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

Abstract

This letter 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 $\hat{I}¼\mathrm{s}$ time frame 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, ø$63\mathrm{mm}\times 63\mathrm{mm}$ 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, $4\ddot{I}€$, thermal neutron flux of 1 $\mathrm{n}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, the absolute sensitivity of the NaI detector was found to be $6.5\pm 1\mathrm{counts}{\mathrm{s}}^{-1}$ with a background of $0.8\mathrm{events}{\mathrm{day}}^{-1}$ and a delay time frame of $\hat{I}”\mathrm{t}=1\hat{I}¼$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${}^{-8}\mathrm{n}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$.

accepted for publication / NIM A