Simultaneous investigation of fission fragments and neutrons in 252Cf (SF)

Abstract

Abstract The gridded twin ionization chamber developed at CBNM is used to measure the kinetic energy-, mass- and angular distributions of the fission fragments of the spontaneous fission of 252Cf. Together with a neutron time-of-flight detector this experimental arrangement permits to measure the correlations between neutron emission, fragment angle, mass and energy of the fission fragments. Without neutron coincidences 40 · 106 fission events were recorded which are evaluated to give mass-, total kinetic energy- and the variance distributions in a broad mass range from mass 67 to 185. About 3 · 106 fission events were recorded in coincidence with a neutron detected in the time-of-flight detector. Angular distributions in the c.m.-system revealed isotropy in the whole fission neutron energy range. This permits the conclusion that fission neutrons are emitted from the fully accelerated fragments and that the hitherto assumed scission neutron component of 15–20% is much smaller, as can be determined from the uncertainty of the second Legendre polynomial coefficient. The average number of neutrons was determined as function of fragment mass and TKE. The mass range for ν (A) was extended beyond that of earlier measurements and revealed two new saw-teeth near masses 80 and 176. The slopes and end points of ν ( TKE ) were also determined for each fragment mass. The fragment center-of-mass fission neutron spectra were determined as function of fragment mass and TKE. These spectra permitted the evaluation of the average neutron energy η (A, TKE ) , the nuclear temperature T(A, TKE), and the λ-factor from the cascade evaporation model. These quantities permitted the evaluation of the level density parameter a(A) in the mass range from 90 to 169.