Residue adsorption—I separation of strontium-90 and yttrium-90

J. lnorg. Nucl. Chem., 1963, Vol. 25, pp. 483 to 499. Pergamon Press l.td. Printed in Northern Ireland

RESIDUE A D S O R P T I O N - - I SEPARATION OF STRONTIUM-90 AND

YTTRIUM-90

H. W. KIRBY Mound Laboratory,* Monsanto Research Corporation, Miamisburg, Ohio (Received 20 August 1962)

Abstraet--A new separation technique is presented, based on the tendency of the hydrolysed species of certain elements to become adsorbed on any available surface. To separate strontium-90 and yttrium-90, a dilute acid solution of the radioelements is evaporated to dryness, and the residue is covered with dilute ammonium hydroxide. The solution is evaporated to dryness, and the residue is washed with distilled water. Yttrium-90 remains adsorbed by the surface, while strontium-90 is separated in the distilled water. The adsorption of yttrium-90 is independent of the nature of the surface, being nearly quantitative on Teflon, glass, stainless steel, tantalum, gold and platinum. The classical (Freundlich) isotherm is followed in the presence of yttrium carrier. The degree of separation (i.e., the fraction of the strontium-90 remaining with the yttrium-90) is dependent upon the drying temperature and probably upon the nature of the surface. Yttrium-90 of any desired degree of purity can be prepared with negligible loss by repeated dissolution, hydrolysis and washing. The method is applicable to the determination of total rare earths in the picogram range. THIS paper presents a new separation technique based on the tendency o f many elements to be adsorbed, under certain conditions, on any available surface such as the walls of a container. This phenomenon, sometimes erroneously called "radiocolloidal" adsorption is not limited to radioactive elements nor even to trace concentrations o f hydrolysable species, although the high sensitivity of radiometric methods has greatly extended the lower limits at which adsorption studies can be carried out. F o r example, a c o m m o n observation in precipitations from homogeneous phase is the strong tendency o f slowly forming precipitates to be adsorbed by the walls o f beakers31~ It has, in fact, been suggested by H. H. WILLARD(2) that an appropriate goal would be to make this tendency quantitative, so that the mother liquor could be poured off without filtration and the precipitate weighed in the reaction vessel. In the method to be described, strontium-90 and yttrium-90 are deposited on a flat plate, evaporated to dryness, and converted to hydroxides. The hydroxides are evaporated to dryness and washed with water. Yttrium-90 remains adsorbed nearly quantitatively, while strontium-90 is separated with the wash water. The separation is independent o f the nature of the surface. The separation of 28 year strontium-90 and its 64 hr daughter, yttrium-90, has been accomplished by carrier precipitation, 13) "radiocolloid" filtration, ~4,'~)electrolytic * Mound Laboratory is operated by Monsanto Research Corporation for the U.S. Atomic Energy Commission under Contract No. AT-33-1-GEN-53 ~1~L. GortDoy, M. L. SALUTSKYand H. H. WILLARD,Precipitation.D'om Homogeneous Solution p. I I. J. Wiley, New York (1959). ~ L. GORDON,Private communication (June, 1961). ~ M. L. SALUTSKYand H. W. KIRBY,Anal)'t. Chem. 27, 567 (1955). 483

484

H.W.

KIRBY

deposition, (8~ liquid-liquid extraction, tT) anion exchange, ts~ cation exchange, tga°~ and paper chromatography, mJ In addition to these selected examples, numerous studies of the behaviour of the rare earth and alkaline earth elements offer the analyst a wide choice of methods from which separations appropriate to his specific needs can be adapted, t12'13~ Thus, there is no scarcity o f methods for the separation of strontium-90 and yttrium-90, and it is not the purpose of this paper merely to add to the list. The strontium-90/yttrium-90 system was selected as a convenient vehicle for the investigation of some of the parameters of the method of residue absorption, which appears to be applicable to rare earth-alkaline earth separations in general and probably to other separations involving elements having significantly different hydrolytic tendencies. Preliminary observations

A solution of strontium-90/yttrium-90 had been obtained from Oak Ridge National Laboratory several years prior to the beginning of this work, and all of the solution had evaporated during the intervening years. The bottle was leached with distilled water until further leaching recovered no additional radioactivity. A sample of the solution was fl-counted. The counting rate increased rapidly over a period of several days, indicating that the yttrium-90 had not been recovered from the original stock bottle. After a period of several months, the water solution of strontium-90 was again sampled and fl-counted. Although the yttrium-90 should have been in secular equilibrium by virtue of its short half-life, the fl-counting rate of the sample increased for several days before atttaining a constant value, indicating that most of the newly formed yttrium-90 was being adsorbed on the sides of the storage vessel. Concentrated acid was added to make the solution approximately one normal in nitric acid. The counting rate of the acidified solution immediately increased to twice its original value and remained stable thereafter. Concurrently, it was observed that, if a solution of protactinium-231 and its decay products (actinium-227, thorium-227, and radium-223) was evaporated to dryness on a fiat plate, most of the radium-223 could be desorbed with distilled water, but dilute acid was required to remove actinium-227. Desorption of thorium-227 could usually be accomplished with hot 8N HNOs, but the addition of a trace of hydrofluoric acid was sometimes necessary for quantitative removal. Recovery of protactinium from the residue required leaching with a mixture of nitric and hydrofluoric acids. t4~G. K. SCHWEITZER,}3. R. STEINand W. M. JACKSON,J. Amer. Chem. Soc. 75, 793 (1953). ~5~j. D. KURBATOV,and M. H. KURBATOV,J. Phys. Chem. 46, 441 (1942). te~ G. LANGE,G. HERRMANNand F. STRASSMANN,J. Inorg. Nucl. Chem. 4, 146 (1957). '7~N. P. RtlDENKO,Zh. Neorg. Khim. S.S.S.R. 4, 220 (1959). c8~S. MISUMIand T. TAKETATSU,J. Inorg. Nucl. Chem. 20, 127 (1961). ,9~ E. V. MARATHE,J. Sci. lndustr. Research (India) 14 B, 354 (1955). ,10~I. J. GAL and O. S. GAL,Proceedings o f the Second International Conference on Peaceful Uses of Atomic Energy, Geneva 28, 24 (1958). ~lx~p. C. STEIN,Analyt. Chem. 34, 348 (1962). c12~D. N. SUNDERMANand C. W. TOWNLEY,The Radiochemistry of Barium, Calcium and Strontium. Natl. Aead. Sciences-Natl. Research Council Report NAS-NS 3010 (1960). tls~ p. C. STEVENSONand W. E. NERVIK,The Radiochemistry of the Rare Earths, Scandium, Yttrium and Actinium. Natl. Acad. Sciences-Natl. Research Council Report NAS-NS 3020 (1961).

