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

This letter explains a new, highly sensitive method for the detection of neutrons, which uses the T${}_{1\mathrm{/}2}=845$ 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 $\mu \mathrm{s}$ 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, \o$, 63 , {\rm mm}\times 63, {\rm mm}$ 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, $4\pi$, thermal neutron flux of 1 $\mathrm{n}{\mathrm{c}\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, the absolute sensitivity of the NaI detector was found to be $6.5 \pm 1, {\rm counts}, {\rm s}^{-1}$withabackgroundof$0.8, {\rm events}, {\rm day}^{-1}$ and a delay time frame of $\mathrm{\Delta }\mathrm{t}=1\mu$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$^{-8}, {\rm n}, {\rm cm}^{-2}, {\rm s}^{-1}$.