Journal of Radioanalytical and Nuclear Chemistry, Articles, 91/2 (1985) 291-296

Y I E L D S O F R A R E E A R T H F I S S I O N P R O D U C T S

I N T H E S P O N T A N E O U S F I S S I O N O F C A L I F O R N I U M - 2 5 2

B.. S. TOMAR~ H.. NAIK, A. RAMASWAMY, SATYA PRAKASH

Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-400085 (India)

(Received August 29, 1984)

Cumulative fission yields of rare earth isotopes have been determined in the spontaneous fission of 252Cf by fast radioehemieal separatio n and gamma-ray spectrometry, The determined yield values are compared with the available literature data.. The yield values for 14~Nd, tSlNd and aS~Pm differ from the reported values. The yield for ~45Ce is determined for the first time.

Introduction

Mass distribution in the spontaneous fission of 2 s 2 Cf has been studied using radiochemical and gamma-ray spectrometric techniques, x -3 The yields of high yield

asymmetric fission products are well established. Few data are available on the yields

of rare earth fission products due mainly to the very low yield and also the difficulty in their purification. In the present study the rare earths are separated as a group from other fission products and assayed gamma-spectrometrically. The yields of rare earths are determined in comparison with 146Ce for which the yield value was well

established in a separate experiment. 4 The determined yield values are compared with the available litreature data. /

Experimental

Californium-252 obtained from Oak Ridge National Laboratory was purified free from its fission and decay products by ion exchange method, s The purified californium was assayed for the amount of 2 s 2 Cf by neutron well coincidence counting as well as fission fragment counting. A thin source containing 3/ag of 2 s 2 Cf was made on

platinum backing by electrodeposition from isopropyl alcohol medium. The fission rate of the deposited source was obtained by collecting recoiling fission products on an aluminium catcher foil and analyzing the foil for different fission products gamma- spectrometrically.

Elsevier Sequoia S. A., Lausanne Akaddrniai Kiad6, Budapest

B. S. TOMAR et ak: YIELDS OF RARE EARTH FISSION PRODUCTS

In the present study this californium source was used as target. The fission products recoiling out of the target were collected on an aluminium foil or ammonium chloride pellet. The time of collection was varied from 5 minutes to 3 days, depending on the fission products of interest. After collection the catcher (A1/NI-I4C1 pellet) was dissolved in dilute nitric acid or distilled water (as the case may be) in presence of lanthanum and cerium carriers. The one step group separation was then carried out.

Chemical separation o/ lanthanides

From the fission product solution lanthanides were precipitated as hydroxides by addition of dilute ammonia. The precipitate was dissolved and fluoride precipitation was carried out by adding dilute hydrofluoric acid dropwise. The fluoride precipitate was dissolved in dilute nitric acid containing a small amount of boric acid. Lanthani- des were again precipitated as hydroxides and dissolved in minimum amount of nitric acid. The entire chemical operation could be finished within two minutes after removal of the catcher foil.

Counting

The separated lanthanide solution was taken in a standard counting vial and the volume was made up to 5 ml. The vial was counted on a precalibrated 45 cm 3 HPGe detector coupled to a TN-1700 Tracor Northern 4K analyzer. The gamma-spectra of the lanthanides were taken initially for 100 seconds each and the spectra were recorded on a magnetic tape. After the end of counting the spectra were played back and analyzed.

Calculations

The disintigration rate Ax of any fission product (x) formed in the spontaneous fission of 2S2Cf is given by

Ax --- nXs [1 - exp ( - ) , x t ) ] exp (-)~xT) " Yx (1)

where n - the number of atoms of 2s2Cf,

ks - spontaneous fission decay constant, ),x - decay constant of the fission product, t, T - collection and cooling time, respectively, Y x - yield of the fission product x.

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.13. S. TOMAR et al.: YIELDS OF RARE EARTH FISSION PRODUCTS

In Eq. (1) the most uncertain term is nAs, which is eliminated by determining me yield of fission product x relative to a standard fission product (146Ce in the present

ease). The final equation for the calculation becomes:

Ax

A~

Vx [1 - exp ( - xx0] exp ( - X T)

Y6 [ 1 - exp ( - X6 t)] exp ( - ?~6 T) (2)

where the subscripts x and 6 refer to the fission product x and 146 Ce, respectively.