Residue adsorption--I

485

F u r t h e r w o r k with actinium-227, thorium-227, a n d radium-223 in the absence o f protactinium-231 confirmed the q u a n t i t a t i v e differences in the a d s o r p t i v e tendencies o f the three r a d i o e l e m e n t s a n d showed t h a t lead-211 (a short-lived decay p r o d u c t o f radium-223) was a d s o r b e d as t e n a c i o u s l y as thorium-227. N e i t h e r the f o u r - c o m p o n e n t p r o t a c t i n i u m chain, n o r the t h r e e - c o m p o n e n t actinium chain, because o f the c o m p l e x i t y o f their analytical p r o b l e m s , lend themselves well to a systematic investigation o f the s e p a r a t i o n technique. The strontium-90/ y t t r i u m - 9 0 system, chemically a n a l o g o u s to radium-223/actinium-227 a n d b a r i u m 140/lanthanum-140, was therefore selected for this initial s t u d y o f the m e t h o d o f s e p a r a t i o n b y residue a d s o r p t i o n .

Equipment and reagents The strontium-90 solution described in the previous section was diluted with one normal nitric acid to provide a convenient counting rate. A 250/~1 sample of the diluted stock solution contained, by absolute beta counting, 0"867/~c of beta activity, or approximately three nanograms of strontium-90 and 0"8 pg of yttrium-90. fl-bremsstrahlung counting of strontium-90 and yttrium-90 was done with a Nuclear Measurements Corporation PHA-ICA single channel scanning spectrometer equipped with a sodium iodide (thallium-activated) crystal, 2 in. in diameter and k in. thick. The face of the crystal was covered with 0'001 in. aluminium. In this instrument, the gross/3-bremsstrahlung counting-rate of the platinumbacked standard sample was 8-7 × 105 counts per min, uncorrected for coincidence. Above0-61 MeV, the gross counting-rate was 2"5 × 105 counts per min. Evaporations were carried out on a Precision hot plate equipped with a stepless bimetal thermoregulator. Surface temperatures up to 200°C were determined, for each type of backing material, with Tempilstik or Tempilaq melting point compounds (Tempil Corporation, 132 West 22nd Street, New York 11, New York). Higher temperatures were supplied by a muffle furnace with a direct reading temperature indicator. Surface rugosity was measured with a mechanical profilometer (Micrometrical Mfg. Co., Ann Arbor, Michigan) which reported the average depth of surface imperfections in microinches. All reagents, except yttrium carrier, were of analytical grade, and were used as received without further purification. The yttrium oxide (99 + ~Y~O3) was prepared and analysed spectrographically by Y. G. Ishida of this Laboratory. Plastic retaining rings on glass and metal surfaces were prepared by spraying the surfaces, except for a circular masked area 15 mm in diameter, with Krylon Crystal Clear Spray Coating, a mixture of methyl and n-butyl methacrylate polymers dissolved in volatile aromatic hydrocarbons and propelled by fluorinated hydrocarbons (Krylon, Incorporated, Norristown, Pennsylvania). Transfer pipettes were made from commercial medicine droppers whose tips were drawn down to openings approximately 0"5 mm in diameter. Siliclad, a silicone coating (Clay-Adams, Incorporated, 141 East 25th Street, New York 10, New York), was used to coat the insides and tips of micropipets, eliminating the need for rinsing. EXPERIMENTAL

Sample preparation and separationprocedures To provide uniform surface conditions, all experimental samples (except as noted in individual cases) were prepared identically. Evalzorations and other heating were at 90°C (3z 3~C), except as noted. The backing material was cleaned by agitating for 5 or 10 rain in an alcoholic solution of potassium hydroxide. The surface was rinsed thoroughly with distilled water and dried with absorbent paper. A methacrylate retaining ring was provided by masking the surface with an inverted glass vial, 15 mm in diameter, and spraying the exposed area with Krylon coating. The retaining ring was omitted in the experiments with Teflon. The masking vial was removed, and the spray coating was allowed to dry thoroughly in air. The surface was rinsed with distilled water, the excess water was shaken off. and the sample disk was dried on the thermostatically controlled hot plate.

486

H . W . KrRsY

The hot plate was adjusted with reference to the surface temperatures of glass, Teflon, or stainless steel, depending upon the material to be tested. Stainless steel blanks (0"025 in. thick) were used as temperature references for all metals and for Teflon film mounted on stainless steel backings. The surface temperature was taken to be the average of the rated melting points of two Tempilstiks, one molten, the other solid at the stated temperature. The open (i.e., uncoated) area of the sample disk was covered with 0.25 ml of 1N HNO~ and heated for 5 min. The acid was rinsed away with distilled water, the excess water was shaken off, and the disk was dried on the hot plate. The open area was covered with 250 #1 of the strontium-90/yttrium-90 stock solution, and the solution was evaporated to dryness. The residue and the inner edge of the retaining ring were covered with 0-5 ml of recently boiled and cooled distilled water, and the solution was evaporated to dryness. As the liquid evaporated, the aqueous solution contracted away from the retaining ring, and the evaporation was completed entirely within the open area. Heating was continued for 5 rain after the last visible trace of moisture had evaporated. The residue was covered with 0.25 ml of the selected reagent, and the solution was evaporated to dryness. Heating was continued for 5 min after evaporation was completed. The gross counting rate of the sample above 0'61 MeV was determined during a standard 5 min counting and cooling period. A standard sample of strontium-90/yttrium-90 in secular equilibrium and mounted on platinum, stainless steel or glass was counted just before or after the experimental sample to provide a basis for normalizing counting-rates obtained over a period of several days. Two types of separation procedure were used: (A) After being counted, the sample was washed for 60 sec in a stream of distilled water delivered by gravity from a storage bottle 6 ft overhead through an orifice 1.5 millimeters in diameter. The total volume of water used to wash the sample was 560 ml. The wash water was discarded, the excess water was shaken off, and the sample was dried at 90°C. (B) After being counted, the sample was returned to the hot plate, and the residue was covered with 0'5 ml of recently boiled and cooled distilled water. The solution was heated for 5 min, and the remaining liquid (usually about 0-2 ml) was transferred by means of a transfer pipet to a second disk of the same material. The residue was again covered with 0-5 ml of water, and the heating and transfer were repeated. Both fractions were evaporated to dryness. Type A separations permitted rapid evaluation of many variables while minimizing potential manipulative errors. Type B separations permitted the determination of material balances and multistage purifications of the separated radioisotopes. When the sample was dry, its fl-bremsstrahlung counting rate above 0.61 MeV was determined. Differential spectrometry was found to be unsatisfactory for routine use because of the inconvenience of preparing and maintaining standard samples of yttrium-90 having both adequate counting rates and high radio-chemical purity. Instead, quantification of strontium-90 and yttrium-90 was accomplished routinely by the method of differential decay, t14~ based on the yttrium-90 half-life of 64.029 hr TM and the strontium-90 half-life of 28 years.C15' To simplify the differential decay calculations, the lower discriminator of the spectrometer was set on the 0'61 MeV y-photopeak of bismuth-214 (radium C), and the upper discriminator was disabled. Thus, only beta particles and bremsstrahlung due to yttrium-90 were counted.~X~ The spectrum of an equilibrium mixture of strontium-90 and yttrium-90 is shown in Fig. 1. Spectra of an equilibrium mixture and of the separated radioisotopes are shown in Fig. 2. It is evident that 0.61 MeV is well beyond the maximum fl-energy of strontium-90, as detected by the sodium iodide crystal. The more probable maximum energy of the strontium-90 fl-particles and bremsstrahlung is 0"54 MeV. ~16) This uncertainty does not, however, affect the reliability of the differential decay calculations, and the use of the higher energy limit insures that no strontium-90 activity will be counted directly.

