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

Abstract

Radionuclide Identification Devices (RIDs) or Backpack Radiation Detection Systems (BRDs) are often equipped with NaI(Tl) detectors. We demonstrate that such instruments could be provided with reasonable thermal-and fast-neutron sensitivity by means of an improved and sophisticated processing of the digitized detector signals: Fast neutrons produce nuclear recoils in the scintillation crystal. Corresponding signals are detectible and can be distinguished from that of electronic interactions by pulse-shape discrimination (PSD) techniques as used in experiments searching for weakly interacting massive particles (WIMPs). Thermal neutrons are often captured in iodine nuclei of the scintillator. The gammaray cascades following such captures comprise a sum energy of almost 7 MeV, and some of them involve isomeric states leading to delayed gamma emissions. Both features can be used to distinguish corresponding detector signals from responses to ambient gamma radiation. The experimental proof was adduced by offline analyses of pulse records taken with a commercial RID. An implementation of such techniques in commercial RIDs is feasible.

Fast- and thermal-neutron detection with common NaI(Tl) detectors

“…The stripping procedure deployed for pileup detection and decomposition provides separate energies and trigger points for all detected sub-pulses. It is described in reference [14]. To identify the origin of the structures, we defined cuts in the PSD plot (Figure 1, right panel) and had a look the corresponding pulse shapes.…”

“…Meanwhile, the delayed-coincidence method has been refined and complemented with a triple-coincidence technique [14]. Gamma-ray cascades from 127 I (𝑛, 𝛾) 128 I sometimes involve not only the mentioned but one more isomeric level, the 167.4 keV (6 -) state of 128 I with 175 ns half-life, which feeds the 137.9 keV (4 -) level with a strongly converted 29.5 keV transition.…”

“…The half-lives of these states, and, consequently, the delays between subpulses of a corresponding signal, are comparable with the light decay time of NaI(Tl). An algorithm decomposing pulse pileups (instead of rejecting them) turned out to be the key for using delayed double-and triple-coincidence events for thermal-neutron detection in the presence of ambient gamma radiation [14].…”

“…The right panel of Figure 4 presents delay-time distributions between sub-pulses of double-or triple-coincidence events as shown in the right panel of Figure 2. The delays were derived from the individual sub-pulse trigger times provided by the pileup decomposition algorithm [14]. The time distributions comprise exponential components with half-lives in correspondence with the lifetimes of the isomeric states in 128 I discussed in Section 2.…”

“…This confirms their interpretation as neutron-capture events in 127 I involving one or both isomeric levels. Refence [14] explains in detail how such events could be selected and counted, supposed all pulse records were searched for delayed signal components and subsequently decomposed in their constituents. The latter is, however, computationally expensive and could, if implemented in the instruments' firmware and applied to all signals, reduce the achievable throughput.…”