SrI₂ scintillator for gamma ray spectroscopy

We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of SrI₂(Eu)-based gamma ray spectrometers. SrI₂(Eu) offers a light yield in excess of 100,000 photons/MeV and light yield proportionality surpassing that of Ce-doped lanthanum bromide. Thermal and x-ray diffraction analyses of SrI₂ and EuI₂ indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy. We have demonstrated energy resolution with SrI₂(4-6%Eu) of 2.6% at 662 keV and 7.6% at 60 keV with small crystals, while the resolution degrades somewhat for larger sizes. Our experiments suggest that digital techniques may be useful in improving the energy resolution in large crystals impaired by light-trapping, in which scintillation light is re-absorbed and re-emitted in large and/or highly Eu²⁺ -doped crystals. The light yield proportionality of SrI₂(Eu) is found to be superior to that of other known scintillator materials, such as LaBr₃(Ce) and NaI(Tl).