Results from an investigation of the physical origins of nonproportionality in CsI(Tl)

The relative scintillation response per energy deposited by Compton electrons, or nonproportionality, has traditionally been considered an intrinsic scintillator property. However, such an interpretation is inconsistent with recent results that show nonproportionality to depend on external factors such as shaping time, temperature and supplier. Apparently, at least some of the overall nonproportionality has an extrinsic origin. In this work we describe the results from a suite of measurements designed to test the hypothesis that nonproportionality in CsI(Tl) material has an extrinsic component that correlates with impurity levels. Our choice of material was motivated by the excellent energy resolution observed in one bulk crystal (6.4%)—a marked departure from that measured with conventional CsI(Tl) stock (8–8.5%). Six bulk CsI(Tl) crystals were procured and diced into 44 wafers. Using X-ray fluorescence techniques no conclusive evidence for impurities was found in any of the wafers at the 1–50 ppm level. One crystal exhibited a distinct correlation among energy resolution, decay lifetimes, nonproportionality and a very low level of Tl doping.