Lithium alkali halides - New thermal neutron detectors with n-γ discrimination

We are investigating two promising new families of materials derived from proven, low-cost, well-understood, versatile scintillator hosts CsI and NaI. These are modified by incorporating Li ions to achieve the desired spectroscopic properties, producing high quality, combined n/γ sensors. Specifically we report on the synthesis and characterization of Li₃Cs₂I₅ (LCI) and Li_xNa_1-xI (LNI), both of which demonstrate pulse shape discrimination (PSD) and pulse height discrimination (PHD) for effective suppression of gamma background from neutron signals. In the case of LCI, the primary decay time for thermal neutron interactions is faster than for gamma interactions, and is on the order of 250 ns for neutrons and 500 ns for gamma rays. LNI is opposite to LCI in this respect, which shows slower, 210 ns, decay for neutron interactions and relatively faster, 180 ns, decay for gammas. The measured light yield for LCI is ~40,000 to 55,000 photons/neutron, which corresponds to an electron equivalent energy of 2.1 to 2.8 MeV. Whereas LNI demonstrates a much brighter yield of over 100,000 photons/neutron and electron equivalent energy per neutron interaction of over 4.5 MeV, very close to the ⁶Li(n,α) Q value of 4.7 MeV. Relatively narrow emission bands with a peak at 450 nm for LCI:Eu and at 420 nm for LNI:Tl make these sensors well matched to the quantum efficiency of conventional photodetectors such as PMTs. Our data show that significant red shift in emission can be achieved by doping LCI with Tl and LNI with Eu, making these materials well suited for use with such sensors as solid state photomultipliers (SSPMs). In addition to their excellent scintillation properties, the use of widely available, proven host materials, and a possible vapor deposition method for their synthesis, are promising features of this development. This combination allows mass production of large-area, high-performance n- utron sensors in a uniquely time efficient manner.