Publications

Thermal neutron detection using alkali halide scintillators with Li-6 and pulse shape discrimination

Brubaker, Erik B.; Dibble, Dean C.; Mengesha, Wondwosen M.; Yang, Pin Y.

An ideal ³He detector replacement for the near- to medium-term future will use materials that are easy to produce and well understood, while maintaining thermal neutron detection efficiency and gamma rejection close to the ³He standard. Toward this end, we investigated the use of standard alkali halide scintillators interfaced with ⁶Li and read out with photomultiplier tubes (PMTs). Thermal neutrons are captured on ⁶Li with high efficiency, emitting high-energy and triton (³H) reaction products. These particles deposit energy in the scintillator, providing a thermal neutron signal; discrimination against gamma interactions is possible via pulse shape discrimination (PSD), since heavy particles produce faster pulses in alkali halide crystals. We constructed and tested two classes of detectors based on this concept. In one case ⁶Li is used as a dopant in polycrystalline NaI; in the other case a thin Li foil is used as a conversion layer. In the configurations studied here, these systems are sensitive to both gamma and neutron radiation, with discrimination between the two and good energy resolution for gamma spectroscopy. We present results from our investigations, including measurements of the neutron efficiency and gamma rejection for the two detector types. We also show a comparison with Cs₂LiYCl₆:Ce (CLYC), which is emerging as the standard scintillator for simultaneous gamma and thermal neutron detection, and also allows PSD. We conclude that ⁶Li foil with CsI scintillating crystals has near-term promise as a thermal neutron detector in applications previously dominated by ³He detectors. The other approach, ⁶Li-doped alkali halides, has some potential, but require more work to understand material properties and improve fabrication processes.