Energy dependence of nuclear charge distribution in neutron induced fission ofZ-even nuclei

Abstract

For the first time the distribution of nuclear charge of fission products with mass numbers 87, 88, 89, 91, 93, 94, 95, 137, 138, 139, and 140 and their complementary products have been studied for neutron induced fission of $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ in the energy range from thermal up to 1.2 MeV. The energy dependences of the cumulative yields of $^{87}\mathrm{Br}$, $^{88}\mathrm{Br}$, $^{89}\mathrm{Br}$, $^{91}\mathrm{Br}$, $^{93}\mathrm{Kr}$, $^{94}\mathrm{Rb}$, $^{95}\mathrm{Rb}$, $^{137}\mathrm{I}$, $^{138}\mathrm{I}$, $^{139}\mathrm{I}$, and $^{140}\mathrm{I}$ have been obtained by delayed neutron measurements. The most probable charge ${Z}_{P}(A)$in the appropriate isobaric \ensuremath{\beta}-decay chains was estimated. The results were analyzed in terms of the deviation $\ensuremath{\Delta}{Z}_{P}({A}^{'})$ of the most probable charge of isobaric \ensuremath{\beta}-decay chains from the unchanged charge distribution before prompt neutron emission (nuclear charge polarization) and they are compared with experimental data of other authors and with predictions from Nethaway's ${Z}_{P}$-formula and Wahl's ${Z}_{P}$-model. We show that the nuclear charge polarization of primary fission fragments $\ensuremath{\langle}\ensuremath{\Delta}{Z}_{P}({A}^{'})\ensuremath{\rangle}$ before prompt neutron evaporation decreases as the excitation energy of the compound nucleus increases. This decrease is more pronounced for fission of $^{235}\mathrm{U}$. The energy dependencies of $\ensuremath{\Delta}{Z}_{P}({A}^{'})$ and $\ensuremath{\Delta}{Z}_{P}({Z}_{P})$ obtained in the present work show an attenuation of the odd-even effects in the charge distribution as the excitation energy of the compound nucleus increases.