A scintillation counter with neutron and gamma-ray discriminators

Abstract

Certain organic scintillators, notably anthracene, stilbene and quaterphenyl crystals and oxygen-free liquid scintillators, show an effectively longer scintillation decay time for heavily ionizing particles such as alpha-particles or protons than for electrons. A scintillation counter is described which distinguishes fast neutrons from gamma-rays by means of the different decay times of recoil proton and Compton electron scintillations respectively. Measurements of the proton-electron resolution for different scintillators are described. It is found, for example, that using a one inch thick stilbene crystal, 2 MeV neutrons may be detected with 9.5% efficiency while the detection efficiency for 2 MeV gamma-rays is reduced by decay time discrimination to less than 0.007%. An application of the counter for fission cross section measurements by fission neutron detection is described. The decay time properties of the scintillators are discussed and the longer decay time for protons (or alpha-particles) is interpreted in terms of slow (10 −7 sec) ion recombination processes in the scintillators; these processes produce a slow component in the scintillation decay and in a proton scintillation the proportion of slow component is more than in an electron scintillation 4).