Determination of Radioactive Cesium THOMAS H. HANDLEY and CECIL L. BURROS Anulytical Chemistry Division, Ouk Ridge Nutianal laboratory, Oak Ridge, Tenn.
b Cesium is separated from other fission products by precipitation with the organic reagent sodium tetraphenylborate, following scavengings with hydroxide and carbonate. Good decontamination factors and chemical yields are obtained. After separation y-ray spectrometry is used to distinguish between cesium-1 36 and cesium- 1 37.
A
method for the separation and determination of cesium136 and cesium-137 from mixed fission products was needed. The classical method of separation as cesium perchlorate (8) was time-consuming and required reaction with perchloric acid, which is hazardous and necessitates a special fume hood. Numerous applications of sodium tetraphenylborate as an analytical reagent have been published (3, 4, 6). In the determination of radioactive cesium, chemical interference of potassium, ammonium, and rubidium may be eliminated by a preliminary precipitation of the cesium as the silicotungstate. Usually, these are not present in fission product solutions, or the dilution factor is so great (often 103 or greater for safe handling in the laboratory) that interference on the chemical yield is negligible. This step also gives a good decontamination from other activities and is a specific separation of cesium from rubidium, ammonium, and the other alkalies. I n the analysis of fission product solutions more than several hours old, contamination from radionuclides of rubidium is unlikely, because the significant rubidium activities formed by fission are short-lived. The initial Separation, if necessary, is followed by “scavenging” with ferric hydroxide and barium carbonate precipitations. Cesium is finally precipitated as cesium tetraphenylborate, which is dried, weighed, and mounted for counting. Considerable difficulty was experienced in filtering the fine precipitate of cesium tetraphenylborate as precipitated from an acid solution. Filtration or centrifugation from a basic solution also proved to be impractical. Therefore, following precipitation from an acid solution and separation of the precipitate by centrifugation, the precipitate was dissolved in a minimum of N IMPROVED
332
ANALYTICAL CHEMISTRY
acetone and reprecipitated from an alcoholic solution. This formed a precipitate easily filtered and in appearance like a cotton batting on the filter paper. Following determination of chemical yield, cesium-136 and cesium137 content are determined by gamma scintillation spectrometry (1). The 0.661-m.e.v. photopeak is measured for determination of cesium-137 and the l.04m.e.v. peak for determination of cesium-136 (19). It is necessary to subtract the Compton background produced by the 0.82, 1.04, and 1.25 m.e.v. peaks of cesium-136, from the 0.661-m.e.v. peak of cesium-137 (see Figure 1) (9). INSTRUMENTS A N D REAGENTS
Sodium Tetraphenylborate Solution. Dissolve 4 grams of sodium tetraphenylborate and 1 gram of aluminum chloride hexahydrate in 100 ml. of water. Add a few drops of phenolphthalein indicator solution and then add 6 M sodium hydroxide dropwise, until the solution is just alkaline. Allow to stand several hours. Filter, dilute the filtrate t o 200 ml., and store in a cool location. The solution will gradually decompose a t room temperature ( 7 ) , as noted by the appearance of a precipitate or turbidity, but will keep for several months in a refrigerator. The following solutions were obtained for testing the procedure: strontium-90, ruthenium-103, zirconium-95 (in equilibrium with niobium-95) , and cerium144. A mixed fission product solution more than 1 year old and a mixed fission product solution less than 1 month old mere also used to check the procedure. Standard Cesium Carrier Solution. Dissolve 6.4 grams of cesium chloride in water and dilute t o 1 liter with water. Pipet 5-ml. portions of the carrier solution into a beaker and dilute t o 50 ml. Cool in a n ice bath. Add 10 ml. of sodium tetraphenylborate dropwise while stirring and allow it to stand for about 10 minutes. Filter onto a sintered-glass filter of fine porosity and wash with several small portions of n-ater. Dry a t 110’C. for 15 minutes, cool, and weigh. Four standardizations should agree within 0.5Yob. For detection of electromagnetic radiation, a y-ray spectrometer ;,as used. The detector consisted of a 3 X 3 inch cylindrical, “thallium-activated” sodium iodide crystal. This crystal was mounted on a DuMont 3-inch No. 6263 multiplier phototube. Pulses from the
