The Energy Distribution of Delayed Fission Neutrons

Abstract

A high-pressure ³He proportional counter has been developed with good energy resolution for fast neutrons, and detailed descriptions are given of its construction and calibration and the determination of its response function. The spectrometer was used to measure the delayed-neutron energy distribution for samples of ²³²Th, ²³³U, ²³⁵U, ²³⁸U, and ²³⁹Pu repetitively irradiated in the Israel Research Reactor-1. Irradiation and counting times were optimized to accentuate group 2 and group 4 delayed neutrons. Corrections were applied for the finite energy resolution, for the energy-dependent detection efficiency and for spectrum distortion due to gamma rays. An approximate energy spectrum was obtained for group-1 delayed neutrons, and more precise spectra were obtained for group-2 delayed neutrons from each of the samples investigated. Preliminary results are also given for group-4 delayed neutrons from ²³² Th and ²³⁵ U.