Partition of total excitation energy between fragment pairs in asymmetric and symmetric fission modes

Primary and secondary fragment mass and fragment total kinetic energy were measured by the time-of-flight technique in the $12\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ proton induced fission of $^{232}\mathrm{Th}$. The average neutron multiplicity was obtained from the difference of the primary fragment mass and the secondary one as a function of mass and total kinetic energy of fission fragments. Simulation calculations were applied for correction of observed fission data affected by instrumental fluctuations in measurements of flight time and kinetic energies of fragments. Anomalous behaviors of the average neutron multiplicity as a function of total kinetic energy in the mass region of $98--107$ for the lighter fragment and $126--135$ for the heavier were observed and they were found to correlate with two fission modes, symmetric and asymmetric fission. The overall trend of the average neutron multiplicity was discussed in terms of the two fission modes. Partition of the total excitation energy between fragment pairs deduced from the neutron multiplicity data was evaluated for each fission mode for the first time.