Nature of the adsorbent surface The materials listed in Table 1 were tested to determine the effect of the nature of the substrate on the degree of separation. All of the surfaces were prepared as described above, except that the Teflon

~14~H. W. KIRBY, Analyt. Chem. 26, 1063 (1954). ~15~W. H. SULLIVAN,Trilinear Chart of Nuclides, U.S.A.E.C., January (1957). ,1G~D. STROMINGER,J. M. HOLLANDERand G. T. SEABORG,Revs. Mod. Phys. 30, 585 (1958).

Residue adsorption--I

B E T A - BREMSSTRAHLUNG OF" srgO_ y90

487

SPECTRUM

E E

(.3

0'06

0.10

0.25

0-50

Energy.

1.0

1.5

2.0

5.0

MeV

FIG. 1

BETA-BREMSSTRAHLUNG srgO+ y9O

.¢_ E

c)

Energy FIG. 2

---..-

SPECTRA

H. W. Kmnv

488

TABLE I.--ADSORPTION or STRONTIUM-90AND YTTRIUM-90HYDROXIDES Effect of Substrate Material (Maximum Temperature 90°C)

Number

Substrate

Sepn. type

281-G 241 281-H 269 252 281-E 255 256 281-A 239 259 281-F 281-D 240 294-B 281-]3 258 281-C 250-B 251

Soda-lime glass Soda-lime glass Borosilicate glass (Pyrex) Borosilieate glass (Pyrex) Polytetrafluoroethylene (Teflon) Teflon film on stainless Teflon film on stainless Teflon film on stainless Stainless steel Stainless steel Stainless steel Gold Gold-plated stainless Gold-plated stainless Gold-plated stainless Tantalum (Annealed, dull) Tantalum (Annealed, dull) Tantalum (Unannealed, bright) Platinum Platinum

A B A B B A B B A B B A A B B A B A A B

% Retained 9°Sr 90y 18"1 9.7 19.0 18.5 23.8 1.7 10.7 4.1 7.9 27-7 11.9 3.6 2.9 2.1 4.0 10.5 27-6 12.9 14.0 9-2

95.3 98.7 96'4 94.1 96.4 63"9 74.6 94.2 96.4 98.2 99.2 94.2 83.3 99-1 99.9 95.9 99.2 95-7 98.5 96.8

Separation* factor 5.3 10.2 5.1 5-1 4-1 37.6 7.0 23.0 12.2 3"5 8.3 26.2 28.7 47.2 25.0 9.1 3-6 7.4 7.0 10"5

* Separation factor = %9°Y/%9°Sr film cemented to a stainless steel backing was cleaned with ethyl alcohol instead of alcoholic potassium hydroxide. In each case, the "selected reagent" consisted of 0.2 ml of previously boiled and cooled distilled water and one drop (0.05 ml) of concentrated ammonium hydroxide. (This mixture is referred to hereafter as 2M NH4OH.) No significant differences were observed in the results obtained by the two separation methods. Except for two Teflon surfaces and one gold surface, over 90 per cent of the yttrium-90 was retained regardless of the nature of the surface material or of the type of separation. All surfaces adsorbed significantly less strontium-90 than yttrium-90. Although the degree of separation varied widely even among similar surfaces, the percentage of yttrium-90 retained by the washed surface was at least 3"5 times as great as the percentage of strontium-90.

Effect of temperature and desiccation The same procedures were used to study the effect on lime glass, stainless steel, gold and platinum, of exposure to various temperatures. For experiments at 90°C or above, samples were counted before the addition of 2M NH4OH to the dried residue. After the ammonium hydroxide solution had evaporated to dryness, the sample was heated for an additional five minutes at 90°C, then transferred to a second hot plate or to a muffle furnace previously set at a higher temperature. After 5 rain at the elevated temperature, the sample was returned to the 90 ° hot plate for 5 min of temperature equilibration before type B separations were carried out. For type A separations, samples were cooled to room temperature for at least five minutes before being washed. Four separations of type A were carried out on platinum at temperatures below 90°C. Modifications were made in the standard waiting periods between reagent additions to compensate for the slower evaporation rates. Since evaporation of 0"5 ml of water at 55°C required 65 min (vs. 12 min at 90°C), heating was continued for 30 min instead of the usual five. Evaporations at room temperature

Residue adsorption--I

489

(27-28°C) were usually carried out overnight since 0.5 ml of H 2 0 required 4-~ hr for complete evaporation. Drying of two of the three room temperature samples was completed in an evacuated bell jar in the presence of anhydrous magnesium perchlorate. All materials tested retained yttrium-90 essentially quantitatively at 90°C and above. The adsorption of strontium-90 increased with increasing temperature, but the temperature at which this increase became significant was different for different materials. TABLE 2.---ADSORPTION OF STRONTIUM-90 AND YTTRIUM-90 HYDROXIDES ON PLATINUM EFFECT OF TEMPERATURE Maximum temperature (°C)

