Investigation of the Spectral Fluence of Neutrons from Spontaneous Fission of ²⁵²Cf by Means of Time-of-Flight Spectrometry

A precision multidetector neutron time-of-flight (TOF) spectrometer and low-mass paral-lel-plate ionization chambers with a ²⁵²Cf deposit on the inner electrode are used to measure the spectral fluence of neutrons from the spontaneous fission of ²⁵²Cf in the 2- to 14-MeV energy range. Various methodological problems are carefully investigated. The influence of anisotropic efficiency in detecting the fission fragments is determined experimentally and numerically. Formulas are derived for an iterative analysis of the measured TOF spectra taking into account an inverted time scale and a nonextended dead time in the stop chain. On the basis of calculated neutron detection efficiencies, which were recently confirmed by n-p scattering experiments and proton recoil telescope measurements, it was possible to extract the spectral fluence of fission neutrons in absolute scale. The experimental data show significant deviations from a Maxwellian distribution with an energy parameter E₀ = 1.42 MeV, but reasonable agreement can be achieved with cascade evaporation model calculations.