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Electrostatic Radionuclide Separation A New Version of Rutherford's "Thorium Cow" Marcus Eiswirth, Robert Schwankner, Fritz Weigel,' and Victor Wishnevsky Radiochemistry Laboratory, Institute of Inorganic Chemistry, University of Munich, Meiserstrasse 1, D-8000 Munich 2 As early as 1900, E. Rutherford ( I ) reported on the discovery of a gaseous a-radioactive suhstance emanating from thorium compounds. In a second paper in the same journal ( 2 )he described a simple device to concentrate the daughter products hy precipi~a;ion on a charged electrode [Fig. I 1. 'I'his device, which was later called "thorium cuu~,"mavbe used in a number of instructive experiments in the undergraduate laboratory. These experiments, which will be described hereafter, produce results which are of practical and theoretical value in undergraduate instruction. They may also he used in lecture demonstrations. Experiment 1
Successive a-decay of the partial chain 220Rn 56.6 s
216p0 A 2 0.15 a
1
~
~
~
Part of the T h (4n) decay series relevant to this work is given in Figure 2. 220Rndecays with a half-life of 55.6 s to 216Po.This in turn decavs with a half-life of 0.15 s to 212Ph.If therefore a carrier-gas containing 2 2 ~isnintroduced into a cloud chamber these two decay events show up as two a-tracks, forming a "V."
Figure 1. Rutherfad's first depositing apparatus (thorium cow) from reference
(2).
Procedure Approximately 5W mg Tho2 (see below) is placed into a small glass tube (ID 6 5 mm, 5 cm long) and kept in place by means of two cotton or glass wool wads. One end of theglass tuhe is connected to a rubber hose which leads into the sensitive reeion of a continuous cloud chamher. The other end of the tuhe is ron&cted to a small rubber hall. the 220Rnwhich has accumulated over the thorium salt. After initial turbulence around the gas entry, the cloud chamber atmmphere calms down, and a large number of a-tracks are observed (Fig. 3), which spread across the whole interior of the chamber. Because of the short half-life of 220Rn,the number of a-tracks diminishes rapidly within the next 2-3 min, so that individual events may he studied more leisurely (Figs. 4 and 5). The V-shaped tracks observed are explained easily below. T h e a-decay observed occurs according to the following general scheme where A = number of nucleons, andZ = atomic number. The reaction energy Q may be calculated as follows:
Q = (rn.[$, XI - m,[$,:$ Y]- m.[$ He]). ci where also
(2)
I n accordance with the classic laws of preservation of momentum and energy, E , and Ey may be defined as follows: E,=Q.
m.
m.[$ He]
19:
Y1
+ rn.[$:$
Y]
and
Numerical evaluation of eqns. (4) and (5) shows that approximately 98%of the energy Q is dissipated to the a particle, but only approximately 2% to the resulting daughter nucleus. This daughter nucleus undergoes a recoil; however, its track is so short that it is barely visible in the cloud chamber before the second a-decay takes place. Even though the a-decay energies of 220Rn(6.288 MeV) and 21BPo(6.7783 MeV) differ by only 0.5 MeV, the tracks of the two successive a's usually appear to have a different length,prohahly due to the different
' To whom all correspondence should be directed.
608
Journal of Chemical Education
Figure 3. Cloud chamber photograph with many cr-tracks
Figure 4. Cloud chamber photograph taken 2 mi" later than Figure 3-few n-tracks, one of them V-shaped.
