Modeling scintillation light absorption and re-emission in SrI2(Eu) scintillators

Europium-doped strontium iodide has recently attracted interest as a scintillator for gamma-ray spectroscopy. Excellent energy resolution (2.6% FWHM at 662 keV) with SrI2(Eu) has been reported for small (<1 cm3) crystals. With larger crystals, however, substantial energy resolution degradation has been found. Proposed as a mechanism for explaining this phenomenon, “light trapping” suggests that scintillation photons generated in SrI2(Eu) could be absorbed back into the excited states of the Eu2+ activators and then re-emitted at a later time, thus prolonging the pulse decay time as well as increasing the probabilities of losses within the crystal bulk and at the interface of the crystal with outer reflectors. Varying pulse decay times and absorption losses would then produce varied pulse heights from pulse processing electronics with a fixed shaping time. In this paper we report an approach to modeling this “light trapping” mechanism, hoping to produce an explicit description of the connection between the trapping rate and increases and/or variations in decay times. Our model shows that decay times depend strongly on the ratio R of SrI2(Eu)'s optical absorption length to crystal dimension, with smaller R values producing longer decay times. Further, decay curves for small R values were best characterized by two decay components, whereas those from larger R values had only one. Using these results, we devised a digital pulse processing algorithm that can correct for decay time variations with better than 2% accuracy, offering an approach to recovering SrI2(Eu)'s inherently excellent energy resolution.