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 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, ø 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, 4π, 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 ∆t = 1 µs. 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 .

Neutron Detection in Mixed Neutron-Gamma Fields with Common NaI(Tl) Detectors

“…The plot shows a crest at 𝐸 * ≈ 150 keV with fading intensity towards rising 𝑡 *) in accordance with a half-life around 1 𝜇𝑠. Delayed coincidence events as observed by Yakushev et al [16] should appear as a ridge around 𝐸 * =138 keV. The shift towards larger energies is caused by the scintillator's nonproportional light yield.…”

“…A second method, discussed by Yakushev et al in 2017 [16], exploits delayed coincidences of consecutive signals in a single NaI(Tl) crystal. As known from previous work by Sakharov et al [17], gamma-ray cascades following neutron capture in 127 I sometimes involve the 137.8 keV state in 128 I, which has a long half-life of 845 ns and decays to ground state via sequential emission of low-energy gamma rays or conversion electrons [18].…”

“…The method failed, however, in gamma radiation fields causing random delayed coincidences between gamma-ray signals that finally cover the neutron signature. One obvious reason was the independent handling of prompt and delayed signals combined with the 1.8 𝜇s trigger hold-off [16], which reduced the rate of detectible delayed signals with respect to the original rate at the given 845 ns half-life by a factor of 4-5.…”

“…The paper demonstrates that neutron detection with common NaI(Tl) detectors based on Yakushev's delayed coincidence method [16] can be effectively improved by decomposing piled-up signals into their constituents instead of considering only non-overlapping pulse pairs. It also introduces and substantiates a complementary method based on delayed triple coincidences, distinguished by higher selectivity and better robustness against external gamma load.…”

Thermal Neutron Detection in Mixed Neutron-Gamma Fields With Common NaI(Tl) Detectors

“…• The first method, proposed in 2009 and named Neutron Capture Detector (NCD) [9]- [10], considers a high energy deposition in the scintillation crystal as distinctive feature. [14].…”