I 2.d ] Nuclear Physics A198 (1972) 228--236; (~) North-HollandPublishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

I N D E P E N D E N T Y I E L D S O F 233, 13$Xe

I N T H E F I S S I O N O F 23SU, 238U A N D 239pu

by 14 M e V N E U T R O N S

PETER ALEXANDER t

Teledyne Isotopes, 50 Van Buren Ave., Westwood, New Jersey *t

Received 19 July 1972

(Revised 19 September 1972)

Abstract: The direct fission populations of states in 133.135Xe have been measured for fission induced by 14 MeV neutrons. Targets consisting of metal foils of 235U, 23sU and 239pu were separately encapsulated and irradiated for periods of 1 to 8 rain in a flux of 1.5 × 10 H n/cm2 • s. The xenon isotopes of interest were extracted from the bulk fission products within 3 to 12 min after irradiation. The xenon 7-rays were then examined with a Ge(Li) detector and the popula- tions of the 133,13SXe isomer and ground states were deduced. Computer analysis techniques were employed to extract the independent fission yield values from this data.

E l NUCLEAR REACTION, FISSION 23su, 238U, 239pu(n,f), E = 14 MeV; ] I measured 133. laSXe independent fission yields, isomer ratios. [

1. Introduction

Some features o f the fission process such as the neu t ron- induced f iss ion-product

mass -d i s t r ibu t ion curve have been accura te ly de te rmined for a variety o f target nuclei

over a range o f neu t ron energies. Or thogona l to this curve at each mass number lie

the f i ss ion-product charge d is t r ibu t ion curves. In general the detai ls o f these curves

are not accurate ly determined. I n f o r m a t i o n concerning the charge d is t r ibu t ion curves

are i m p o r t a n t factors in developing a comprehens ive theory o f fission.

This work is concerned with measuremen t o f charge d i s t r ibu t ion and i somer ra t io values in the neu t ron- induced fission o f 235U, 238U and 239pu. The 133Xe and 135Xe

popu la t i ons direct ly p r o d u c e d th rough the fission process were determined. Measure- men t o f the shape o f the 133,135Xe p o p u l a t i o n ingrowth curves fol lowing fission also

suppl ied in fo rma t ion abou t the init ial f i ss ion-product charge d is t r ibu t ion o f the xenon

precursors . These effects were s tudied using 14 MeV neut rons genera ted by the 3H(d, n )4He react ion.

"In brief, encapsu la ted metal l ic fissile targets were i r rad ia ted for shor t per iods in a

high-intensi ty neu t ron flux. As soon as possible after i r r ad ia t ion the targets were dis-

solved and the xenon fission gas ext rac ted and counted on a Ge(L i ) detector . The

* Present address: NISC, 4301 Suitland Rd., Washington, DC. ** This work was supported by the Advanced Research Projects Agency under Contract DASA-

01-69C-0057 with the Defense Atomic Support Agency.

228

235, 2aaU, 2a9pu(n ' f) 229

counting data were analyzed with the aid of several newly developed computer pro- grams to yield the independent fission yield and isomer ratio values.

Wahl et al. 1) have summarized much of the experimental information relating to independent fission yields and to fission-product charge distribution functions for thermal neutron fission of 235U. They indicate that this curve can be approximated by a Gaussian of FWHM w and charge value Zp at the centroid. The factor Zp varies markedly with both fissile target material and energy of the neutron inducing fission 2). Analysis of the experimental data 3) suggests that the quantity w does not appear to vary appreciably with fissile target species or neutron energy in neutron- induced fission. Wahl et al. have deduced a universal value for w equal to 1.50___0.12 charge units. Based on the limited amount of published experimental data it appears that significant deviations from Wahl's value for w occur in the mass chains near mass numbers A = 132 and 136 [refs. i, 4)]. These perturbations are probably asso- ciated with the closed proton shell at Z = 50 and the closed neutron shell at N = 82.