phototube were fed into a linear amplifier and a multichannel differential pulse analyzer. EXPERIMENTAL STUDIES
During initial study of the procedure an attempt was made to precipitate cesium tetraphenylborate from an alkaline solution in the presence of complexing agents. However, filtration or centrifugation of the precipitate proved impossible. Also an attempt was made to extract the salt into an organic phase. Extraction of macro amounts of cesium was not feasible. Tracer amounts of cesium could be readily extracted by several different organic solvents, among them amyl acetate. Finston (6) has used this method for the determination of cesium and assumes complete recovery. The precipitate from a mineral acid solution is easily centrifuged. It is difficult to filter by the usual radiochemical technique using a Hirsch funnel, a small filter disk, and suction. However, if the precipitate from a mineral acid solution is dissolved in a minimum of acetone and recrystallized by addition of absolute alcohol, a filterable precipitate is obtained, of the same composition as that made from a mineral acid solution. This step also eliminates possible decomposition products of the reagent from the precipitate. The chemical yield is usually 7501, or better. A decontamination factor >lo6 was obtained for each radionuclide tested. Decontamination may be improved by repetition of each step. I n testing for decontamination from specific nuclides a known amount of the nuclide was added to the solution in step 1 and the separated cesium in step 5 was checked by counting in a well type scintillation counter or with an end-window GM tube in the case of strontium-90. During testing for decontamination from ruthenium103, the purified cesium showed a consistent gamma count, indicating there might be some carry-over of ruthenium with the cesium, an average of 80 counts out of a possible lo8 counts added. However, a careful y-ray spectrum showed this carry-over to be cesium-137, indicating the ruthenium tracer was contaminated with cesium-137. This is mentioned to give some idea as to the extent of decontamination, when 80
Whatman or a N u n k t e l l KO.00 filter disk and wash with several small portions of alcohol. Dry a t 110” C. ( I S ) for 15 minutes and weigh as cesium tetraphenylborate. Mount the precipitate for beta counting, or place in a suitable tube for gamma counting.
Figure 1. Photopeaks of cesium isotopes
PULSE H E l G H l
counts per minute can be separated from IO6 counts per minute. Following purification of the ruthenium tracer a decontamination factor of 2 lo6 was obtained. T o check the procedure further, cesium was separated from old mixed fission products. A y-ray spectrum of the separated cesium showed the presence of cesium-137 only. The y-ray spectrum of a typical separation from a young fission product mixture showed the presence of only cesium-136 and cesium-137. A decay curve showed only these two radionuclides present. PROCEDURE
Step la. Steps l a a n d 2a should be used if macro amounts of rubidium a n d potassium are present. To a n aliquot of t h e sample, in a 50-ml. centrifuge tube, add 1 ml. of a standardized cesium carrier. Add 15 ml. of 6iM hydrochloric acid and 2 ml. of silicotungstic acid (1 gram per ml.). Digest for 10 minutes. Centrifuge and discard t h e supernatant liquid. K a s h the precipitate with 5 ml. of GJi hydrochloric acid. Step 2a. Dissolve t h e precipitate of cesium silicotungstate in 0.5 ml. of 6 M sodium hydroxide, add 20 ml. of 6 M hydrochloric acid, and discard t h e yellow precipitate. Add 2 ml. of silicotungstic acid and digest for 10 minutes; centrifuge and discard t h e supernatant liquid. Wash t h e precipitate with 5 ml. of 6 M hydrochloric acid. Dissolve t h e precipitate of cesium silicotungstate in 0.5 ml. of 6 M sodium hydroxide, warm, if necessary, to dissolve it. Begin with Step 1, omitting the addition of a standard cesium carrier.
