GROSS ) , - A C T I V I T Y OF F I S S I O N P R O D U C T S OF U ~ss

V. N. S a k h a r o v a n d A. I . M a l o f e e v

Measurements were made of the y-ac t iv i ty Q(t), representing the average for the products of a single fission of U m~, in the time interval 1 < t < 1000 hours after fission.

The value Q was determined by comparing the y -ac t iv i ty J of specimens of U ~5 and Na m after simul- taneous irradiation in a homogeneous stream of slow neutrons emitted fromthe heavy-water reactor' of AN SSSR.

Comparison of the y-act ivi t ies was carried out using a Geiger counter, the construction of which was described In work [1]. This counter possessed a constant sensitivity to y-rays over a wide range of energies. Therefore, in standard conditions of measurement the ratio of the y-act ivi t ies of the specimens JU/JNa was equal to the ratio of the counting rates CU/ CNa independent of variation in the emission spectrum of the U m5 fission products with time.

,,.

r r \ \ &

,% \

\

,o I ,o / . ' " i o o " '

t hours

The ratio of y-act ivi t ies of the specimens in the conditions of the experiment was equal to

JU PU .ANa Zfiss 1 e ~ , t Q ( t ) ' -J~q~-= PNa A U z~ct q~-

Gamma-act iv i ty , representing the average for the products of a single fission of U m~. I) Experimen- tal data of authors: II) experimental data of [5]; III) experimental data of[6] ; IV) calculated

values of [7], [8]. The values of Q for the interval 1 < t < 1000 hours are given on the figure. The accuracy of

determining Q throughout the whole time interval examined is estimated at * 20 %.

where PU/PNa is the ratio of weights of irradiated specimens; A Na/A U = 23/235 is the ratio of atomic weights of Na'~ and UrnS; k = 1.27.105 sec -l is the decay constant of Na~'t; q=4.2 Mev " e n e r g y of y-radiat ion emit ted in a single decay of Na~; O fis s / / o act. = 945- ra t io of fission cross section of U ~ t o the activation cross section of Na m.

For determining this ratio we used data on the spectral distribution of slow neutrons emitte~d from the reactor [2] and data on the deviation of the fission cross section of U ~5 from the 1/v law [3]. The activation cross section of Na m was taken equal to 0.565 barns. This value was found by comparison with the activation cross section of Au 1~t. The ratio of the activation cross sections

of gold and sodium was obtained in the same way as in [4] but the activation cross section for gold was taken equal to 96 barns, i. e., somewhat great- er than in [4].

Q(t) can be represented in the form

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Q = 23t.i,4s _ Mev sec. fission

�9 for 1< t< lOhrs.

Q = 0.76 t "1"1~ Mev sec. fission

for 10< t< 1000hrs.

The values of Q(t) determined from these formulas do not differ by more than :~ '/% from the results given on the figure.

Values of the function Q(t) obtained by other authors are also plotted on the figure, The results of our measurements are in good agreement with those of Borst [5] whose work gave experimental values of Q for the time interval 20 rain < t < 240 hrs. The results of Sugarman and others [6] may be compared with ours in the interval 1 < t < 2 hrs. The values which we obtained for Q are 2-3 times greater than those of Sugarman.

The figure also gives values of the function Q(t) obtained by summing the y-activit ies of separate fission products of U m5 according to the tabular data of [7], [8]. The results of these calculations with t < 10-20 hrs give values of Q(t) 2-3.5 times smaller than the experimental ones, This discrepancy can be accounted for by our incomplete knowledge of the y-emission of the separate fission products. With increase o f t the experi- mental and calculated values approach one another.

The authors thank O. I. Leipunskii for suggesting the theme and for guidance in this work.

L I T E R A T U R E C I T E D

[1] V. N. Sakharov, J. Atomic Energy (USSR) 3, 61 (1957).

[2] Iu. G. Abov, Session of AN SSSR on the Peaceful Uses of Atomic Energy, July 1-5, 1955 (Session of the Section of Phys.-Math. Sciences) [in Russian] (Izd. An SSSR, 1955), p. 294.

[3] D. Popovic, and N. Rajsic, J. Nucl. Energy 1, 2, 170 (1954).

[4] R. M. Bartolomew, R, C. Hawkings, W. F. Merritt and L. Yaffe, Can. I. Chem. 31,204 (1953).

[5] L. B. Borst, Nat. Nuel. Energy Ser., div. IV, 9, book I, McGraw-Hill, N. Y., 1951, p. 344.

[6] N. Sugarman, S. Katcoff, B. Finkle, N. ElUott and I. D. Knight, Ibid. p. 371.

[7] J. Moteff, Nucleonics 13, 5; 28 (1955).

[8] M. Ia. Gen, M. S, Ziskin and E. L Intezarova, Private Communication.

Received May 16, 1957

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