Measurements of fragment mass, charge and kinetic energy distributions in thermal neutron fission of 235U with a back-to-back ΔE- E detector system

Abstract

A simultaneous measurement of mass ( M), charge ( Z) and kinetic energy ( E k) distributions of fragments in thermal-neutron fission of 235U has been carried out using a back-to-back ΔE- E detector system. A pair of gridded ionization chambers filled with P-5 gas measured the energy losses ΔE 1, ΔE 2 of the complementary fragments in the gas, and the residual fragment energies were measured with a pair of semiconductor detectors. The four-parameter data were analysed to obtain fragment Z-distributions using the mass-momentum conservation relations to obtain M and then using the dependence of ΔE on E/ M and Z. The charge resolution for the light-fragment group was determined at the gas pressures of 44, 150, 270 Torr and the best Z-resolution was obtained at 270 Torr. The data at this latter gas pressure were analysed to obtain the variances σ 2 z of the fragment charge distributions for different E k windows and this was compared with the measured variances σ 2 A of the fragment mass distribution as a function of E k as obtained by twoparameter measurements of the correlated fragment energies. The dependence of the measured variances σ 2 A , σ 2 Z on E k is considered with a view to learning about the degree of neutron-proton correlations in the nucleon exchanges taking place during the mass division in fission, in analogy with that used in the case of heavy-ion deep inelastic collisions (DIC). The variation of σ 2 A σ 2 z with E k observed in the present work, which is somewhat similar to that seen in heavy-ion DIC, implies that the fragment mass division in fission is governed by nucleon-exchange processes, where the degree of neutron-proton correlations is dependent on the total kinetic energy E k. While for very large E k the neutron-proton motion in the exchange processes is found to be uncorrelated, for lower E k values the motion is seen to be correlated. Implications of these results regarding the applicability of fission models such as statistical model are pointed out.