In the A = 133 mass chain the independent yields ofSn, Sb, I and Xe are measured [ref. 5)] for thermal neutron fission of 235U. In the A = 135 mass chain both I and Xe yields are measured 6-s), for thermal neutron fission of 235U. These measure- ments are not all consistent with a Gaussian charge dispersion function with w = 1.50. Although the consistency of the independent fission yield data for thermal neutron fission of 235U requires further examination, the behavior of the charge dispersion function for fission induced by 14 MeV neutrons has not been investigated at all. This is the rationale for undertaking these measurements which are part of a broader program which includes like measurements using thermal neutrons and reactor- produced fast neutrons.

2. Experiment

The Cockroft-Walton accelerator of the Inorganic Chemistry Institute of the Guten- berg University in Mainz, Germany was used to generate 14 MeV neutrons through the 3H(d, n)4He reaction. A deuteron beam current of 4mA at 550 keV produced a 14 MeV neutron flux of 1.5 x 1011/cm 2 • s on target. Tritium targets were repeatedly changed in order to minimize any neutron energy distribution distortion from deuterium build-up in the target.

Encapsulated fissile targets were manually mounted on the accelerator terminal so that the fissile material was located 5 mm from the face of the tritium target. The fissile material was multiply encapsulated. The outer encapsulation consisted of a 0.1 cm thick cadmium shell. The innermost encapsulation consisted of a gas-tight gelatin capsule. Irradiation periods ranged from 1.0 to 8.0 min. Typical masses of the irra- diated fissile targets were: 235U, 300 mg; 238U, 200 mg; 239pu, 20 mg. A total of ten fissile targets were irradiated and processed. Immediately after the end of each irra- diation the target was manually transported as quickly as possible to the radio- chemistry hood area some 30 m away.

230 P. ALEXANDER

The 235U target material consisted of metal foil 0.050 cm thick enriched to 97.68 ~ . The 238U target material was metal foil 0.025 cm thick depleted in z35U to 40 parts per million. The 239pu targets consisted of 10 mg strips of metal enriched to 99.12 %.

Following each irradiation the encapsulated fissile target was rapidly recovered and the gas-tight gelatin capsule was placed in a sealed dissolver vessel containing an acid solution. The air was removed from the vessel prior to dissolution of the gelatin cap- sule, and the vessel was backfilled with carrier xenon. The acid solution in the dissolver vessel consisted of a mixture of 3 ml of 7N hydrochloric acid and 5 ml of 7N nitric acid to which had been added 0.5 mg each of potassium iodide and potassium di- chromate. The dissolver vessel solution was continually frothed by a magnetic stirring bar promoting dissolution and carrier-gas exchange. After the fissile target was com- pletely dissolved, the active xenon plus carrier xenon were slowly pumped for several minutes through a trap held at - 15 ° and then through a second trap held at - 195°C. The purpose of the first trap was to catch any iodine which was not held back by the potassium iodide and potassium dichromate in the dissolver solution. The second trap was used to collect the xenon. From the second trap the xenon was transferred to a small thin-walled glass vial which was sealed for counting. When processing irradiated samples 8 dissolver vessels, traps, and stirrers were assembled as a unit and fed into a common gas collection tube.

Several short (2 min) irradiations were employed in conjunction with fast extrac- tion of the fission gas in order to provide an optimal basis for calculating the xenon isomer ratio values. In these samples the xenon was separated from the bulk fission products within 5 min after the beginning of the irradiation. These samples were not used to provide independent fission yield data as the rapid gas extraction procedure could not be expected to produce a high xenon recovery efficiency. A typical time sequence for an irradiation designed to provide independent fission yield data was as follows: t = 0, beginning of irradiation; t = 8 rain, end of irradiation; t = 10 rain, insertion of gelatin capsule into dissolver vessel; t = 13 min, target dissolved; t = 15 min, extraction completed. This procedure resulted in a xenon recovery efficiency which was > 97 °/o. The small amount of xenon absorption which occurs on various portions of the system could be neglected since 2-3 cm 3 of carrier xenon was used for each extraction. Observation of the collected xenon revealed no trace of iodine lines. Alternatively no residual xenon was found to be caught in the first trap. The effectiveness of exchange between carrier gas and the active xenon in the dissolver solution was also investigated. Examination of the xenon activity in the extracted gas and the active xenon residue in the dissolver vessel led to the conclusion that the xenon exchange efficiency was > 95 ~ .

