Branching ratio of 133Sb decay to 133Te isomers

J. inorg,nucl.Chem.,1969,Vol.31,pp. 585to590. PergamonPress. PrintedinGreat Britain

BRANCHING

RATIO OF 133Sb D E C A Y TO 133Te ISOMERS*

B A H M A N PARSAt, G. E. G O R D O N and A L L E N WENZEL:~ Department of Chemistry and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge. Mass. 02139

(First received 31 May 1968; in revised form 5 August 1968) A l ~ t r a e t - A n t i m o n y samples were quickly separated from fission products and, after a short decay period, Te daughters were isolated and their y rays observed with a Ge(Li) detector. The fractions of decay of 2-7 min lz3Sb to 12.5 min 133UTeand 55 rain laamTe were found to be 0.58___0.06 and 0.42___0.06, respectively. The cumulative yield of 133Sb in thermal neutron fission of 235U was measured as 1.7 0.4 per cent. An upper limit of 0.29 per cent was determined for the yield of an 11 sec ls4Sb in thermalneutron fission of 2a~U. INTRODUCTION

THE GENETIC relationships for the fission-product mass chains 131,133 and 134 are given in Table 1. From estimates of absolute fission yields, Pappas [3] gave Table 1. Genetic relationships for fission product chains A = 131, 133, and 134 Chain yield

235U(nth,f) (%)

Genetic relationships a 131mWe

30*r < 2-min 131Sn -.o 23-rain 1315b93"2%N'~"

4,[ 18% / / , 8"05-day 1311~ 131X....~e

2"93

25-min lal~Te ~55-min 55-sec la3Sn ~ 2.7-rain la3Sb

laCtaTe x , ~ %

/

| 13%

20,8-hr 1331~ la3Xe

6"62

1 -- F~'~'N 12.5.min 13~T l e / < l-5-sec 134Sb or

42-rain 134Te --+ 52-rain la4I --+ 134Xe

Z 1 l-sec la4Sb

8.06

References to original work are given by Lederer et al.[1] or are given in Table 2. ~[2]. *This work was supported in part by the U.S. Atomic Energy Commission under Contract No. AT(30-1)-905. ¢Present address: Tehran University Nuclear Centre, Tehran, Iran. ~:Present address: Merck Corp., Chemical Division, Rahway, New Jersey. 1. C. M. Lederer, J. M. Hollander and I. Perlman, Table of Isotopes, 6th Edn. Wiley, New York (1967). 2. H. Farrar and R. H. Tomlinson, Nucl. Phys. 34, 367 (1962). 3. A. C. Pappas, Massachusetts Institute of Technology Laboratory for Nuclear Science Technical Rep. No. 63, AECU-2806 (1953). Unpublished. 585

586

B. PARSA, G. E. G O R D O N and A. W E N Z E L

the value of F1 = 0.92 for the fraction of decay of lazSb to laZmTe. He made the calculations using a value of 2 min for the half life of 'zagTe. Later Wahl measured the fraction of laZl formed via the 133roTe path to be 0-72 _ 0.02 in thermal-neutron fission [4]. He used samples separated 8 to 16 min after the end of irradiation from which he reasoned that most of the "2 min" 13agTe, but little of the 133mTe,had decayed. More recently, Prussin and Meinke [5] found the half life of 'zagTe to be 12.45-+-0.28 min. This result has been confirmed by Bemis [6] and Parsa e t al.[7] Consequently, previous determinations of the branching of laaSb decay to the la3Te isomers, based on the 2 min half life of laa°Te, are most likely in error. We have measured the laaSb branching fractions by separating Sb from fission products shortly after irradiation of 235U with thermal neutrons. The Sb sample was allowed to decay for a short time and then the Te daughters were separated and the 3' rays of laXCFe, '33¢Fe and laamTe observed with a Ge(Li) detector. From the intensities of the observed lines it was possible, with the use of standard growth and decay equations, to calculate the branching fractions of ~azsb. In addition, it was possible to extract from the data an upper limit for the fission yield of the reported [8] 1 1 sec la4Sb and a value for the cumulative yield of ~azsb.

