A coincidence method of measuring a flux of fast neutrons

A method is described in which the rate at which fast neutrons cross unit area is measured by counting protons projected into a small solid angle in the forwards direction from thin and thick layers of polythene. The protons are detected by triple coincidences between three proportional counters mounted coaxially behind the hydrogenous layers. The method is applicable to neutrons of energies from 1 MeV. upwards, and can be used in the presence of intense γ-rays. The flux of these neutrons is calculated in terms of the rate of detection of the protons, the solid angle for proton collection, the mass per unit area of the polythene layer, and the neutron-proton-scattering cross-section. A study of the behaviour of proportional co-axial counters, used in this manner, has been made. A determination of the angular distribution of fast neutrons produced by a deuteron-deuterium source has been made by our coincidence method, and the results compared with those obtained by the ionization chamber method by Bretscher and French. Absolute values agree to within 10%.