Number

Separation type

700Retained 90Sr 90y

297-A 298-A* 298-Bt 297-B

A A A A

28 28 28 55

2.6 2.0 3.3 8.8

69.0 71 "9 81.0 90.7

250-B 251 280-19 253-1 253-3++ 280-17 253-2++ 280-18

A B A B B A B A

90 90 200 300 400 500 500 1000

14.0 9'2 8"2 12.6 50-0 64.1 75.7 79.7

98.5 96'8 99-7 99.9 99.8 95.1 100.3 98.4

* Vacuum desiccated 1 hr. I" Vacuum desiccated 19 hr. ~: Plastic retaining ring burned off and surface became very hydrophilic. Washing was carried out with two 0'25 ml portions of HzO instead of standard 0-5 ml portions. Lime glass and stainless steel showed increased retention of strontium-90 at 150°C, but the retention of strontium-90 by gold and platinum did not increase significantly between 90 ° and 300°C. The results for platinum over the range 28 ° to 1000°C are given in Table 2. Evaporation at room temperature resulted in lower retention of yttrium-90, but the retention was significantly enhanced by vacuum desiccation. The highest separation factor achieved (relative retention of yttrium-90 and strontium-90) was 36-3 for a sample which was vacuum desiccated for 1 hr at room temperature, but the yttrium-90 yield was only 71-9 per cent. A yield of 81.0 per cent and a separation factor of 24.9 was found for a sample which was vacuum desiccated at room temperature for 19 hr.

Effect of the residual anion Seven volatile and five non-volatile reagents (Tables 3 and 4, respectively) were studied with platinum at a maximum temperature of 90°C. Separation type A was used throughout. The total amounts of strontium-90 and yttrium-90 in the carrier-free residue were approximately 3 × 10 -s and 1 × 10-11 mmole, respectively. Nevertheless, the concentration of the added reagent was significant, even for the volatile reagents. Furthermore, when the residue from an acid reagent was covered with distilled water and re-evaporated, the retention of yttrium-90 was enhanced, suggesting that residual acid had been redissolved and volatilized. There was no consistency among the several reagents in the relationship between concentration and per cent adsorption. With hydrofluoric acid, an increase in concentration resulted in increased adsorption of both strontium-90 and yttrium-90, but the reverse was true for hydrochloric, nitric and and acetic acids. The results with NH4HCOs, which decomposes at about 60°C to yield NHz and CO2, were the

490

H. W. KIRBY TABLE 3.--ADSORPTION OF STRONTIUM-90 AND YTTRIUM-90 ON PLATINUM AT 9 0 ° C EFFECT OF VOLATILE REAGENTS

Number

Reagent

250-A 283-20* 275-6* 303-23 303-24 250-B 293-27 293-26 250-D 277-11 278-15 277-9 257-D 278-13 257-C 274-3 302-9 278-14 257-F

H~O (1 M HNOs) + 2HcO (6 M HC1) + 2H20 0.01 M NH~OH 0.1 M NH4OH 2 M NH4OH 0.001 M NHaHCO3 0'1 M NH4HCO3 0-01 M H F 0'1 M H F 1 M HF 0.02 M HNO3 1 M HNO3 0'01 M HCI 1 M HC1 6 M HCI 0"1 M CH3COOH 0"15 M CHaCOOH 1'5 M CH3COOH

Retained coSr coy 1.6 0.5 2.3 10.5 13.0 14.0 1'4 9'9 7'4 14"4 19-6 1-0 0"7 2"0 4'5 3"4 8'2 15'7 0'7

57.0 44.5 14.7 87-3 96.1 98.5 63'3 89'3 29'3 71"6 64'9 25"6 15"1 10-8 4-2 7-0 52"8 29-2 8"6

Separation factor 35-6 89.0 6.4 8.3 7.4 7.0 45"2 9"0 4"0 5"0 3"3 25"6 21"6 5"4 0"9 2"1 6"5 1"9 12"3

* After evaporation to dryness, the acid residue was covered with 0'5 ml HcO and dried, covered again with 0.5 ml HzO, dried and washed.

TABLE 4.--ADSORPTION OF STRONTIUM-90 AND YTTRIUM-90 ON PLATINUM AT 90°C EFFECT OF NON-VOLATILE REAGENTS

Number

Reagent

250-E 277-12 304-25* 283-30 283-29 293-28 250-C 277-10 250-F 278-16 283-22 283-23 285-1 285-25 283-24

0'001 M NH4HcPO4 0"01 M NH4H2PO4 (0'1 M NH4HcPO4) + 2HcO 0'01 M N a O H 0"1 M N a O H 1 M NaOH 0"001 M (COOH)2 0.01 M (COOH)2 0-001 M (NH4)~SOa 0"01 M ( N H 4 ) 2 S O 4 0"000001 M Na2EDTA 0'00001 M NacEDTA 0.00002 M Na~EDTA 0.00005 M Na2EDTA " 0-0001 M NacEDTA

Retained coSr coy 19'2 6'8 0.2 4-7 1"4 0'8 27'8 1.2 1"5 0'4 10.0 12'3 1.8 4.5 2.2

60'7 68.5 4"7 50'4 49'0 0-7 46'2 12.4 22'0 4"8 51.8 45.9 34.4 4'5 1-2

Separation factor 3"2 10'1 22.0 10-7 35-0 0.9 1'7 10'3 14"7 3'2 5.2 3.7 19'1 1.0 0.5

* After evaporation to dryness, the phosphate residue was covered with 0.5 ml HcO and dried, covered again with 0-5 ml H~O, dried and washed.

