Roshchenko V.A., Piksaikin V.M., Korolev G.G., Balakshev Yu.F.
Russian Federation State Scientific Centre — A.I. Leipunsky Institute of Physics and Power
Engineering, Obninsk, Russia
The most of data on nuclear charge distribution were obtained by radiochemical methods of determination of fission product primary and cumulative yields. These results were
related to the thermal neutron-induced fission of 233U, 235U, 239Pu and spontaneous fission of 252Cf. And only recently the data on charge distribution in neutron-induced fission of 229Th, 237Np, 249Cf were obtained. These new data have enlarged to a considerable extent our mind about the most probable charge of fission fragments, odd-even effects and dispersion of their charge distribution. However, up to now a lot of unsolved problems remain. One of them is related to the character of charge distribution shift of fission fragment as the excitation energy of fissioning nucleus increases.
At the present work the dependence of fission fragment charge distribution on the compound nucleus
excitation energy has been studied experimentally for the fission of even-charged nucleus 238U and oddcharged
nucleus 237Np. The method for determination of the fission fragment most probable charge used in
the present work is based on the experimentally stated fact that the primary distribution of fission fragments
IY (A, Z) in a given isobaric chain A can be described by Gauss distribution characterized by the most
probable charge ZP, and dispersion σ. And if it is remembered that the cumulative yield of separate fission
product in a given isobaric chain is uniquely determined by the primary charge distribution, then, knowing
the cumulative yield of individual member of isobaric chain CY (A, Z) and the primary charge distribution
width σ, one can obtain the most probable charge. In the present work the experimentally obtained data on
cumulative yields of delayed neutrons precursors were used to determine the most probable charge in isobaric
β-decay chains with mass numbers: 87, 88, 89, 91, 93, 94, 95, 137, 138, 139, and 140 for neutron induced
fission of 237Np and 238U in the energy range from 0,5 and 1,2 MeV respectively up to 5 MeV. A comparison
of present work data with available experimental data of other authors and with data obtained in assumption,
that the shift of fission fragment charge distribution as excitation energy of a compound nucleus increases
caused mainly by prompt neutron emission, was done. Data for 237Np have been obtained for the first time.
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