Knowing the disintegration rates, Ax and A6 and also the collection time t and the

cooling time T the yield of fission product x is obtained relative to the yield of 146Ce" The determination of the disintegration rate of the fission product involves the

determination of the area under the photopeak and also the gamma-ray abundance and

the efficiency of the detector system. The gamma-ray abundance values were taken

from the most recent compilation by BLACHOT and FICHE 6 and are shown in

Table 1. The gamma-ray efficiency values used are from an earlier calibration. The

Table 1 Nuclear data for the fission products

Nuclide H a l f - l i f e Gamma-ray energy (KeV)

Gamma-ray intensity

Ref. 6 ReL 7 ReL 8, 9

141Ce 32.50 d 145.4 48.5 49.3 48.2 + 0.3 z4SCe 33.0 h 293.3 42.0 46.5 43.2 • 2.0 ,4 SCe 2.98 m 723.6 59.0, 69.0 59.0 • 8.0 14~Ce 14.2 m 218.3 11.8 18.0 20.5 • 3.2 * 4 ~ Nd 10.98 d 91.1 27 .9 28 .3 27 .9 • 0.5 t 49 Nd 1.73 h 211.3 27.3 24.0 25 .9 + 1.3

i S l N d 12 .44 m 116.3 46.8 25.0 46.8 • 3.6 i s i Pm 28.40 h 340.1 22.4 21.0 22.3 • 0.5 ! SaSm 46.70 h 103.2 28.3 28.2 28.3 • 1.2 15 s Sm 22.10 m 104.3 74.0 73.0 -

photopeaks of interest were followed as a function of time and their purity was

ascertained by seeing their decay. Each of the selected gamma-ray photopeaks

decays with the proper half-life showing thereby that the photopeaks were free

from any interference due to other gamma-rays.

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R e s u l t s a n d d i s c u s s i o n

The f ission yields for the rare ea r th f iss ion p r o d u c t s were o b t a i n e d relat ive to the

4 6 Ce fission yield. T he yield o f * 46Ce was d e t e r m i n e d in a d i rec t way in ou r

earlier w o r k a and was f o u n d to be 4 .08. Tab le 2 gives the d e t e r m i n e d yie lds o f the

rare e a r t h f ission p roduc t s . T he l i t e ra tu re values b y F L Y N N et a l . l , T H I E R E N S et

al. 2 and T O P P A R R E et al.3 are also s h o w n for compar i son . The s t a n d a r d dev ia t ion

given w i t h the e x p e r i m e n t a l values is the p rec i s ion for th ree m e a s u r e m e n t s . T h e

p re sen t da ta are in good ag reemen t w i t h t he l i t e ra tu re values e x c e p t for ~ r

I s t Nd and ~ s 1Pm. T he yield o f * 4 s Ce is r e p o r t e d for the first t ime .

Table 2 Mass yields of rare earth isotopes from spontaneous fission of 2 s 2 Cf

Nuclide Present work Ref. I Ref. 2 Ref. 3.

14iCe 6.00 ~ 0.67 6.00 6.13 +--0.36 -- t43Ce 6 .20~0.71 6.22 6.13 -+0.31 - -

t 4 s C e 4.18 • 0.7J - - - - -

t 4 * C e 4.08 • 0.41 - 5.I8 +- 0.32 3.82 -+ 0.10 t4~Nd 3.30+0.37 4.26 4.10 • -- *49Nd 2.42 +- 0.27 2.71 2.74 + 0.20 2.42 +- 0.10 t st Nd 0.94 +- 0.12 - -- 1.72 +- 0.10 , s t Pm 1.40 • 0.14 1.81 1.99 +- 0.13 - -

t S 3 S m 1.35 • 1.33 1.31 - * 0 . 0 8 - -

t S,Sm 0.70 +- 0.07 - 0.838 +- 0.035 --

Table 3 Error analysis for the determined yieM values

Nuclide Determined Statistical Error on Total error, yield error, % 7-abundance, % %

14, Ce 6.00 0.96 0.62 i1.19 t 43 Ce 6.20 0.4I 4.76 11.47 14 s Ce 4.18 3.19 13.56 17.11 t 47 N d 3 . 3 0 0 . 9 8 1 . 8 0 1 1 . 3 2

t 4 �9 Nd 2.42 2.23 4.21 11.25 ! s t Nd 0.94 2.61 7.70 13.57 �9 s, Pm 1.40 1.35 2.23 10.67 �9 s 3 Sm 1.35 0.87 4.24 11.95 t s s Sm 0.70 3.08 - 11.18

The error on detector efficiency varies from 3 to 5%. The error on t 4 ~ Ce yield is 10%.