Residue adsorption--I

491

only ones comparable in yttrium-90 yield to those with NH4OH. The presence of carbonate ion did not significantly change the degree of strontium-90 retention. Because of the vigorous evolution of CO~ and the consequent danger of spattering, it was not feasible to test NH~HCO3 at concentrations above 0'1 molar. Separations factors less than unity were obtained at some HC1 concentrations, but were not reproducible. In all such cases, yields of strontium-90 and yttrium-90 were below 10 per cent, and the low ratios were probably fortuitous. Increasing concentrations of non-volatile reagents in general gave lower yields of yttrium-90. With the disodium salt of ethylene diamine tetra-acetic acid (Na2EDTA), desorption of yttrium-90 wasnearlyquantitativewhenthetotalamountofNa~EDTAwasapproximatelyl'25 × 10-'~'millimole. Except with ammonium dihydrogen phosphate, the highest yttrium-90 yield achieved with non~olatile reagents was lower than that with distilled water. ADSORPTION OF Y T T R I U M - 9 0 -

EFFECT OF pH

I00 90 Q 80 IM z I.-

70

~ 5o ONH 4 I-.I.-

30

~

2G

/

['~- 0.25M

GH3COON H 4

-~- - O, I I d

CH3 COOH

~-NH40H

1 I

1 2

I 3

[ 4

1 5

I 6

I 7 pH

I 8

I 9

I 10

I li

I 12

FiG. ]

l~ffect qf pH Standardized ammonium hydroxide and acetic acid were mixed in various proportions to provide 0-05 and 0'25 molar ammonium acetate containing excess acid or base and 0'1 molar acetic acid containing various concentrations of ammonium acetate. Procedure A was followed throughout. Fig. 3 shows the percentage of yttrium-90 adsorbed on platinum at 90°C as a function of the initial pH. Results for three concentrations of ammonium hydroxide are included for comparison. In general, increasing the initial pH resulted in higher retention of yttrium-90, but an increase in salt concentration at a given pH inhibited the adsorption. Strontium-90 retention (not shown) varied, for rcagentscontaining ammonium acetate, from 0.2 per centat pH 6.7 to 5.3 percent at pH 9'1 with separation factors of 207 and 13, respectively. In pure ammonium hydroxide, the strontium-90 retention increased from 10.5 to 14-0 per cent with increasing pH.

Effect of carrier additions Strontium-90/yttrium-90 aliquots were mounted on platinum and evaporated to dryness. Strontium and/or yttrium nitrate carrier in 1 M HNO3 was added, and the solution was again evaporated to dryness. The residue was covered with 0.5 ml of water, and Procedure A was followed, with 2 M NH~OH as the selected reagent. A logarithmic plot of the amount of yttrium adsorbed versus the amount added (Fig. 4) yielded an adsorption isotherm which was linear with up to 25/tg of added yttrium. The maximum retention of yttrium was approximately 12/~g with 25, 50 and 100/~g added. However, yttrium retention dropped sharply when larger amounts were added. Only 1"6/~g of yttrium was retained when one milligram of yttrium was added. Addition of strontium carrier alone or in combination with yttrium carrier had, in general, an

492

H . W . KIRBY

inhibiting effect on the retention of yttrium. However, the effect of strontium addition on strontium retention was ambiguous and, within experimental limits, appeared to be random (Table 5). The addition of strontium had approximately the same effect as the addition of sodium (Cf. Table 4). ADSORPTION

OF YTTRIUM

HYDROXIDE ON PLATINUM

/

I-

G

C

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W

if)

-~--

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=~ - 5

[ - : l - Y + ioo#~ ~

Sr

ADDED

_1 -6 s

/

I

I

-s

-s

I -4

I

I

I

I

I

I

I

-3

-z

- I

o

I

z

3

LOG HICROGRAHS YTTRIUH ADDED

F1o. 4

Surface rugosity and area With carrier-free strontium-90]yttrium-90, there was a significant difference between the adsorptivity of mill-finished or mechanically buffed bright platinum and platinum which had been handpolished with abrasives to a dull finish. The dull surface retained more yttrium-90 than did the bright surface, when the same reagent was tested on both by procedure A (Table 6). However, when 100/Lg of yttrium was added to the carrier-free samples, approximately the same amount of yttrium was retained by both the bright and the dull platinum surfaces. Similar tests on other surfaces indicated that the apparent surface area was more significant t h a n the rugosity in determining the amount of yttrium that could be adsorbed (Table 7).

Beaker and crucible separations A sample of 9°Sr/YS° was evaporated to dryness in a one-milliliter Pyrex beaker, and 0.25 ml of 2 M NH4OH was added. The solution was evaporated to dryness, and 0"25 milliliter of 10 -4 N HNOs was added. This solution was evaporated to dryness. The beaker was leached three times with 0.25 ml milliliter portions of previously boiled and cooled distilled water, heated for 2 min before each transfer. All three leach solutions were deposited on a single stainless steel disk and evaporated to dryness. The beaker was leached twice with 0.25 ml portions of hot 1 M HNOs, and the solutions were transferred to a second stainless steel disk. The distribution of g°Sr and 80y was as follows:

Leached with H20 Leached with 1 N HNOs Totals

00Sr

soy

(%)

(%)

48.8 53.7

4.1 96.3

102.5

100.4

Residue adsorption--I

493

TABLE 5.--ADSORPTION OF STRONTIUM AND YTTRIUM HYDROXIDES ON BRIGHT PLATINUM AT 90°C EFFECT OF CARRIER ADDITIONS Carrier added

Retained

~ug) Number 299-1 299-2 299-3 301-35 299-4 299-5 299-6 299-7 299-8 299-9 299-31 301-2 299-30 299-10 299-29 299-32 301-3 299-26 299-27 299-28 301-34 301-33 301-1 301-32 301-4 301-5 301-6 301-7 301-8

Sr

Y

Sr(To)

1000 500 250 250 100 50 25 10 5 2"5 2-5 2-5 1-5 1'0 0'5 0"25 0"25 0'1 0"01 0"001

100

100

100

0"02 0'12 0"33 0"49 1'5 2"0 5"5 3"5 5"0 9"0 11'9 15"7 7'9 10-8 8-8 16"3 5"8 4"4 5"5 5"2 0-2 1"2 3"3 10"2 16'0 1"3 3"0 2'5 4"6

1000 250 100 50 10 1 0-1 0-01

Sr~ug)

0-49

15"7

5'8

1"8 3'0 3'3 5'1 1'6 0'01 0'003 0"0003

Y(~)

Y~g)

0'16 0'13 1'7 10"5 I 1'5 23"5 49'1 41-0 45'5 35"3 45'7 32"2 47"9 43'4 47-7 46"4 50'8 75"3 93"4 94"0 32'3 68'6 65"9 83-9 8l-2 84-8 87"1 84"1 91"6

1'6 0"6 4"2 26'3 11 '5 11"8 12"3 4"1 2'3 0"9 1"1 0"8 0"7 0-4 0"3 0"1 0"1 0'08 0'009 0'0009

