Analysis has shown that there are very few experimental data on the yield of fission products from the spontaneous

fission of even-even plutonium isotopes. Only one publication was found [1]. However, knowledge of this parameter is of

great scientific and practical interest, for example, for the development of a γ-spectrometric method for determining the mass

of plutonium samples [2].

Peaks due to spontaneous-fission products had previously been observed in the γ-ray spectra of californium sources

and plutonium samples [2]. It is well known that γ-spectrometry is often used to determine the yield of fission products [3].

Thus, it might be possible to use γ-spectrometry to measure the yield of certain products of the spontaneous fission of 240Pu.

Two spherical samples of plutonium metal in a 2-mm thick double shell made of corrosion-resistant steel were used for the

experiments described below. The plutonium samples were certified as state standard samples of neutron flux and activity due

to spontaneous fission (SP-1, -2). The plutonium mass was 82.93 g in the first sample and 41.46 g in the second one. The

radii of the spheres were 1.05 and 0.83 cm, respectively. The 238Pu content was <1·10–4%, 239Pu = 7.86 ± 0.01%.240Pu = 91.07 ± 0.01%, 241Pu = 0.88 ± 0.01%, and 242Pu = 0.17 ± 0.01%. The 241Am content at the time of the measure-

ments was ~3% of the plutonium mass.

A virtue of these samples is their high content of 240Pu (>90%), so that the contribution of other isotopes to sponta-

neous fission was negligible, and the probability of active fission was low. However, because the mass of the samples is large

γ-rays are absorbed inside their volume, which makes it necessary to introduce corrections into the results of the measurements.

Peaks due to plutonium isotopes and 241Am are observed in the spectra of the samples (see Fig. 1), and peaks due to

certain fission products are observed at energies above 700 keV. Lead and cadmium filters, which absorb low-energy radiation,

were used to unload the measurement channel and increase the relative probability of detecting γ-rays from the fission products.

The yield of spontaneous-fission products was determined as the ratio of the number of counts in the γ peaks of the

fission products and the γ peak at 642 keV of 240Pu using the relation

YS T I

S T I

K

KK Kfp

fpsf

fp fpsa

safp if sf

40 1 240 40 40

40 1 240

40

= /,

/,

* ,γ γ

αγ γ

ε

ε

Atomic Energy, Vol. 102, No. 3, 2007

YIELD DETERMINATION OF 240Pu SPONTANEOUS

FISSION PRODUCTS

SCIENTIFIC AND TECHNICAL COMMUNICATIONS

A. V. Bushuev,1 V. N. Zubarev,1

A. F. Kozhin,1 E. V. Petrova,1

T. K. Ragimov,2 A. M. Petrov,2

G. N. Vlaskin,2 V. I. Timonin,2

and A. A. Samoilov2

UDC 539.173.7

Translated from Atomnaya Énergiya, Vol. 102, No. 3, pp. 189–190, March, 2007. Original article submitted April 20,

2006.

1063-4258/07/10203-0232 ©2007 Springer Science+Business Media, Inc.232

1 Moscow Engineering Physics Institute.2 A. A. Bochvar All-Russia Research Institute of Standardization in Machine Engineering.

where S40 and Sfp are the number of counts in the 240Pu and fission-product peaks. The GammaVision program was used for

the mathematical analysis of the spectra; T1/2α,40 and T1/2

sf,40 are, respectively, the α-decay and spontaneous-fission half-lives of240Pu [4]; Iγ

fp and Iγ40 are, respectively, the γ-ray yields due to the decay of the fission products and α-decay of 240Pu [4, 5];

εγfp and εγ

40 are, respectively, the detection efficiencies for γ rays from fission products and 240Pu (the ratio of the efficiencies

is used; its error does not exceed 0.5%); Ksa40 and Ksa

fp are, respectively, the ratio of the corrections for the self-absorption of240Pu and fission-product γ-rays in the sample and the absorption in the shell of the sample; Kif is a correction which takes

account of the contribution of the induced fission to the total number of fissions; K*sf is a correction for the spontaneous fis-

sion of other isotopes (238Pu, 239Pu, 241Pu, and 242Pu) and equals 0.995% in our case.

It is important to obtain an accurate estimate of the self-absorption of γ rays and the additional contribution from

neutron-induced fission (active fission) products and to take account of both effects on the results obtained. The MCNP-4B

program was used to determine both corrections. The contribution of the fission products from active fission of 240Pu was

assumed to the same as that of spontaneous fission. The corrections are small (0.06 for SP-2 and 0.08 for SP-1) and this

assumption could not have a large effect on the values obtained.