observation angle, or because the particle causing the shorter track escapes from the sensitive region of the cloud chamber. Experiment 2. Carrier-free Isolationof 220RnDaughters Since the recoil nuclei formed in the 220Rn a-decay are electrically ~ h a r g e d they , ~ may be collected on a suitable surface, having the appropriate charge. The yield is different, depending on whether the wire or the body of the chamber (Fig. 1)is chosen as the cathode. A carrier-free isolation of the Z20Rn granddaughter 212Pb,and its greatgranddaughter 212Bi mav be carried out usine the amaratus develoned bv B. ~ e i n r i c h(3)or that by R.?3chwa&er (4,5). sin& the 2 i " ~ n manddauahter mav be continuouslv "milked" from the setuo. .. the latteris somet&nes referred to-as a "thorium cow." An Erlenmeyer flaskwith narrow neck is coated on its inside surface with strips of aluminum foil. The individual strips have to be in electrical contact with each other, and should project far -2 cm beyond the rim of the flask's neck. The"col1ar"formed in this manner is turned down on the outside and is fixed to the wall of the flask with a clean wire loop. An anode wire is soldered to this loop, and then the whole aluminum collar is insulated with plastic tape. The exehangeable, well-insulated cathode assembly consists of a rubber stopper equipped with a well-centered hole, into whieh a banana plug is inserted and connected to the cathode wire. A crocodile clamp connected to a piece of Pt foil is clamped to this plug (see Fig. 6). For safety reasons. the reouired hieh voltaee .. 10.1 to 1 kV1, is connected to the appamtus hy nl'eans uf 100 k!! rwlator which is rowwrwd in line with the 3ppRmf114.Finely divided igruund) Tho2 is reec,mmmdrd a* the emanating mnrerml: conrss'l'hO, or other Th compuundz hnvr a considerably lower emanating power, so that they have to he eonverted to a highly emanating source (5). Since the emanating power of the ThOt may differwithin a wide range depending on its previous history and on several other parameters (deposition voltage, geometry of the assembly, size of Pt-eleetrode), we have constructed an apparatus which allows a continuous measurement of the 212Pbdeposit and its daughter. This apparatus may be considered as a modified "thorium cow," which is equipped with a side m through which a continuousmeasurement and reading of the cathode deposit activity is possible throughout the whole deposition period. This technique also allows us to measure the equilibrium value (saturation value), whieh is attained by competition between deposition value and decay value for the predetermined parameters.
a
.
~
~~~~~~
~
Figure 5. Cloud chamber photograph with
another typical V-shaped n-track.
Using a 200-ml wide-necked Erlenmeyer flask, we obtained the following optimum values for the individual parameters: DC high voltage: 500-600 V; emanating substance: -10 g of ThOz; area of cathode: -3 cm2; saturation time -70 hr; recommended minimum deposition time 24 hr. If more than one experiment is planned with the 220Rndaughter products deDue to strip-off effect by collision with air molecules. Volume 59
Number 7
July 1982
609
Figure 6. Exploded view of depositing apparatus.
Figure 7. Five depositing units in operation.
posited, several of the "thorium cows" may be connected in parallel to a high voltage line (Fig. 7). Using the "thorium cow" we obtained carrier-free 212Ph-an ideal nuclide for many lahoratorv experiments on the undergraduate level. One has to ..take -into - - ~ nrcount. -~~~ ~,however. that after -8 hr this nuclide is in equilibrium with its daughters. For this reason, a Pt electrode which has been exoosed for sufficient time, and is then left off the "thorium cow" for approx. 8 hr may he used for identification of the 212Pb.After this time. the B actlvitv of the deposit, which may he measured in a Geiger counter, is proportional to the ZL2Phamount present. The half-life ohtained from the fl measurement is 10.6 0.5 br (recommended measuring time: 5 days, 2 3 measurements per day). Residual activity observed after 5 days is due to contamination with T h o ? dust and is to he subtracted from the individual measurements. An electrode exposed in this manner and "cooled" for a few hours may also he used to demonstrate the 20sT1recoil. Approximately one-third of the 212Biformed on the electrode surface undergoes a-decay [6.05, 6.09 MeV], in which the daughter nucleus undergoes a relatively large recoil, which may he calculated from the law of conservation of momentum:
thousand 208T1nuclei at the most are collected), it is recommended to use a low background counter, if such is available. Using the 212Phisolated from the "thorium cow," a number of classical microchemical experiments are possihle, in which the nuclide is used both with and without carrier (5). The experiment described here is the confirmation of Hahn's adsorption rules. One has to keep in mind that a precipitation reaction with a very small amount of a radioactive nuclide cannot he carried out, unless a large amount of a suitable carrier is oresent. or co-orecioitation, adsorption or similar . . phenomena are utilized in scavenginga carrikr free radioisotope. This is easily explained by the following rough estimation. An electrode exposed for a longer period of time in the "thorium cow" may collect up to 0.5 pCi 212Ph,if 10 g ThOz is used as the charge. With a specific activity given by the following formula
~
~~
+
(6)
leads to the quantity of 212Pbcontained in 0.5 pCi where ER= energy of recoil daughter nucleus, m R = mass of recoiling nucleus, E, = energy of emitted a-particle, and m, = mass of emitted a-particle. In the particular case discussed here 4
ER = -. 6 MeV = 0.115 MeV (7) 208 According to B. Heinrich (3, p. 160) this energy is sufficient to rooe el the dawhter - nucleus over a distance of up to 0.2 mm in ihe-air. The loaded electrode is wrapped tightly in thin aluminum foil. After 30 min the foil is removed and is placed under the counter. The inside, which was exposed to the 208TIrecoil nuclei, is placed facing the sensitive area of the counter. The 208T1 is identified from its decay curve with the half-life of 3.1 f 0.7 min. Since the recoil activity collected is small (a few
610
Journal of Chemical Education
If this amount is carefully dissolved from the Pt electrode and concentrated to approximately 1ml solution, the 212Pbconcentration is calculated to he 1.7 . 10-l2 Molfl. Since the soluhility product of PhSOa is
K.,PbSO.
