A Hemispherical Fission Chamber for the Neutron-Flux Monitoring

Abstract

A fission ionization chamber has been designed to monitor neutron fluxes produced by the proton beam of the Van de Graaf accelerator in the ⁷Li(p, n)⁷Be nuclear reaction. A target producing neutrons is placed at the center of the chamber, which has a hemispherical shape. The neutrons escaping from the target are detected within the limits of a space angle a little larger than 2π sr. A ²³⁵U isotope is used as a fissile material. A U₃O₈ layer with a thickness of 300 μg/cm² and an area of 80 cm² has already been deposited on the surface of the inner hemisphere using electrolysis. The chamber is filled with a gas mixture of 97% Ar + 3% CO₂. The detection efficiency of the chamber for neutrons with 1- to 150-keV energies is found to be 1.6 × 10^–5. Using this chamber, it is possible to measure (with a statistical accuracy of 1%) the integrated neutron yield in a thick target exposed for ∼20 min to a proton beam with a current of ∼3 μA and an energy of 1942 keV, which is 60 keV above the threshold of the ⁷Li(p, n)⁷Be reaction. The design of the fission chamber is described, and the test results are presented.