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B. S. TOMAR et al.: YIELDS OF RARE EARTH FISSION PRODUCTS

The total error on the determined yield values will be due to the errors in different factors used in the calculation. The error on the disintegration rates will be due to the errors on the peak area determination, the error on gamma-ray abundance values and that on the detector efficiency. The error on photopeak area determination is statistical in nature. Enough counts were accumulated under the

desired photopeak and hence the error on the peak area was always within +1%.

The error on gamma-ray efficiency is systematic in nature since the values were used as constants in the present experiment. The efficiency values were obtained by counting calibrated standard gamma-ray sources under a fixed geometry. The accuracy of calibration varied with the gamma-ray energy. Table 3 gives the evaluated

errors on the determined yield values. The most significant contribution to the errors arises from the gamma-ray abundance values used. In Table 1 the gamma-ray abundance values from different recent compilations 6- 9 are also shown. It can be

seen that the errors quoted on the abundance values range generally from 1 to 5%. The error on the abundance values for 14 s Ce and 14 6 Ce gamma-rays is very high

up to 15%. Also it can be seen that the gamma-ray abundance values differ in different compilations. In view of this it can be stated that in gamma-spectrometric measuremen~ of fission yields the major error is due to the gamma-ray abundance

values used. This may possibly explain the large difference in the determined yield values for I s 1 Nd as compared to the available literature values. The variation

in yield value is much less than that seen in the abundance values quoted in two different compilations. This shows the need for more accurate determination of gamma branching ratios for the fission product nuclides. The determined yield values will have to be updated as and when more accurate data on gamma-ray abundances are made available. No error evaluations on the literature yield values were done. The literature data for rare earth yields by THIERENS 2 and TOPPARE a give only the error on counting statistics.

In the present work the rare earths are separated as a group. No individual separation was tried. This makes the separation faster but again the yields of many low yield rare earths could not be determined due to interference from other

rare-earth gamma-rays. This restricts the number of rare earth yields which can be determined by this method. Also the group separation by hydroxide and fluoride precipitation causes contamination from high yield barium isotopes. To overcome all this an ion exchange group separation of rare earths prior to counting is recommended. Recent studies on fast individual separation of rare earths should be

adopted to obtain the undetermined yields. Work is in progress in this direction using the extraction chromatographic method.

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The authors are grateful to Dr. M. V. RAMANIAH, Director Radiological Group and Dr. P. R. NATARAJAN, Head, Radiochemistry Division for their encouragement and helpful suggestions during this work.

References

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DERYTLER, NucL Instr. Methods, 134 (1976) 299. 3. L. TOPPARE, H. N. ERTEN, N. K. ARAS, Technical Journal, Turkish Atomic Energy

Commission, 8 (1980) 8. 4. A. RAMASWAMY, B. K. SRIVASTAVA, ALOK SRIVASTAVA, S. B. MANOHAR, SATYA

PRAKASH, DAE Symposium on Nuclear Cheanistry and Radiochemistry, Banaras, India, Nov. 3-7,1981.

5. L. I. GUSEVA, B. F. MYASOEDOV, G. S. TIICHOMIROVA, L. I. PAVLENKO, G. G. BABICHEVA, J. Radioanal. Chem., 13 (1973) 293.

6. J. BLACHOT, Ch. FICHE, Ann. Phys., 6(1981) 3. 7. G. ERDTMANN, W. SOYKA, J. Radioanal. Chem., 26 (1975) 375. 8. J.K. DICKENS, J. W. McCONNELL, Phys. Rev., C73 (1981) 331. 9. J.K. DICKENS, J. W. Mc'CONNELL, Phys. Rev., C74 (1981) 192.

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