Nonlinear quenching rates in SrI₂ and CsI scintillator hosts

Abstract

Using 0.5 ps pulses of 5.9 eV light to excite electron-hole concentrations varied up to 2x10²⁰ e-h/cm³ corresponding to energy deposition within electron tracks, we measure dipole-dipole quenching rate constants K₂ in SrI₂ and CsI. We previously reported determination of K₂ directly from the time dependence of quenched STE luminescence in CsI. The nonlinear quenching rate decreases rapidly within a few tens of picoseconds as the host excitation density drops below the Förster threshold. In the present work, we measure the dependence of integrated light yield on excitation density in the activated scintillators SrI₂:Eu²⁺ and CsI:Tl⁺. The “z-scan” method of yield vs. irradiance is applicable to a wider range of materials, e.g. when the quenching population is not the main light-emitting population. Furthermore, because of using an integrating sphere and photomultiplier for light detection, the signal-to-noise is substantially better than the time-resolved method using a streak camera. As a result, both 2^nd and 3^rd orders of quenching (dipole-dipole and Auger) can be distinguished. Detailed comparison of SrI₂ and CsI is of fundamental importance to help understand why SrI₂ achieves substantially better proportionality than CsI in scintillator applications. The laser measurements, in contrast to scintillation, allow evaluating the rate constants of nonlinear quenching in a population which has small enough spatial gradient to suppress the effect of carrier diffusion.