EXPERIMENTS Irradiations and chemical separations Samples containing 560 mg UO2(NO3)2 • 6H~O of natural isotopic composition were dissolved in one ml of water and irradiated in the M.I.T. reactor for 25 sec with a flux of 2.3 x 10'3 neutrons/cm2 sec. Following irradiations, the target was transferred to the laboratory within 6 sec via the pneumatic sample-transfer system described by Bemis and Irvine [9]. Antimony was rapidly separated via a stibine-generation method [ 10]. The sample carrier and the vial containing the sample were pierced with hypodermic needles and the fission-product solution was drawn by vacuum into a dropping funnel containing 20 nag Sb(III) and Sb(V) carders. This solution was added, over a 10-12 see period, to 20 ml o f 5 N NaBH4 solution in a distillation flask. The flask was cooled with ice to make the reaction less vigorous than at room temperature and the solution was stirred with a magnetic-stirring apparatus. The H~ and SbHa generated were swept out of solution by continuous suction and passed through an empty bottle (safety trap), 2 traps containing 0.5 N NaOH, another safety trap, and a drying tube containing Drierite and Ascarite. The latter was used in the drying tube to remove any iodine contamination remaining after passage through the N a O H traps. The SbHz was collected by passing the gas through a flask containing 40 ml of 6 N HCI saturated with liquid Brs. The Sb samples were allowed to stand for several minutes and then Te daughters were separated. Forty mg of Te(IV) and Te(VI) carriers were added and the solution transferred to a flask containing 10 ml of 15% N2H4" 2HC1 solution. The solution was heated by a heat lamp, while the addition of SOs gas precipitated Te metal. The metal was filtered with a micro-filter apparatus, washed with 3 N HC1 and HsO, and mounted on a sample card for counting. Gamma-ray spectrometry The "y-ray spectra of the samples were taken with the use of a 1.2-cm a Ge(Li) detector in the con-

4. A. C. Wahl, Phys. Rev. 99, 730 (1955). 5. S. G. Prussin and W. W. Meinke, Radiochim. Acta 4, 79 (1965). 6. C. E. Bemis, Jr., Ph.D. Thesis, Massachusetts Institute of Technology, Department of Chemistry, Sept. 1964. Unpublished. 7. B. Parsa, G. E. Gordon andW. B. Waiters, Nucl. Phys. All0, 674 (1968). 8. A. A. Delucchi, A. E. Greendale and P. O. Strom, Phys. Rev. 173, 1159 (1968). 9. C.E. Bemis, Jr. andJ. W. Irvine, Jr., Nucl. lnstrum. Meth. 34, 57 (1965). 10. M. P. Menon, N. K. Aras and J. W. Irvine, Jr., J. inorg, nucl. Chem. 27, 767 (1965).

Branching ratio of laaSb decay to lS3Te isomers

587

figuration previously described in [11] and [12]. Pulses were sorted a n d stored in a 4096-channel pulse-height analyser. This system gave a full-width at half-maximum of 2.3 keV for the 662 keVT ray from ~aTCs. The energy calibration and the relative photopeak efficiency curve were determined by the methods of [12]. Following separation of the Te, the samples were counted for two consecutive 30 min periods. A typical spectrum obtained is shown in Fig. 1. In order to avoid confusion, only very prominent lines of the species present in the samples are identified in Fig. 1. More detailed analysis and identification of the lines is given in [7]. Relative amounts of the various species present in the samples were determined by computing the areas under the photopeaks of the strong ),-ray lines listed below in Table 2. Table 2. Fission yields and decay properties of species used in analysis of results

Nuclide

H~flife

Re~

lndep, or cum.(#)yield a 235U(nrh,f)(%)

13~Sn 133Sn ~z4Sn

1-32 ±0.23 min 55 sec Assumed v. short 23 min 2.67±0.33 min 11.1±0.8 < 1.5 sec 25.0±0-1 min

[13] [13]

1.28___0-21# < 0.015#

131roTe 3 0 h r lSSUTe 12.45±0.28 min

[1] [5]

~z~Sb ~s3Sb ~s4Sb ls~¢I'e

[6] [13] [8] [ 15] [16]

~33"Te 5 5 . 4 ± 0 . 4 m i n

[17]

'34Te

[19]

41.5±0.8 min

Branching fraction to

Re~

7 rays per decay

Ref

1.66±0.40 ~zlmTe,6"8±0-1% [14] 3.05_.+0.39 lss"Te, 4 2 ± 6 % b 0.32±0.04 c 150-keV, 0.67 ± 0.08 '31°Te,

18%

'33oTe, 13±3%

[16]

[1]

[18]

312-keV, [7] 0.73 -----0.07 913-keV, [17] 0.32 ± 0.03 767-keV, 0.29 [19]

~Reference [ 13] unless otherwise noted. °Determined in this work.