Step 1. To a n aliquot of t h e sample add 1 ml. of t h e standardized cesium carrier and about 5 mg. of iron, barium, lanthanum, and zirconium carriers a n d dilute t o 15 ml. Add 1IM sodium hydroxide until t h e solution is just basic t o phenolphthalein and then 1 ml. of 3 M sodium carbonate. Warm t o coagulate the precipitate. Centrifuge and discard t h e precipitate. Step 2. Make t h e supernatant solution just acid with 6 M hydrochloric acid, and add 5 mg. each of iron, barium, lanthanum, and zirconium carriers. Repeat t h e precipitation with sodium hydroxide and sodium carbonate. Step 3. Make t h e supernatant solution just acid with l M hydrochloric acid. Cool in a n ice b a t h , add 4 ml. of sodium tetraphenylboron dropmise with stirring, let stand for 10 minutes. Centrifuge and discard t h e supernatant liquid. Wash t h e precipitate with approximately 5 ml. of water, centrifuge, and discard the wash solution by decanting it. Step 4. Dissolve t h e precipitate in a minimum of acetone, add 1 ml. of 1N hydrochloric acid, and dilute t o 10 ml. with m-ater. Cool, a d d 4 ml. of sodium tetraphenglborate dropwise and let i t stand for 10 minutes. Centrifuge and discard t h e supern a t a n t liquid. Wash t h e precipitate n ith water, centrifuge, a n d discard t h e supernatant liquid. Step 5. Dissolve t h e precipitate of cesium tetraphenylborate in a minimum of acetone, usually 1 ml. Add 10 ml. of absolute ethyl alcohol containing 0.501, by weight of sodium tetraphenylborate and cool in a n ice b a t h for 10 minutes n-ith occasional stirring. Filter onto a tared KO.42
Counting Procedure. K h e n old mixed fission products are being analyzed and cesium-137 is the only cesium radionuclide present, the separated cesium may be counted on a well-type scintillation counter previously calibrated n i t h a cesium-137 standaid of a known disintegration rate. Young mixed fission products t h a t contain cesium-136 and cesium-137 are counted on a multichannel y-ray spectrometer. The technique used has been described by Lyon and Kahn ( I O ) and Heath (9). A typical y-ray spectrum is shown in Figure 1. Cesium136 is determined by integration of the 1.04-n1.e.v. photopeak following subtraction of the Compton distribution underlying it from the 1.25-n1.e.v. photopeak. For calculation it is assumed that cesium-136 dccays 100% through the 1.04 ganinia (11, 12). Cesium-137 is determined by integration of the 0.661-m.e.v. photopeak following subtraction of the Conipton distribution from the 1 5 5 , 1.04-, and 0.82-111.e.\-. gammas. Radionuclides of near13 the same energy are counted under identical conditions, to establish the correct Compton distribution for subtraction from the si ectruni of the unknown saml,le. For the 0.82-m.e.v. peak a standard of manganese-54 was used. For the 1.04-11i.c.v.peak zinc-65 and for the 1.25-n1.e.v. peak sodium-22 n a s used. Of course, a pure source of cesium-136 would be ideal for this subtraction, but no such source was available. Also the y-ray spectrum as published by Heath (9) may be used if identical geometry and mounting techniques are used. The annihilation peak found in the spectrum of zinc-65 and sodium-22 does not interfere in this case, because it is too low in energy. The more energetic gamma rays associated with cesium-136 are so low in intensity that they do not contribute significantly to the Conipton distribution and may be ignored. A factor of 0.82 was used for calculating the disintegration rate of cesium-137. This factor includes correction for branching and internal conversion of the 0.861m.e.1’. gamma of cesium-137. To obtain the disintegration rate, use is made of photopeak-to-total ratio as a function of y-rav energy and source distance as determined by Heath (9) and Bell, Davis, and Lazar ( 2 ) . Using their convention the disintegration rate of a specific radionuclide will be given by the following relationship : VOL. 31, NO. 3, MARCH 1 9 5 9
333
Dis. rate where Arp
=
NPD Et PABY
B
=
Y
=
~
integrated area under the photopeak, counts per second D = dilution factor Et = total absolute detection efficiency for source detector geometry used =
(4)
P = appropriate value for the peak-to-total ratio ( 4 , 9 )
A = correction factor for absorption in source and any beta absorber used in the measurement. Tables for absorption coefficients are in (4)
correction for branching ratio and internal conversion of gamma measured chemical yield
LITERATURE CITED
(1) Bell, P. R., “Scintillation Method,” in “Beta- and Gamma-Ray Spectroscopy,” ed. by K. Seigbahm, Interscience, New York, 1955. (2) Bell, P. R., Davis, R. C., Lazar, N. H., Oak Ridge Natl. Lab., Rept. 72 (1957’1. (3) B&&d, A. J., Jr., Chemist Analyst 44, 104 (1955). (4)De La Rubin, P. J., Blasco, L. R. F., Ibid., 44, 58 (1955). (.5 ,) Finston. H. L.. Brookhaven National Laboratofy, private communication.
(6) Glass, G. H., Chemist Analyst 42, 50 (1954). (7) Glass, G. H., Olson, B., Ibid., 43, 70 (1954). \ - - - - I
(8) Glendenin, L. E., Nelson, C. M., National Energy Series Div. IV, 9, Paper 283, McGraw-Hih, Tern York, 1951. (9) L.. Atomic E n e r u Comm. . Research , Heath. R. and Development Kept., I D 0 16408 (July 1, 1957). (10) Kahn, B., Lyon, W. S., Szicleonics 11, S o . 11, 61 (1953). (11) O’Kelley, G. D., Oak Ridge National Laboratory, private communication. (12) Olsen, J. D., O’Kelley, G. D., Phys. Rev. 9 5 , 1539 (1954). (13) Wendlandt, W. LT‘., .\SAL. CHEM. 28, 1001 (1956). RECEIVEDfor review June 11, 1958. Accepted October 13, 1958.