The corrections for self-absorption are larger. To show that they were determined correctly, we compared the fol-

lowing:

• the ratio of the corrected intensity of the peaks at 642 and 1436 keV for the samples SP-1 and -2, for which the

self-absorption coefficients differ by almost a factor of 2, and

• the coefficients obtained from the calculations performed with the MCNP-4B program and the formula given in [4]

for spherical samples. Agreement within 1.5 and 2% was found in both cases. This confirmed the correctness of the

method used to introduce corrections.

The measurements were performed using a coaxial germanium detector protected from the neutron radiation by borat-

ed polyethylene, lead, and cadmium. The detection efficiency was 60% and the resolution was 1.9 keV for Eγ = 1332 keV.

A DSpec digital spectrometer was used. One of the spectra measured is shown in Fig. 1.

The samples SP-1 and -2 are strong sources of neutrons; they emit about 8·104 and 4·104 neutrons/sec and could pre-

sent a danger for the operation of germanium detectors. The insertion of a layer of borated polyethylene between the sample

and the detector decreased the neutron irradiation of a detector substantially.

To determine the yield of fission products using γ-spectrometry requires data on the relative detection efficiency of

radiation with different energy. The energy dependence of the γ-ray detection efficiency for a germanium detector with borat-

ed polyethylene protection (40 mm) and lead (4 mm) and cadmium (2 mm) filters was determined using the sources of 152Eu

and 226Ra, which were placed during the measurements into the same position as the plutonium samples. It was determined

233

3 4 5 6

1

2

34 5 6 7 8 9

Channel number, 103

Channel count, 103 counts1000

100

1

10

Fig. 1. Gamma-ray spectrum of a sample of 240Pu metal (SP-2), keV: 642.4 240Pu (1),

847.1 134I (2), 973.9 132Sb (3), 1072.6 134I (4), 1260 135I (5) 1383.9 92Sr (6), 1427.794Sr (7), 1435.9 138Cs (8), 1596.2 140La (9).

that the efficiency decreases rapidly at energies below 300 keV; this made it possible to unload the spectrometric channel

and accelerate data acquisition in the γ peaks studied. In addition, the filters equalized the efficiency in the energy range

600–1800 keV and provided a ratio εγfp /εγ

40 close to 1.

The results for the yield of 240Pu spontaneous-fission products are presented in Table 1.

In summary, data on the yield of 10 products from the spontaneous fission of 240Pu were obtained in this work. In

a previous experimental work devoted to solving this problem, the yield of 14 spontaneous-fission products was determined

radiochemically [1]. One of these products was 135I, whose yield was 6.4 ± 0.7%, which agrees with our value.

REFERENCES

1. J. Laidler and F. Brown, “Mass distribution in the spontaneous fission of 240Pu,” JINC, 24, No. 12, 1485–1492

(1962).

2. A. V. Bushuev, A. L. Bosko, A. F. Kozhin, et al., “Development of the method for characterization of samples con-

taining spontaneously fissioning nuclides using fission products gamma-spectroscopy,” JNMM, 31, No. 1, 59–62

(2002).

3. H. Dencshlag, “Measurements of cumulative and independent fission yield,” in: Proceedings of a Specialists

Meeting on Fission Product Nuclear Data, Tokai, Japan (1992), pp. 256–270.

4. D. Reilly, N. Ensslin, H. Smith, Jr., and S. Krelner, Passive Nondestructive Assay of Nuclear Materials [Russian

translation], Moscow (2000).

5. Nuclear Data Sheets “NuDat 2.1,” National Nuclear Data Center, Brookhaven National Laboratory (2005).

6. N. G. Gusev and P. P. Dmitriev, Radioactive Transformation Chains, Énergoatomizdat, Moscow (1994).

234

TABLE 1. Yield of Products from Spontaneous Fission of 240Pu

Fission product Energy of the γ rays measured Quantum yield of γ rays, % Yield from 240Pu spontaneous fission, %

92Sr 1383 90.0 2.92 ± 0.1494Rb 1309 87.1 1.23 ± 0.0494Sr 1428 94.2 3.94 ± 0.06

96mY 1107 48.1 1.13 ± 0.16

1751 88.2133Sb 1096 43.0 3.26 ± 0.18

1132 22.6135I 1260 28.7 8.13 ± 0.15

1796 7.7136I 1321 24.8 3.65 ± 0.40

138Xe 1768 16.7 7.60 ± 0.29138Cs + 138mCs [6] 1436 76.3 + 19.0 6.85 ± 0.18

140La 1596 95.4 5.70 ± 0.18