TABLE 6.--ADSORPTION OF STRONTIUM-90 AND YTTRIUM-90 ON PLATINUM AT 90'~C EFFECT OF SURFACE FINISH

Number

Finish

Reagent

250-A 292-21 293-27 292-2 293-26 292-1

Bright Dull Bright Dull Bright Dull

H~O H20 0.001 M NH4HCOa 0"001 M NH4HCO3 0-1 M NH~HCO3 0-1 M NH4HCO3

~o Retained goSr 9oy 1"6 7.9 1.4 12.3 9.9 9.9

57'0 84"5 63'3 85"8 89"3 93-2

Separation factor 6"9 l'l 5"0 21 '6 7'4

11"7 9"0 11-9 9"2 3"9 3'9 2'1 6'1 4'0 6"6 2"8 8'8 17-3 17'0 18"2 180 57"8 20'0 8"2 5-1 66"5 29"5 33"1 t9"8

H. W. KIRBY

494

TABLE 7.--YTTRIUM RETAINEDBY VARIOUSSURFACES (Yttrium added--100t~g; Reagent--2 M NH~OH)

Number

Surface

Apparent area (cm ~)

300-A 299-4 306-4R 300-B 300-C 300-D

Soda-lime glass Bright platinum Dull platinum Teflon film Stainless steel Stainless steel

1.8 1"8 1"8 0"4 1.8 4.9

Y Retained Q~g/cm~)

Relative retention

Relative rugosity

6.4 6"5 6"9 9"3 11-6 I 1.4

1.0 1 '0 1"1 1"5 1.8 1.8

1.0 2"9 5-0 2"9 1-2 1.2

Similar results were obtained when the separation was repeated in 5 ml Pyrex beakers, including one which was coated internally with Siliclad. Experimentation indicated that the loss of strontium-90 resulted from the thermal gradient between the top and the bottom of the beaker. To test this conclusion, separations were carried out in a soft glass vial (coated internally with Siliclad), a stainless steel crucible, and a platinum crucible. All three vessels were placed in a drilled aluminium block which was set on the hot plate. The temperature was measured with Tempilaq in a glass vial heated in the same block. Aliquots of the strontium-90/yttrium-90 solution were transferred to the three jacketed vessels and evaporated to dryness at 90°C. One-half millilitre of previously boiled and cooled distilled water and one drop of concentrated ammonium hydroxide was added to each dry vessel, and the solutions were evaporated to dryness. One-half millilitre of 10 -~ N HNO3 was added to each dry vessel, and the solutions were evaporated overnight at 90°C. Each vessel was leached with two 0.5 ml portions of previously boiled and cooled distilled water, which were heated for five minutes at 90 ° before being transferred to stainless steel disks. The vessels were then washed with two 0-5 ml portions of 0'01 M HNO3, also heated for 5 min before transfer to second stainless steel disks. The results showed separations comparable to those obtained on flat disks (Table 8). TABLE 8.--SEPARATION OF STRONTIUM-90 AND YTTRIUM-90 HYDROXIDES IN JACKETED GLASS, STAINLESS STEEL, AND PLATINUM VESSELS

Number

Vessel

272-1 272-2 272-3

Glass vial Stainless crucible Platinum crucible

Leached with H20 90Sr( ~ ) 90Y(~o) 82.5 90.8 88"5

3'2 7'7 7'9

Leached with 0.01 M HNO8 90Sr( ~ ) 90y(~) 16-0 4.1 7.1

91 '1 96.1 88"1

Cleaning and masking The cleaning and masking steps used in the experimental procedures were adopted to provide uniformity of experimental conditions. It was of interest to determine the extent to which these steps contributed to the results obtained. A stainless steel disk was cleaned with acetone and benzene until no grease or gum was visible. The customary cleaning with alcoholic K OH, the plastic retaining ring, and the preliminary treatment with hot 1 M HNO3 were omitted. A 250 pl aliquot of Srg°/Y ~° in 1 M HNOs covered an area approximately 25 millimeters in diameter instead of the usual 15 mm defined by the retaining ring. When the hydroxide treatment was completed and the residue leached with distilled water by separation procedure B, it was possible to heat for only 2 min instead of the usual five because of the relatively rapid evaporation from the larger surface area.

Residue adsorption--1

495

Nevertheless, the residue retained 96'3 per cent of the yttrium-90 and 14-3 per cent of the strontium90 indicating that neither the cleaning solutions nor the methacrylate ring were necessary for the separation except for their intended purposes.

Purification of stronium-90 and yttrium-90 Although the hydroxide residues were washed with 560 ml of water in procedure A and with 1 ml in procedure B, the results were comparable. No further improvement in purity could be expected from additional washing with water. However, both fractions could be purified by redissolution in nitric acid followed by hydroxide metathesis and water leaching. A platinum-clad stainless steel disk was cleaned with distilled water and acetone, then masked and sprayed with methacrylate as usual. The strontium-90/yttrium-90 mixture in 1 M HNO3 was transferred to the disk, evaporated to dryness, covered with 2 M NH4OH, and dried at 90°C. The residue was leached with three 0.25 ml portions of hot distilled water, the leachings being transferred to a second disk. The residue on the first disk was covered with 0.25 ml of 0.1 M HNO3, evaporated to dryness, covered with 2 M NHaOH, dried and leached as before. The leachings were transferred to a third disk. The redissolution, hydroxide treatment and leaching with water were repeated, and the leachings transferred to a fourth disk. Finally, the residue was redissolved, treated with hydroxide, dried and leached with three 0.25 ml portions of 10 4 M HNOa, the leachings being transferred to a fifth disk. The results below were computed by differential spectrometry, based on the assumption that the original residue of yttrium-90 was free of strontium-90 after four successive separations : Fraction First H 2 0 Second H20 Third H20 l0 a M HNO3 Original disk (residuaL) Totals

a°Sr(~/o)

9oy(%)

96.7 5.0 0.4 0.15 0.0"

0.9 1.3 1-6 4.6 90.6

102.2

98.9

* Assumed Purification of the strontium-90 fraction can be accomplished in a similar manner. The strontium90 is readily transferred from one disk to the next, leaving behind 90 per cent or more of any yttrium-90 previously transferred or produced by decay. However, since yttrium-90 grows initially at the rate of approximately 0.1 per cent in 5 min, decontamination of the strontium-90 much below that order of magnitude is impractical.