"[8]. RESULTS AND DISCUSSION

~z3Sb branching fractions The 133Sb branching fractions, FI and 1 - F1, to the isomers of 133Te were determined from the areas of the photopeaks produced by the 3 12 and 9 i 3 keV y rays of 133~Te and 133roTe, respectively, in the first 30 min spectra taken after Te separation. The decay properties used for these species in analysing the data are 11. G. Graeffe, C. -W. Tang, C. D. Coryell and G. E. Gordon, Phys. Rev. 149, 884 (1966). 12. R.C. Ragaini, G. E. Gordon and W. B. Waiters, Nucl. Phys. A99, 547 (1967). 13. P. O. Strom, D. L. Love, A. E. Greendale, A. A. Delucchi, D. Sam and N. E. Ballou, Phys. Rev. 144, 984 (1966). 14. D. G. Sarantites, G. E. Gordon and C. D. Coryell, Phys. Rev. 138, B353 (1965). 15. C. E. Bemis, G. E. Gordon and C. D. Coryell, J. inorg, nucl. Chem. 26, 213 (1964). 16. W. B. Waiters, C. E. Bemis, Jr. and G. E. Gordon, Phys. Rev. 140, B268 (1965). 17. V. Berg, K. Fransson and C. E. Bemis, Jr., Ark. Fys. (to be published). 18. T. Alvager and C. Oelsner, Ark. Fys. 12, 319 (1957). 19. V. Berg, K. Fransson and C. E. Bemis, Jr., Decay Properties of 1sere (to be published).

588

B. PARSA, G. E. G O R D O N A N D A. W E N Z E L .I

I

I

I

I

I

I

I

131gTe -

150

7tSa~Te 312

8

133gTe

I~m're

134Te 787

\

913

767

.

il

r

I

200

J

J-

~

-5-

400 600 Channel Number

t

i

800

Fig. 1. Gamma-ray spectrum of Te sample milked from Sb fission products. Observed with a Ge(Li) detector for 30 min starting about 8 min after the end of irradiation. Most of the small, unidentified photopeaks are associated with the decay of 12.45 rain laagI'e and are discussed in detail in [7].

Table 3. Experimental data for la3Sb branching-ratio determination Quantity Length of irradiation Time intervals: End of irrad, to transfer of solution to reaction flask Start of reaction to end of Sb separation End of Sb separation to Te separation Te separation to start of 3' spectrometry Length of first count Ratio of photopeak areas: 312 keV y ray/913 keV 1' ray Ratio of photopeak efticiencies: 312 keV/913 keV Branching fractions: F1 (to laamZe) 1 -- F1 (to laaCFe)

First expt.

Second expt.

25 sec

25 sec

16 sec 20 sec 6.8 min 1.9 min 30 min

18 see 14 sec 3.3 min 3.1 min 30 min

57.1 ± 1.0

57.0±1"2

8.6±0.5

8.6±0.5

0.42_+0.7 0.58----.0"07

0.43_+0.07 0.57+0.07

Branching ratio of ~33Sb d e c a y to l ~ T e isomers

589

given in Table 2. In Table 3 we list the experimental data used as input for calculation of the branching fractions. The value of F1 obtained from two experiments is 0.42___0.06. The uncertainty of the mean value includes contributions from the experiments (e.g. statistical fluctuations and timing errors) as well as uncertainties in the decay properties (absolute y-ray intensities and half lives) of the species involved. It should be noted that only - 3 per cent of the ~33OTeactivity results from decay of 133roTe; therefore, uncertainty of the I.T. fraction of 133roTe (13---3 per cent) does not contribute seriously to overall uncertainty. Contamination in the chemical separations was not a problem. The contamination factor for breakthrough of Te activity in the Sb separation was found to be < 1.8 per cent from the upper limit of '34Te in the spectra as discussed below. The contamination factor for break-through of Sb activity in the Te separation was 0.2 per cent as determined from tracer experiments.

