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

An ideal He-3 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 He-3 standard. Toward this end, we are investigating the use of standard alkali halide scintillators interfaced with Li-6 and read out with photomultiplier tubes (PMTs). Thermal neutrons are captured on Li-6 with high efficiency, emitting high-energy alpha and triton (H-3) 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 inorganic scintillating crystals. We constructed and tested two classes of detectors based on this concept. In one case Li-6 is used as a dopant in polycrystalline NaI; in the other case a thin Li foil is used as a conversion layer. We present results from these investigations, including measurements of the neutron efficiency and gamma rejection for the two detector types.