New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI₂, CaI₂, SrBr₂, BaI₂ and BaBr₂-SrI₂ is the most promising. SrI₂(Eu) emits into the Eu²⁺ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.

Homeland security applications often require detection of both neutron and gamma radiation. A combination of two detectors registering neutrons and gammas separately is typically used. Recently, a number of scintillators from the elpasolite crystal family were proposed, that provide detection of both types of radiation. The most promising are Cs ₂ LiYCl ₆ , Cs ₂ LiLaCl ₆ , and Cs ₂ LiLaBr ₆ . All are doped with Ce ³⁺ . They are capable of providing very high energy resolution. The best values achieved for each material are 3.9%, 3.4%, and 2.9% at 662 keV (FWHM), respectively. Since ⁶ Li has an acceptable cross-section for thermal neutron capture, these materials also detect thermal neutrons. In the energy spectra, the full energy thermal neutron peak typically appears above 3 MeV gamma equivalent energy. Thus very effective pulse height discrimination can be implemented with these materials. The CLLC and CLYC emissions consist of two main components: Core-to-Valence Luminescence (CVL; 220 nm to 320 nm) and Ce emission (350 to 500 nm). The former is of particular interest since it appears only under gamma excitation. It is also very fast and decays with less than 2 ns time constant. The CVL provides a significant difference to temporal responses under gamma and neutron excitation thus it may be used for effective pulse shape discrimination.

Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr₃) and cerium-activated lanthanum tribromide, their commercial availability and application are limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. This investigation employed aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca²⁺) was used as a dopant to strengthen CeBr₃ without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca²⁺ dopant were grown, i.e., 1.9% of the CeBr₃ molecules were replaced by CaBr₂ molecules, to match our target replacement of 1 out of 54 cerium atoms be replaced by a calcium atom. Precisely the mixture was composed of 2.26 g of CaBr₂ added to 222.14 g of CeBr₃. Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca²⁺-doped CeBr₃ exhibited little or no change in the peak fluorescence emission for 371 nm optical excitation for CeBr₃. The structural, electronic, and optical properties of CeBr₃ crystals were studied using the density functional theory within the generalized gradient approximation. Calculated lattice parameters are in agreement with the experimental data. The energy band structures and density of states were obtained. The optical properties of CeBr₃, including the dielectric function, were calculated.

In recent years, a number of materials from the elpasolite crystal family have been under development for either or both gamma ray and neutron detection. The scintillators show good energy resolution and thermal neutron detection efficiency. The latter is achieved due to the fact, that the selected compositions contain Li-6 ions. In order to effectively and reliably register both types of radiation, it is necessary to separate them through particle identification schemes. This can be accomplished using either pulse height or/and pulse shape discrimination, with the latter being more reliable. In this paper, we summarize our work and provide current status of pulse shape discrimination in the selected elpasolite scintillators. These include Cs₂LiYCl₆ (CLYC), Cs₂LiLaCl₆(CLLC), Cs₂LiLaBr₆(CLLB), and Cs₂LiYBr₆(CLYB).