The fission decay sequences for the A = 133, 135 mass chains are presented in fig. 1. In these experiments the 133 and 133mXe populations were deduced from the intensity of the 81 keV transition in 133Cs fed by the decay of 127.5 h 133Xe and from the intensity of the 233 keV transition generated by decay of the 52.58 h iso- meric state. The 135Xe and 135mXe populations were deduced from the intensities of

235, 23sU, 239pu(n ' f) 231

the 250 keV transition in 135Cs fed by decay of 9.14 h 13SXe and from the 527 keV

transition generated by decay of the 15.2 min xenon isomeric state. Two different germanium detectors were used to record these transitions. The first detector had a volume of 8 cm 3 and a resolution (FWHM) of 1.5 keV at 233 keV. This detector

55.4 min 2.2 d

55s 2.7 min 12.5 min 21h 5.7d STABLE

Sn Sb Te I Xe Cs

16 min

1.9s 29s 6.7h 9.1h 2x 106y STABLE

Sb Te I Xe Cs Bo

Fig. 1. The A : 133 (top) and A = 135 (bottom) fission chain decay sequences.

was used to record 7-rays from all high-activity samples. The second detector had a volume of 45 cm 3 and a resolution of 2.2 keV at 233 keV. This detector was used to study low-activity samples. Pulses from these diodes were recorded using a 4096- channel analyzer system.

3. Data analysis

The experimental counting data were analyzed using the computer program ISIS-2. The details of this program are described in refs. 9, lo). Briefly, the program deter- mines the parameters Zp and w which characterize the Gaussian charge distribution which best reproduces the experimental counting data; Zp is the charge value at the peak of the independent yield distribution and w is the F W H M of this distribution. The input data required for this program consists of the names of the states, their half-lives, and the pertinent branching ratios, as well as initial 'guess estimates' for Zp and w. It is also necessary to supply an indentification of the members of the decay chain which are removed from the bulk fission products, the pertinent extrac- tion times, the start and stop count times, and the counting data. Multiple counts

232 P. ALEXANDER

of the same sample are permitted and loss of one or more of the extracted samples does not invalidate the analysis. Provision is made for folding into the input data the values in internal conversion coefficients, germanium detector efficiency, and isomer ratio values known from other experiments. Using the above information, the program generates the t = 0 populations for all chain members and from this calculates the expected number of counts for the extracted members over the counting intervals specified. The differences between the calculated and measured counts are used to construct X 2 which is essentially a function of Zp and w. At the option of the user, either one or both of these parameters may be varied according to the Gaussian iterative method until Z 2 is minimized. From this analysis the values for the xenon independent yields can be extracted. Estimates for best-fit Zp and w-values are also generated. The internal conversion coefficients employed in the analysis of the data are a T = 11.4, 1.58, 0.27 and 0.062 respectively for the transitions observed in the decay of 133mXe, 133Xe, ~35mXe and 135Xe. The direct plus indirect fl-decay

feeding of the isomeric state was assumed to be 2.9 % for 133Xe and 16.0 % for a35Xe.

There is provision in the ISIS-2 program to obtain a xenon isomer ratio fit value from the experimental data. In these experiments this technique has only marginal sensitivity to the isomer ratio value. The isomer ratio values in this experiment were extracted from the data by a different method. The isomer ratio at the time of ter- mination of the first xenon gas extraction was calculated from the measured xenon activities. Next a correction was applied for the distortion of the isomer ratio due to the effects of xenon dilution from chain ingrowth during the period prior to termina- tion of the gas extraction. This correction was calculated using the initial population values determined by the ISIS-2 solution for the independent yield parameters. Finally the corrected xenon isomer ratio at t = 0 was computed.

4. Results

The 235U, 23Su and 239pu targets were irradiated and processed to extract inde-

pendent fission yield and isomer ratio data. In these experiments the isomer ratio is defined as the population of the isomer divided by the population of the ground state. The independent fission yields of la3Xe and 135Xe from neutron-induced fission of 2aSU, 23Su and 239pu are given in table 1. The tabulated xenon values are given as a

percentage of the total experimental yield of that mass chain. The errors shown in

TABLE 1

Fractional xenon independent fission yields

Mass chain Yield (%)

235 U 23a U 2 3 9 p u

133 2.2 --0.7 (2) 0.30±0.13 (3) 12.0~0.1 (2) 135 26 : 1 (3) 5.6 ±3.7 (3) 46 ~3 (2)

23~. 23su, 239pu(n ' f) 233

table 1 represent only the rms error for repeated measurements. The number in brackets is the number of targets considered. It is felt that a realistic appraisal of the error sources involved in the data analysis establishes a minimum error in the range 10 to 30 ~ which must be attached to all independent yield values unless the rms error shown is larger than this value.

