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

Abstract This work explains a new, highly sensitive method for the detection of neutrons, which uses the T 1 / 2 = 845 ns delay in the decay of 128 I 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 μ s 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, o 63 mm × 63 mm NaI(Tl) scintillator and those from a low-background proportional 3 He counter experimentally demonstrates the efficacy of this neutron detection method. For an isotropic, 4 π , thermal neutron flux of 1 n cm − 2 s − 1 , the absolute sensitivity of the NaI detector was found to be 6.5 ± 1 counts s − 1 with an accidental coincidence background of 0.8 events day − 1 for any delay time window of Δ t = 1 μ s . 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 − 8 n cm − 2 s − 1 .