.
= [PbZ+][SO:-] = l.8.10-8 [MoI2A2]
(12)
the amount of [SO:-], which theoretically would he required to reach this value of K., a t the Ph2+-concentration given, would he
It is immediately clear, t h a t this value [more than 1 ton of SO:- per liter!l cannot he realized. o n e of the pbssihle ways to sravenge minute amounts of a radionuclide present in the solutiun is hv the adsorotion on t h e surface of some suitable inactive earrier However, this phenomenon may also lead to erroneous results in a precipitation. For instance, in 1930 0. Erbacher and B. Nikitin (7) found a value of 1.40. 10-4g/100 ml HzO a t 20°C for the solubility of RaS04. Two years later B. Nikitin and P. Tolmatscheff ( 8 ) found this value t o he too low and, by correction for adsorption on glass surfaces, arrived a t 2.1 10-4g/100 ml H z 0 a t 2O0C-a much higher value. Otto Hahn (9-11) studied t h e co-precipitation, co-crystallization, and surface adsorption phenomena in great detail and formulated his well-known precipitation rules, which were discussed in detail in his b w k "Applied Radiochemistry" (12). I n this hook Hahn formulated his precipitation law a s follows:
.
An ion, at any desired dilution, will be adsorbed by a precipitate if that precipitate has acquired a surface charge opposite in sign to the charge of the ion to he adsorbed, and if the adsorbed compound is slightly soluble in the solvent involved. T h e 212Pbisolated in Experiment 2 may b e used t o demonstrate this rule a s described below. Experiment 3 If 212Pb is t o he co-precipitated on the surface of AgCl by addition of AgN03 and KC1 t o theZ12Pbsolution, two differe n t surface conditions of the precipitating AgCl may be achieved (Fig. 8), depending on whether Ag+ or C1- is added in excess. T h e 212Ph2+-adsor~tion should b e much better on e surface would carry the surface of t h e ~ g ~ l - ~ r e c i & t ifa tits neaative charae. T o test this assumotion. the following experiments may he performed.
Immobile Immobile Double Free Double Free Cr stal Layer Solution Cr stal Layer Solution A rc~A ~ + cf- Ag+ C1CI- AgC NO, Agf C1K+ Ag+CI- Ag+ CI- Ag+ C1CI- Ag+ NO, Ag+ CIK+ Ag+CI- Ag+ C1- Agf C1C1- Ag+ NO, Ag+ CIKt Ag+ CIC1- Ag+ C1- Agt Ag+CI- Agt A + C1cf- Agf CIC1- Ag+ NO, Ag+ C1\ Ag+CIAgCl with Charge of the inner part of AgCl with double layer excess of AgN03 excess of KC1
A~YCI-
10 ml dist. H20, dried by washing with acetone-ether, and counted with an end window GM counter. (If the filter assembly is not available, a larger filter crucible may be used instead; for counting, the GM end window counter is inserted into the crucible). The following observation will be made in the experiments type A through E:
T y p e A. Equivalent amounts of Ag+ and CI- are employed. The surface of the precipitate (resp. precipitated crystals) stays neutral, only little 2L2Pbadsorption is observed: Low counting rate. T y p e R. Excess of Ag+ causes positive charge on surface of precipitate; noZ12Ph-ionswill be adsorbed. Only small counting rate due to inclusions is observed. T y p e C. Excess of C1- causes negative charge on surface of precipitate; 212Pbion adsorption is strongly favored. Counting rate is strongly increased. T y p e D and Type E. In these experiments, the 212Pbsolution is added after the AgCl precipitation has been made. The 21Tb S0lution is kept in contact with the precipitate for -5 min, preferably with stirring. Then, the solution is filtered, the precipitate washed and counted as described before. In experiment D (Ag+in excess) similar results are observed as in experiment B;experiment E (CI- ion in excess) shows results similar to those obtained in experiment C. However, the effects observed in Band Care larger than those in D and E since in experiments D and E only the surface of the precipitate is involved in the adsorption,and no inclusion takes place, as in B and C. The same experiments may also he carried out with KBr instead of KCI. The results are even better than in the chloride precipitation. However, if KI is used instead of KBr or KCI, a strong excess of KI causes a decrease of activity, probably due to the formation of the complex [212Pb4]2-(3, p. 162).