Metathesis of chloride residues Of the reagents listed in Table 3, the most effective in inhibiting the adsorption of yttrium-90 was hydrochloric acid at all concentrations. To test the permanence of this effect, an aliquot of the strontium-90/yttrium-90 stock solution was evaporated to dryness on a platinum disk and the residue covered with 6 M HCI. The solution was evaporated to dryness and the residue covered with 2 M NH4OH. The solution was evaporated to dryness, and the residue was leached twice with 0-5 ml of previously boiled and cooled distilled water as in procedure B. The strontium-90 fraction contained 2.1 per cent of the yttrium-90, and the yttrium-90 fraction contained 3'9 per cent of the strontium-90.

Autoradiography An aliquot of the strontium-90/yttrium-90 stock solution was mounted on each of two goldplated stainless steel disks. 3

496

H.W. KIRBY

Both samples were carried through the standard procedure with 2 M NH4OH, except that only one of the samples was leached with water by procedure B. The other sample was retained as a standard. The three disks were autoradiographed on Ilford X-ray film (Fig. 5), and their beta-bremsstrahlung spectra were recorded (Fig. 2). There was a distinct differencein the distribution of the yttrium-90 and strontium-90 hydroxides. Whereas the unseparated mixture covered the entire open area (15 mm in diameter) of the gcl J surface, the strontium-90 concentrated at the edge of the plastic in a ring 3 mm thick, while the yttrium-90 covered an area approximately 10 mm in diameter, in the center of the gold surface. DISCUSSION AND CONCLUSIONS It seems evident from the foregoing experimental results that the separation of yttrium-90 and strontium-90 hydroxides is based on qualitative differences in chemical behaviour rather than on quantitative or kinetic differences. The nearly identical adsorption of yttrium-90 hydroxide by substrates of widely differing chemical compositions (Table 1) is characteristic of physisorption, which is always non-specific {17~. Strontium-90 adsorption, on the other hand, ranged from an average of 2.9 per cent on gold to 18.8 per cent on Pyrex. Although the strontium-90 results were erratic (extremes of 1.7 and 23-8 per cent on Teflon), they indicate that the nature of the substrate is significant and, hence, that the mechanism of strontium-90 adsorption probably differs from that of yttrium-90 in kind as well as magnitude. The same conclusion can be derived from the results obtained at different temperatures (Table 2). Yttrium-90 adsorption was nearly independent of temperature over the range 28°C-1000°C. The low value (69.0 per cent) found at 28°C can be attributed to the presence of a film of adsorbed moisture, as shown by the substantial increase in adsorption under the influence of prolonged vacuum desiccation. Strontium-90 adsorption was a function of both substrate and temperature, again indicating chemisorption. Nearly all precious metal surfaces were readily decontaminated with hot 6 N HCI after the methacrylate ring had been removed with acetone. However, for complete removal of beta activity from surfaces which had been heated above 90°C, brief exposure to aqua regia was necessary. One platinum disk which had been heated to 1000°C retained 0.6 per cent of the original activity (subsequently identified as strontium-90) even after the surface was deeply etched with aqua regia. This observation supports the conclusion that strontium-90 retention is due to the formation of a surface chemical compound or solid solution. Finally, autoradiography showed striking differences in the drying patterns of the two radioelements (Fig. 5). The estimated area covered by the yttrium-90 was only about twice as great as that covered by the strontium-90; yet the per cent retention by the gold surface was twenty-five times as high for yttrium-90 as for strontium-90. There is, apparently, no synergic effect and, from the standpoint of yttrium-90 yield, the presence of strontium-90 is almost irrelevant. The addition of relatively massive amounts of strontium carrier had some effect on the degree of adsorption of yttrium-90 (Table 5), but this can be ascribed to the fact that the dried strontium hydroxide acted as a water-soluble substrate for deposition and subsequent desorption of yttrium-90. ~17~s. J. Grtsco, The Surface Chemistry of Solids. Reinhold, New York (1961).

Residue adsorption--I

497

The data on the effect of residual anions (Tables 3 and 4) are somewhat more equivocal, particularly with regard to the non-volatile reagents, where the presence of a water-soluble residue with a relatively high specific surface would inhibit retention by the permanent substrate. In general, however, the data suggest that yttrium-90 is not adsorbed as a simple cation but as a molecule or coordination compound, and both the nature and concentration of the residual anion exert a significant influence. BELLONI et al., (is) working with lanthanides under equilibrium conditions, have assumed that adsorption of cations is due to the electrostatic attraction of a primary layer of adsorbed anions, and that the effect of pH is explained by competitive adsorption of H ÷ ions. While the present work is not strictly comparable because of the nonequilibrium conditions, there seems no reason to prefer a second-order ionic mechanism to a first order adsorption of undissociated molecules. The degree of adsorption of yttrium-90 is at least a qualitative function of the solubility of the yttrium salt. Those reagents which are commonly used in the precipitation and gravimetric determination of yttrium also significantly enhance the retention of yttrium-90 platinum. Quantitative data relating to the solubilities of yttrium fluoride, carbonate, and phosphate are unavailable, but the solubility product of yttrium oxalate is reported as 5-34 × 10-29 (19)and that of yttrium hydroxide as 5.2 × 10-22. (20) Since the adsorption of these compounds is in reverse order, the hydroxide retention being approximately twice that of the oxalate, there is evidently no simple relationship between the solubility of macroscopic amounts of yttrium salts and their adsorption. More probably, the role of the precipitant lies in producing a molecular aggregate of suitable crystal structure, dimensions and charge. The difference between the effects of nitric and hydrochloric acids is striking (Table 3). Both yttrium nitrate and yttrium chloride are highly soluble compounds, yet the adsorption of yttrium-90 from a nitrate residue was significantly higher than that from a chloride residue. This difference was magnified by repeated redissolution and evaporation with water. Adsorption is an exothermic process, (17~and it is possible that partial pyrolysis of the Y(NOa)3.6H20, even at the moderate temperature of 90°C, results in the formation of basic yttrium nitrate, Y(NO3)3.Y(OH)3.3H20. t~ll Under equilibrium conditions, adsorption increases with pH, reaching a maximum at a pH value, usually between three and six, which is characteristic of the radio element. (22.23~ No such maximum was found in the present work; instead, as in ultrafiltration, the adsorption of yttrium-90 continued to increase with pH until nearly quantitative adsorption was attained (Fig. 3). The pH results are particularly interesting with regard to the difference in the adsorption from 0.05 and 0.25 molar ammonium acetate. The rapid convergence of the two pH curves suggests that yttrium hydroxide forms instantaneously to the ~ls~j. BELLONI,M. HAISSINSKYand H. N. SALAMA,J. Phys. Chem. 63, 881 (1959). ~19~A. M. FEIBUSH,K. ROWLEYand L. GORDON,Analyt. Chem. 30, 1610 (1958). t20~T. MOELLERand H. E. KREMERS,Chem. Rev. 37, 97 (1945). 121)R. C. VICKERY,Chemistry of the Lanthanons, p. 279. Academic Press, New York (1953). ~z~ I. E. STARn%Khim. Nauk. Prom. S.S.S.R. 4, 448 (1959). lea) j. BELLONI-COFLER,Doctoral Thesis, Facult6 des Sciences de l'Universit6 de Paris (1961).

