Experimental and computational results on exciton/free-carrier ratio, hot/thermalized carrier diffusion, and linear/nonlinear rate constants affecting scintillator proportionality

Abstract

Models of nonproportional response in scintillators have highlighted the importance of parameters such as branching ratios, carrier thermalization times, diffusion, kinetic order of quenching, associated rate constants, and radius of the electron track. For example, the fraction η eh of excitations that are free carriers versus excitons was shown by Payne and coworkers to have strong correlation with the shape of electron energy response curves from Compton-coincidence studies. Rate constants for nonlinear quenching are implicit in almost all models of nonproportionality, and some assumption about track radius must invariably be made if one is to relate linear energy deposition dE/dx to volume-based excitation density n (eh/cm 3 ) in terms of which the rates are defined. Diffusion, affecting time-dependent track radius and thus density of excitations, has been implicated as an important factor in nonlinear light yield. Several groups have recently highlighted diffusion of hot electrons in addition to thermalized carriers and excitons in scintillators. However, experimental determination of many of these parameters in the insulating crystals used as scintillators has seemed difficult. Subpicosecond laser techniques including interband z scan light yield, fluence-dependent decay time, and transient optical absorption are now yielding experimental values for some of the missing rates and ratios needed for modeling scintillator response. First principles calculations and Monte Carlo simulations can fill in additional parameters still unavailable from experiment. As a result, quantitative modeling of scintillator electron energy response from independently determined material parameters is becoming possible on an increasingly firmer data base. This paper describes recent laser experiments, calculations, and numerical modeling of scintillator response.

Auger recombination in sodium-iodide scintillators from first principles

“…Comparing with the coefficients fitted to experimental data, we note that our calculated rate is several orders of magnitudes smaller than the value of 1.07 Â 10 À20 cm 6 s À1 obtained by Bizarri et al 8 We note however that recently more improved sets of rate equations and boundary conditions have been developed. 16,17,31 Comparing to more recent results from fits (C ¼ 3.2 Â 10 À29 cm 6 s À1 ) to z-scan experiments, our calculated values are off by a few decades. 16,17 This illustrates the difficulty in fitting a rate equation model to a very complex set of events and interdependent mechanisms.…”

“…16,17,31 Comparing to more recent results from fits (C ¼ 3.2 Â 10 À29 cm 6 s À1 ) to z-scan experiments, our calculated values are off by a few decades. 16,17 This illustrates the difficulty in fitting a rate equation model to a very complex set of events and interdependent mechanisms. While one possible cause for the difference between our results and experimental data is thallium doping present in NaI samples that were studied in experiment, we note that Refs.…”

“…AR is thought to reduce the highpower efficiency of light emitting diodes 13,14 and also may play an instrumental role in the non-proportionality problem of scintillators. 11,[15][16][17] In all of these cases, understanding the microscopic mechanism of AR is a prerequisite to improving the device performance.…”

“…On the other hand, a much smaller coefficient of 3.2 Â 10 À29 cm 6 s À1 was reported when modeling z-scan experiments using a pulsed laser. 16,17 FIG. 1.…”

Radiation response of inorganic scintillators: insights from Monte Carlo simulations

“…18,22,25 As described by Williams and co-workers in their scintillator physical "decision tree", 21 electrons and holes persist as free particles during thermalization in materials such as CsI and NaI due to their low-energy phonons. The resultant slow cooling rates, in turn, lead to third-order nonlinear quenching in regions with high electron-hole pair densities (dominant at low incident γ-ray energies) and extensive charge separation in low-density regions (dominant at high incident γ-ray energies).…”