It has been demonstrated t h a t even some of the oldest mdiochemistry experiments may be updated with modern eauiomeut and are found t o he useful in the laboraton, courses o;ii the student demonstrations. Because of t h e sdort halflives of the isotopes involved and very low activities needed for these experiments, n o additional precautionary measures are necessary. Acknowledgment We wish t o express our thanks to Mr. Reinhard Marquart of our laboratory for preparation of the photographs and Mr. H. Kress (Leyhold Heraeus GmbH & Co. KG) for his constant support and advice in the apparatus section. Literature Cited (1) Rutherford, E., Phil. Mag.. 49 [5],1-11 119W). 121 Rutherford,E.,Phil. Mog.,49 Bl, 161-192 119W). 131 Heinrich. B., "Radiochemiseha Demon~trnlio~uersuch.." Aulis Verlsg Deuhner & Co. KG KBn,W. Germany, 1968,p. 142ff. 14) schvsnkner. ~ . , ~ l a rder is~ o t u ~ (chemisi, ~ ~24854s ~ ~(1977). s ~ ~ I51 Schwsnkner, R., "Radiochemioprokiikum-Eiifnhhhw in das Kern- und rodiochamisehr C~undprokfrkum [Uniuersildbfoschmbuch]: Sehbningh-Verlag
f
Figure 8.Surface charge conditions in adsorption precipitation of 2"Pb (canier precipitate AgCI).
Paderborn, 1980. 16) Dunean, J. F.,andCmk,G.B.,"Jmtopeinder Chemh."(Gennantransktor:wn WeigDl F.1 Goldmann Verleg Milnehen, W. Gennsny, 1971 (data not in the Endish edi-
Procedure A platinum electrode exposed in the "thorium cow" for at least 24 hr is leached with 12 ml warm HN03 (4 M). For each experiment, 2 ml of this well-mixed solution are used. The solution is combined with 0.2 M AgNO~salutionand 0.2 MKCI solution in thesequence given by the Roman numerals I, 11, and 111in the table, and by adding the amount in milliliters given in this ml,le. After thwouyh m~xing,the solulions are iiltercd through a radiorhemical f~ltrraisrmldy wlth r~movnblcrhimnryi131,'washed with
(71 Erbseher.O.,andNikitin,B..Z. Phys. Chem.,A 158,216230 11932).
tion). (81 Nikitin,B.,andTolmatscheff.P.,Z Phys. Chem..A 167,260-272(1933). 19) Hahn.0.. Nolurw, 14.11961192S). (101 Hnhn.O.,Rer. Dtsrh. Chem Ce8.59.2014 (1926). (11) Hahn.O.,andlrnre.L.,Z.Phys Chem Islte Ausgahel 144.161 (19291. Ilnl Hahn. 0.. "Applied Radioehemistry: Vol. 14, George Fireher Baker Nonresident Ledurenhip in Chemistry a t Cornell Universiqv. Cornell University Press,Ithaea, N . Y..,1916 ..... 1131 Choppin, G. R.. "Experimental Nuclear Chemistry: Prentiee-Hall Ine., Englevod Cliffs. N.Y., 1961,Fig. S.p.71.
In German: Hahn'sche Nutsche.
Experiments to Study the Absorption Behavlor of 2'2Pb on AgCl Precipitates A ~ N O J(0.2 M)
Experiment
ml
[sequence]
KC1 (0.2 M)
mi
[sequence]
2'ZPb (in 4MHN03)
ml
Volume 59
[sequence]
Number 7
Surface charge
of precipitate
July 1982
611
h
~
~