498

H.W.

KIRBY

extent of the free hydroxyl ion available in the original solution, and is redissolved only slowly by the free acetic acid resulting from the thermal decomposition of ammonium acetate. This behaviour tends to confirm the assumption that yttrium-90 is adsorbed as a molecule rather than as a cation attracted electrostatically to a primary layer of adsorbed anions. The adsorption of yttrium hydroxide as a function of the yttrium concentration obeys the Freundlich isotherm over a wide range; from 8 × 10-7/~g(carrier-free my) to 25/~g of total yttrium (Fig. 4). The slight deviation from linearity at the carrierfree end of the isotherm is probably attributable to the presence of an indeterminate amount of zirconium-90, the stable end product of strontium-90/yttrium-90 decay. The crystalline arrangement of yttrium hydroxide is such that each yttrium atom is surrounded by nine hydroxyl groups, whose nearest approach to each other is 2.78 A. ~4) If it is assumed that each yttrium atom occupies the peak of a pyramid whose base consists of three hydroxyl groups, the maximum amount of yttrium which can be held in a monolayer on a surface area of 1.75 cm 2 is 0"8/~g. This is the same order of magnitude as the yttrium retained with 0.5 and 1.0 mg of total yttrium (Table 5). The maximum amount of adsorbed yttrium found experimentally was 12.3/~g, or approximately fifteen times as great as that of the hypothetical monolayer. The adsorptive process cannot, however, be considered merely the stacking of fifteen monolayers held by progressively weaker van der Waals forces. This would require that the amount of adsorbed yttrium reach a maximum value and remain at that level despite any further increase in yttrium concentration. As seen in Fig. 4, yttrium adsorption, while tending to reach a constant saturation level, dropped sharply when the total yttrium exceeded 100/~g. Decontamination of yttrium-91 deposited on painted surfaces was found by CHANDLERand SHELBERG (25) to be a function of micelle concentration. For each of two soap solutions, decontamination began at or near the critical micelle concentration t26~ and increased rapidly with micellar concentration up to two or three times the critical value. Decontamination was independent of surface tension. The authors suggested that decontamination resulted from competition between the micellar interface and the original surface. By analogy with the concept of the critical micelle concentration, the sharp decrease in the amount of yttrium adsorption may reflect the existence of a critical precipitate concentration, in this case 0.004 M yttrium (100/zg in 0.25 ml of 2 M NH4OH ), at which point the intermolecular forces of the yttrium hydroxide aggregate would counterbalance the weakly polar forces tending to bond all but the first monolayer to the substrate. This interpretation is supported by the results in Tables 6 and 7. Surface rugosity was a significant factor in the adsorption of carrier-free yttrium-90, but at the 100/,g level there was no correlation between rugosity and adsorption, suggesting that the micelles were too large to be affected by the increase in real surface due to microscopic imperfections in the substrate. ~24~R. W. G. WYCHOFF, Crystal Structures, Vol. 2, Chap. 5, p. 25a (supplement). Interscience, New York (1957). t2~J R. C. CHANDLER and W. E. SHELBERG, J. Colloid Science 10, 393 (1955). t2~J j. W. McBAtN, Colloid Science, p. 251. Heath (1950).

Residue adsorption--1

499

Thus, desorption of yttrium hydroxide, or its failure to be adsorbed at levels greater than the critical concentration, does not imply solubilization but the reverse; namely, incipient precipitation. The linear portion of the adsorption isotherm has an interesting analytical application. KURBATOV and KURBATOVt27), in a study of the adsorption of barium by ferric hydroxide, proposed the use of adsorption isotherms for the determination of stable elements at very low concentrations. A similar technique, using residue adsorption instead of filtration, can evidently be applied to concentrations of yttrium in the picogram and nanogram range. A considerable mystique has grown up around "radiocolloids" since their discovery by PANETH in 1912, and it has become routine to take precautions against the loss of radioactivity by adsorption on the walls of containers. For example, standard solutions of strontium-90/yttrium-90 are prepared by the U.S. National Bureau of Standards with strontium and yttrium added (9/zg/ml of each) "to insure that the 9oy activity remains in equilibrium and is not lost to the glassware by adsorption". (2s) Since the solutions are one normal in hydrochloric acid, it seems unlikely from the present work that such adsorption would occur, or that the small amount of added carrier would serve a useful purpose. The term, "radiocolloid", is itself a misnomer, implying that colloid formation at very low concentrations is a unique property of radioelements which is not shared by their stable isotopes. This misconception arises from two principal sources; differences of many orders of magnitude in the sensitivity of detection of stable and radioactive isotopes and excessive faith in the reported solubility products of slightly soluble compounds. (29) The controversy (8°) as to whether radiocolioids are true colloids or merely radioactive ions adsorbed by impurities appears to STARIK(22) to be of purely historical interest. To this writer it appears to be irrelevant. Nothing in the published literature refutes the concept of colloid formation as a precursor of precipitation or of surface adsorption as a substitute for and competitor with crystal growth. Except at levels where the radiation significantly changes the composition of the solvent, the chemistry of stable and radioactive dements is the same. Residue adsorption provides another technique for studying the mechanism of precipitation at levels far below those at which stable isotopes are detectable. The assistance of Mrs. V. A. WURSTNERin carrying out many of the differential decay computations is gratefully acknowledged.

Acknowledgement

12r)M. H. KURBA'I-OVand J. D. KURBATOV,J. Amer. Chem. Soc. 69, 438 (1947). (28)H. H. SELIGERand A. SCHWEBEL,Nucleonics 12, No. 7, 54 (1954). (zg~M. HAlSSINSKY,Acta Physiochem. (URSS), 3, 517 (1935). (3o)G. K. SCHWEITZERand W. M. JACKSON,J. Chem. Educ. 29, 513 (1953).