Double-energy, double-velocity measurement of fission fragments from thermal neutron induced fission

Number of prompt neutrons as a function of individual fragment mass ν(m*) was measured for the thermal neutron-induced fission of 233U and 235 U. This paper reports that by measuring the velocities and energies of two fission fragments simultaneously preneutron-emission fragment mass m* and postneutron-emission fragment mass m were obtained. Thereafter ν(m*) was deduced by subtracting m from m*. The fragment velocity was measured by the 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 good 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