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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
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