Material parameter basis for major and minor trends in nonproportionality of scintillators
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Material parameter basis for major and minor trends in nonproportionality of scintillators Qi Li, Joel Q. Grim, R. T. Williams Department of Physics, Wake Forest University, Winston-Salem, NC 27109 G. A. Bizarri, W. W. Moses Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Abstract We have previously described a numerical model for carrier diffusion and nonlinear quenching in the track of an electron in a scintillator. Significant inequality of electron and hole mobilities predicts a characteristic “hump” in the light yield vs gamma energy, whereas low mobility of either or both carriers accentuates the universal roll-off due to nonlinear quenching at low gamma energy (high dE/dx). The material parameter basis of the two major trends in nonproportionality of scintillators can be related to the effective diffusion coefficient of excitations and the difference of electron and hole mobilities, respectively. Activator concentration, type of activator, and effect of transport anisotropy are associated with minor trends. The predicted trends are qualitatively consistent with empirical measures of nonproportionality including electron yield curves. Keywords: Scintillators, nonproportionality, carrier diffusion, mobility, nonlinear quenching INTRODUCTION Intrinsic nonproportionality is a material dependent phenomenon that sets an ultimate limit on energy resolution of radiation detectors. In general, anything that causes light yield to change along the particle track (e.g., the primary electron track in Ȗ-ray detectors) contributes to nonproportionality. It is widely accepted that nonlinear quenching is a root cause of scintillator nonproportionality. However, the question remains: “What are the independently measurable and calculable material parameters that control nonlinear quenching?” Payne et al [1] have fit electron yield data from the SLYNCI (Scintillator Light Yield Nonproportionality Characterization Instrument) experiment for a number of scintillator materials using two empirical fitting parameters: a “Birks parameter” meant to characterize how strong the 2nd order dipole-dipole quenching term is, and a fraction Șe/h of the initial electron-hole excitations that branch into independent carriers rather than excitons. They have investigated a number of material trends in SLYNCI electron yield data including dependence on the host crystal characteristic, activator concentration, and activator type. [1,2] We have recently presented a numerical model showing how mobilities ȝe, ȝh [3,4], and corresponding diffusion coefficients for electrons, holes and excitons De, Dh, DEXC [5] give a quantitative account of nonproportionality in oxide scintillators and semiconductor radiation detectors, and a qualitative account of characteristic trends of light yield in halide scintillators. In this paper, we examine whether the empirical trends in nonproportionality and energy resolution described in Refs. [1 and 6] can be described by the transport and nonlinear quenching model. Setyawan et al have investigated the link bet
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