TABLE 2 The 13aXe and 13SXe isomer ratios

Mass chain Isomer ratio

235 U 238 U 239pu

133 > 2 2.64-0.8 135 1.2±0.2 1.4-~:0.7 3 .5±1.6

Isomer ratio values deduced from the experimental data are presented in table 2. The first gas sample milked from the bulk fission product is counted several times. Thus each target yields between 2 and 3 data sets from which isomer ratio values have been calculated. Again the errors shown are only the rms statistical errors between individual data sets.

No value is given in table 2 for the 133Xe isomer ratio due to fission of 23SU. This

is because the independent fission yield was relatively small in this case, making ob- servation of the xenon isomeric transition particularly difficult. When the 233 keV xenon isomeric transition is observed in these experiments, this generally signifies a large 133Xe isomer ratio due to the fact that this transition is highly converted.

Sizeable changes in the isomer ratio value produce only small changes in the popu- lation distribution, when the isomer ratio is large. For example changing the isomer ratio value from 4 to 5 only changes the ground state population from 20 ~ to 16.7 of the total. The larger isomer ratio values tend to 'blow up' when the isomer ratio is evaluated at t = 0. For example a 135Xe isomer ratio of 0.95 observed for a fission gas separation time of 12 min yields a t = 0 value of 4.1 when corrections are made for isomer and ground state decay and for the effects of chain ingrowth feeding of the isomer and ground state from the xenon precursors. The statistical errors evaluated at t = 0 tend to balloon in the same manner as the isomer ratio making difficult an accurate isomer ratio assessment for large isomer ratio values. These relationships are depicted in fig. 2 which gives the relationship between t = 0 isomer ratio values and the isomer value at three later fission gas separation times, t = 4, 6 and 12 min. In this example the 135Xe fractional independent yiel d is taken to be 46 ~o. The t = 0 populations of the other chain members are assumed to be: 135Sb, 0.03 ~/o; 13aTe, 3.82 ~o; 135I, 46.3 ~o; and 135Cs, 3.75 ~ .

A summary of the Zp values derived from this experiment is presented in table 3. These values were obtained from the least-squares data fits of this experiment. For the purpose of these calculations w was fixed at a value of 1.50 as recommended by

234 P. ALEXANDER

Wahl et aL 1). Also presented for comparison in table 3 are the Z~ estimates calcu-

lated using the empirical relation o f Coryell et al. 11) and the 23sU thermal neutron

fission Zp values from Wahl et al. 1).

4 --

3

2

12 min

extraction time

6 r a i n . ~ _ ~ / 4 mln ~

extraction / extraction time time

T = O POPULATIONS

135Cs 3.75 %

135xe 46.08 %

1 3 5 1 46.31%

135Te 3 .82%

135Sb 0 .03%

/, ,/ ,/ , , , , ,

0.6 0.8 I.O 1.2 1.4 1.6 1.8 2.0

ISOMER RATIO AT TIME OF XENON EXTRACTION

Fig. 2. Calculated relationship between laSXe isomer ratio value at t = 0 and at xenon extraction time taking into account the decays of the xenons and their precursors.

The independent fission yield using 14 MeV neutrons has been determined for only one member of the A = 133 and 135 mass chains. It is possible to fit the experimental results using a variety of values for the parameters Zp and w. Once the appropriate

value for w has been fixed then the corresponding Zp value is also uniquely deter-

235,238U, 239pu(n ' f)

TABLE 3

Comparison of experimental and calculated Zp values

235

Zp value

Mass chain 2asU 23su 239pu

exp calc exp calc exp calc

133 52.41 51.97 51.73 51.34 52.84 52.13 135 53.16 52.88 52.55 52.25 53.50 53.04

mined. I f the value for w is chosen as 1.50 then the corresponding Zp values are those given in table 3. The experimental uncertainty associated with these values is estimated to be +0.25 charge units. As described in the sect. 5 there is some reason to question the validity of the w-value, 1.50, used in connection with these two mass chains. This value was used for lack of any better estimate at the present time. The semi-empirical relation used to generate all calculated Zp estimates in table 3, based on 235U thermal

neutron fission Zp values, is thought to be accurate to _+0.2 charge units.

