Sensitive neutron detection method using delayed coincidence transitions in existing iodine-containing detectors

This work explains a new, highly sensitive method for the detection of neutrons, which uses the T1/2=845ns delay in the decay of 128I at the 137.8 keV energy level, resulting from the capture of thermal neutrons by iodine nuclei in NaI and CsI scintillation detectors. The use of delayed coincidence techniques with a several μs delay time window for delayed events allows for the highly effective discrimination of neutron events from any existing background signals. A comparison of ambient neutron measurements between those identified through the suggested method from a cylindrical, ø63mm×63mm NaI(Tl) scintillator and those from a low-background proportional 3He counter experimentally demonstrates the efficacy of this neutron detection method. For an isotropic, 4π, thermal neutron flux of 1ncm−2s−1, the absolute sensitivity of the NaI detector was found to be 6.5±1countss−1 with an accidental coincidence background of 0.8eventsday−1 for any delay time window of Δt=1μs. The proposed method can provide low-background experiments, using NaI or CsI, with measurements of the rate and stability of incoming neutron flux to a greater accuracy than 10−8ncm−2s−1.