by operating It as an energy analyzer reg18tering only counts of energy in the vicinity of the ca. 90 kv. k-x-ray of the astattnelH polonium daughter. *t211 decays in part to the long-lived Bi2W which also decays by electron capture. The ratio of initial At211 x-ray activity to residual Bi2W activity is of the order of 105. The bismuth is usually present In colloidal form — probably adsorbed on duBt particles - and will be csxried along unpredictably through a surprisingly wide vsrlety of chemical procedures. Only distillation of the astatine can be reliei on to-remove all of the Bi2W. Samples x-ray assayed for astatlne some time after purifl- 2W should be recounted after the astatinecation from Bi has entirely decayed away, the resulting B1207 count being subtracted from the original count of the sample. The techniques outlined here for At211 apply gener- ally to the other astatine isotopes, with specific mdifl- catlons srislng from the decay scheme of the partlcula- isotope in question. Thus, for example, At210 may also be assayed by scintillation counting of its 0.25 and 1.2 Mev gamm rays. VII. Collection of Detailed Procedures for Isolation and Purification of Astatlne A. Isolation of Astati.nefrom Targets Methods of two types are available for remoting asta- tlrlefrom bismuth targets--those involving distillation of the astatine from the umlten tsrget and those Involving dissolution of the terget In acid. 9 PROCEDURE 1 DISTILLATION OF ASTATINE FROM MOLTEN BISMUTH IN A~10 For very rapid separation of astatine from the blstith target, a-method Is used which gives astatine of somewhat uncertain purity but which is considered adequate for de- termination of short-lived alpha-emitters. The basis for the method is the distillation of astatlne from molten bismuth. The bismuth target is dropped into a stainless steel crucible fitted on top with a water-cooled steel finger to which a collecting platinum disk is clamped. When the bismuth Is kept slightly above its melting point (as measured by a thermocouple fitted into swell in the crucible),,within a few seconds astatine distills onto the collecting plate. Polonium does not distill in appreciable quantities until considerably higher temperatures are reached. Using a vacuum csrrier system to deliver ”the target, this prooedure permits samples to be in the alpha- puise analyzer within 90 seconds after the cyclotron beam iS shut off. Editor’s Comments: This method was confirmed by the editor and Ralph D. McLaughlin.7 Although the astatine begins to come off the target at the melting point of the bismuth, temperatures as high as 8000C. may be required to effect nearly quantitative removal. Astat@e collected at such temperatures will be contaminated with ’anypolonium which may have been present in the target, and also with some bis- muth . The adherence of the astatine to the collecting plate is strongly dependent on the material of which the plate is made. Deposits on platinum or silver are strongly adherent, while those on aluminum are much less so.11 10 ~.1 (Ccmt’d.) This procedure has also been found suitable for iso- lating astatine prepared by’heavy-lon bombardment of gold. In this case the gold Is dissolved in the mo