5. Conclusion

The independent fission yields of 133, 133mXe and 135, 13 SmXe have been measured

using 14 MeV neutrons produced through the 3H(d, n)4He reaction. There is no published data with which to compare the independent yield and isomer ratio values measured in this experiment.

The quasi-theoretical independent fission yield tabulations such as that of Weaver et al. 12) often incorrectly estimate yield projections even for thermal neutron fission of 235U. This effect becomes even more pronounced in the extrapolation to higher neutron energies and/or other fissile targets. The independent fission yield values presented in table 1 are approximately ten times larger than those predicted in ref. ~2) for 135Xe using 23Su and 239pu targets. The 133Xe yields for z3sU and 239pu targets are three to seven times larger than predicted by ref. 12).

In part the problem of matching up measured and tabulated projections for 133Xe and 135Xe independent fission yields and Zp estimates stems from uncertainty as to the w-value which is appropriate to these two mass chains. Not only are the w-values appropriate to 14 MeV neutron-induced fission of z35 U, z38U and 239pu uncertain,

there is even disagreement 1, 4) as to the w-values appropriate to the A = 133 and 135 mass chains for thermal neutron fission of zasu. Wahl recommends a universal w-value of 1.50. Notea arrives at a value w = 1.26 for the A = 133 chain and w = 1.03 for the A = 135 chain. This difficulty may be linked to shell-structure perturbations caused by the closed neutron shell at N = 82 and the closed proton shell at Z = 50. It is hoped that this measurement and like measurements using thermal neutrons and reactor-produced fast neutrons will shed light on this problem. Details of these

236 P. ALEXANDER

measurements and interpreta t ion o f the resulting fission yield systematics will be

repor ted in ano ther publ icat ion.

The au thor is indebted to Messrs. A. Skaar, T. Dempsey, C. Hansen, M. Ne iman

and C. Rosenberg for their assistance with the data collect ion and analysis. The

au thor especially wishes to thank Prof. G. H e r m a n n and Dr. H. Denschlag for gener-

ously making their neut ron facility available for this experiment.

References

1) A. C. Wahl, A. E. Norris, R. A. Rouse and J. C. Williams, 2rid IAEA Symp. on physics and chemistry of fission, 1969, p. 813

2) C. D. Coryell, M. Kaplan and R. D. Fink, Can. J. Chem. 39 (1961) 646 3) M. E. Meek and B. F. Rider, APED-5398-A(1968) 4) A. Notea, Phys. Rev. 182 (1969) 1331 5) P. O. Strom, D. L. Love, A. E. Greendale, A. A. Delucchi, D. Sam and N. E. Ballou, Phys. Rev.

144 (1966) 984; H. C. Storms, Ph.D. thesis, MIT, 1963; A. E. Greendale and A. A. Delucchi, USNRDL report TR-69-11 (1968)

6) H. O. Denschlag, J. lnorg. Nucl. Chem. 31 (1969) 1873 7) R. C. Hawkings, W. J. Edwards and W. J. Olmstead, Can. J. Phys. 49 (1971) 785 8) A. Okazaki, W. G. Walker and C. B. Bigham, Can. J. Phys. 44 (1966) 237 9) P. Alexander, D. Lightbody and W. A. Patton, Nucl. Instr. 86 (1970) 99

10) P. Alexander, W. Patton, D. Lightbody and C. Hansen, ISIS: A program for computerized fission data analysis, AD721481 (1971)

11) C. D. Coryell, M. Kaplan and R. D. Fink, Can. J. Chem. 39 (1961) 646 12) L. E. Weaver, P. O. Strom and P. A. Killeen, USNRDL-TR-633 (1963)