Dependence of nonproportionality in scintillators on diffusion of excitons and charge carriers

R. T. Williams; Qi Li; Joel Q. Grim; K. B. Ucer

doi:10.1117/12.895038

27 September 2011 Dependence of nonproportionality in scintillators on diffusion of excitons and charge carriers

R. T. Williams, Qi Li, Joel Q. Grim, K. B. Ucer

Proceedings Volume 8142, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XIII; 81420Y (2011) https://doi.org/10.1117/12.895038
Event: SPIE Optical Engineering + Applications, 2011, San Diego, California, United States

Abstract

The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at the root of nonproportionality depend respectively on the 4th or 6th power of the electron track radius if modeled as cylindrical. In an initial time interval τ when nonlinear quenching and diffusion compete to reduce the density of excited states, the track radius expands as (D_effτ)^1/2 where D_eff is the effective diffusion coefficient for the mixture of excitons and charge carriers. The range of D_eff across semiconductor and scintillator radiation detectors is large, illustrated by 8 decades between mobilities of self-trapped holes in CsI:Tl and holes in high purity Ge. We present the functional form of nonlinear quenching predicted by diffusive track dilution and show that the simple model provides a surprisingly good fit of empirical nonproportionality across a wide range of semiconductor and oxide radiation detectors. We also show how diffusion drives nonlinear branching between excitons and free carriers in the track when electron and hole mobilities are unequal, and that this nonlinear branching coupled with linear trapping on defects can produce the "halide hump" seen in electron yield data for activated halide scintillators. Picosecond time-resolved spectroscopy in alkali halides, as well as quantitative comparison of recently measured 2nd order quenching rate constants K₂ and results of K-dip spectroscopy, provide experimental benchmarks for consideration of carrier thermalization and the initial track or cluster radius r₀ from which (nearly thermalized) diffusion is assumed to commence. The ratio of initial rate of 2nd order quenching to that of dilution by diffusion in a cylindrical track is proportional to K₂/D_eff and does not depend on r₀ in lowest order; however, the absolute rates of both processes decrease with increasing r₀.

Citation Download Citation

R. T. Williams, Qi Li, Joel Q. Grim, and K. B. Ucer "Dependence of nonproportionality in scintillators on diffusion of excitons and charge carriers", Proc. SPIE 8142, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XIII, 81420Y (27 September 2011); https://doi.org/10.1117/12.895038

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