ν(m*) measurement for thermal neutron-induced fission of 233U and 235U by double-velocity double-energy method

Number of prompt neutrons as a function of individual fragment mass ν(m*) was measured for the thermal neutron-induced fission of 233U and 235U. By measuring the velocities and energies of two fission fragments simultaneously, preneutron-emission fragment mass m* and postneutron-emission fragment mass m were obtained. ν(m*) was deduced by subtracting m from m*. The fragment velocity was measured by a time-of-flight (TOF) method, and the start time was detected by a very thin plastic scintillator film detector. A silicon surface barrier detector was used to measure the fragment energy, which was also used as a stop detector of the TOF. The result of ν(m*) for 233U(n,f) was in agreement with other data in the heavy fragment region, but was 20 to 50% larger than those in the light one. ν(m*) for 235U(n,f) showed a factor of 1.5 to 2 larger in the light fragment region and smaller in the heavy one than the other data. With the energy balance equation, the total kinetic energy was estimated using the ν(m*)-value and was in good agreement with the experimental result. Also using the energy balance equation, the ν(m*)-values were calculated by assuming the thermal equilibrium at the scission point. These values were quite different from the experimental results in both cases. (author). 17 refs, 9 figs, 1 tab