~33Sbfission yield Although the experiments were designed specifically for determination of the ~33Sb branching ratio, we were able to make a crude measurement of the cumulative yield of 133Sb. The yield was determined from the ratio of ~3a°Te activity to that of ~3~Te (measured by the strong 150keV line in the spectrum of Fig. 1) observed in the samples. The latter is the daughter of 23 rain ~3~Sb, whose fission yield is known (see Table 2). The experimental data pertaining to the yield determinations are given in Table 4 and other quantities needed are listed in Table 2. The mean value of the yield is 1.7___0.4 per cent, considerably lower than the independent yield value, 3.05___0.39 per cent, reported by Strom et a/[13]. The cause of the disagreement is not clear although there are several possible sources. Our measurement is rather indirect, as we measured the yield relative to that of ~3tSb; therefore, our result would be in error if some of the Table 4. D a t a and results on cumulative yields of l~Sb and 134Sb Second expt. a Quantity No. counts in photopeak 131~Te, 150 keV y ray 13a°Te, 312-keV 7 ray 134Te, 767-keV 7 ray

First expt a

First s p e c t r u m

Second s p e c t r u m

3.70 × l0 S 1.82 × 105 < 290

1.56 × 105 1.26 × 105 < 210

8.24 x 104

Ratio o f photopeak efficiencies: 312-keV/150-keV 767-keV/150-keV C u m u l a t i v e fission yields (%): 1335b 134Sb

< 150

0.210-+-0.015 0.0344 ± 0.0023

1-6 _+0.4 < 0.6

1.9_+ 0-5 < 0.36

< 0.29

"These are the s a m e as the corresponding e x p e r i m e n t s listed in Table 3. The second s p e c t r u m of the second experiment was taken for 60 rain starting 11.3 min after the end of the first one.

590

B. PARSA, G. E. G O R D O N and A. W E N Z E L

various decay-scheme properties required or the 1318b yield were in error outside of the stated limits. The measurements by Strom et al. are subject to similar errors as their yields were determined by counting granddaughter I activities. They used the older F1 values of 0.15 and 0.72 for 131Sb and la3Sb, respectively, rather than the values used in this work. With the value of 3.05 per cent obtained by Strom et al.[13] for the yield of 133Sb, the charge-distribution curve for A = 133 is extremely narrow (or = 0"39 vs. the more usual value of tr = 0.62, [20]). Our lower apparent yield for 133Sb would cause the charge distribution curve to be even narrower. Clearly, more work is needed in this area. Upper limit of yield of l I sec 134Sb Bemis, Gordon and Coryell[15] searched for evidence of 134Sb by rapidly separating Sb from fission products (from 235U(nth,f)), allowing the samples to decay, and then separating and counting the granddaughter I species. No 1341 was observed and, from the upper limit of its activity, the half life of la4Sb was found to be ~< 1.5 sec. For this calculation, they estimated the 134Sb cumulative yield as 2.2 per cent by using Zp and o- values suggested by Wahl et al.[20]. Recently, Delucchi et al.[8] obtained a value of 11.1 ___0.8 sec for the half life of 134Sb and measured its cumulative fission yield as 0.32---0.04 per cent. Note that the measured yield is down about a factor of ten from the value expected on the basis of Wahl's Zp and cr values. Delucchi et al. have suggested that the 1 1 sec 134Sb is possibly the longer-lived of two isomers, with the unobserved one having a much higher yield. As 134Sb decays to 41-5 min 134Te, a search was made for the 767 keV T ray, the most intense T ray of ~34Te,[19] in Te spectra. As shown in Fig. 1, no peak of that energy is apparent in the spectra. The maximum amount of ~34Te activity that could be admitted within the statistical uncertainties of the T-ray spectrum was used to set an upper limit for the ~34Sb production rate relative to that of 131Sb. The experimental raw data and the results of these measurements are presented in Table 4. An upper limit of 0.29 per cent was established on the cumulative fission yield of ~34Sbusing a value of 11 sec for its half life. 20. A. C. Wahl, R. L. Ferguson, D. R. Nethaway, D. E. Troutner, and K. Wolfsberg, Phys. Rev. 126, 1112 (1962).