Measurements of fragment mass, charge and kinetic energy distributions in thermal neutron fission of <SUP>235</SUP>U with a back-to-back &#916;E-E detector system

A simultaneous measurement of mass (M), charge (Z) and kinetic energy (Ek) distributions of fragments in thermal-neutron fission of 235U has been carried out using a back-to-back &#916;E-E detector system. A pair of gridded ionization chambers filled with P-5 gas measured the energy losses &#916;E1, &#916;E2 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 &#916;E 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 &#963;2z of the fragment charge distributions for different Ek windows and this was compared with the measured variances &#963;2Aof the fragment mass distribution as a function of Ek as obtained by twoparameter measurements of the correlated fragment energies. The dependence of the measured variances &#963;2A, &#963;2Z on Ek 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 &#963;2A/&#963;2z with Ek 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 Ek. While for very large Ek the neutron-proton motion in the exchange processes is found to be uncorrelated, for lower Ek 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