Strontium-90 by an Ion Exchange Method E. A. BRYANT, J. E. SATTIZAHN, and BUDDY WARREN University of California, Los Alamos Scientific Laboratory, Los Alamos, N. M.
b A procedure is described for the radiochemical determination of strontium-90 in fission-product samples which have decayed a t least 10 days. Strontium and barium are adsorbed on a cation-resin column, and after a suitable growth period, yttrium-90, the daughter of strontium-90, is selectively eluted. The yttrium-90 is then adsorbed on and eluted from a second cation-resin column and counted. The radiochemical yield is greater than 97%; gravimetric measurement of the recovery of strontium and yttrium carriers is not required. The method, with modifications, is applicable to samples that contain 100 mg. of iron or uranium.
A
PROCEDURE has been developed for the radiochemical determination of strontium-90 in fission-product samples which have decayed at least 10 days. Such samples contain two radioactive isotopes of strontium, strontium-89, with a half life of 51 days, and strontium-90, with a half life of 27.7 years. Strontium-91, with a half life of 9.7 hours, has decayed to an insignificant level in 10 days. Determination of strontium-90 in the presence of strontium-89 is based on separation and measurement of the radioactive yttrium-90 which is produced by decay of the strontium-90. A problem in the analysis of fission products for strontium-90 (1-3, 7 ) is interference from the high-yield fissionproduct pair barium-140 and its daughter lanthanum-140, which have
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ANALYTICAL CHEMISTRY
chemical properties similar to those of strontium-90 and its daughter yttrium90. A typical procedure ( 2 ) for the determination of strontium-90 in fission products involves precipitation of strontium and barium as the mixed nitrates from a fuming nitric acid solution, followed by removal of barium by precipitation as barium chromate and separation of yttrium from strontium by precipitation of yttrium hydroxide. Each step must be repeated several times to attain sufficient decontamination for the analysis of fresh fission-product samples, and radiochemical yield must be measured for both the strontium intermediate product and the yttrium final product. The procedure described employs a single separation of yttrium from strontium, barium, and lanthanum instead of the two types of separations required in the conventional procedure outlined above. This single separation is accomplished by elution of yttrium from a cation-resin column with a highly selective eluting reagent (6). The use of this reagent and of an evaporative mounting technique makes possible a relatively simple procedure with a radiochemical yield greater than 9770. The high yield kliminates the need for measurement of recovery of strontium and yttrium. There are four stages in the determination of strontium-90 by this method. The most persistent contaminants-zirconium-95, ruthenium-103, and iodine131-are eliminated by preliminary treatment of the sample. The strontium, barium, and other cations are adsorbed on a cation-resin column (6) and
yttrium, lanthanum, and other rare earths are stripped from the resin. Time is allowed for the growth of yttrium-SO, which is then selectively eluted. The yttrium-90 is further purified by adsorption on and elution from a second cation-resin column and is collected for counting by evaporation of the effluent on a counting plate. APPARATUS AND REAGENTS
Apparatus. Aluminum counting inches. plates, 31/4.x 21/2 X Double-sided Scotch tape. Mylar film, 1.7 mg. per sq. cm. Fritted-glass pressure filter, with a 25- t o 30-ml. reserroir and a drip tip, fine fritted disk. Beta proportional counter, methaneflow, 2 inches in diameter, 4.8 mg. per sq. em. aluminum windor. ION EXCHANGE COLVMSS. Three types of columns are used. The first cation column is prepared by sealing a 12-cm. length of glass tubing 18 mm. in inside diameter to a 9-em. length of tubing 6 mm. in inside diameter, fitted with a drip tip. The second cation column is prepared in a similar manner from a 9-em. length of tubing 5 mm. in inside diameter and a 15-nil. centrifuge cone. The anion column is prepared by sealing a 40-ml. centrifuge cone to a 10-em. length of tubing 10 mm. in inside diameter fitted with a drip tip. Reagents. Sea sand, n-ashed with concentrated hydrochloric acid and rinsed with water. Ammonium hydroxide solution, approximately 0.6F “,OH, prepared by saturating water with ammonia gas and diluting the saturated solution 1 to 25 with water. ELUTING REAGENTS.-4 solution of 0.5F a-hydroxyisobutyric acid (Fair-
