Nonlinear quenching rates in SrI 2 and CsI scintillator hosts
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Nonlinear quenching rates in SrI2 and CsI scintillator hosts Joel Q. Grim, Qi Li, K.B. Ucer, R.T. Williams, Wake Forest University A. Burger, P. Bhattacharya, E. Tupitsyn, Fisk University G. A. Bizarri, W.W. Moses, Lawrence Berkeley National Laboratory 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 2nd and 3rd 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. INTRODUCTION Scintillators used as gamma-ray spectrometers can provide element and isotopespecific identification of material, as well as imaging or spatial localization of the information when used in suitable pixilated configurations. The essential function of the scintillator in spectroscopy mode is to produce a photopeak of detectable low-energy photons whose number is proportional to the energy of the incoming gamma-ray. If the light yield (photons/MeV incident) is a constant, the scintillator material would have perfect intrinsic proportionality. But neither the generation of excited states, nor the subsequent photoelectron path which may branch randomly into lower-energy scattered electron “delta rays”, is the same from event to event. But it is well known that the rate of linear energy deposition along the track is a strong function of the electron energy as it slows. The energy deposition rate starts small at high electron energy, and increases to a very high rate around 100 eV. This general behavior is well known to surface scientists, for example, where it accounts for the so-called “universal curve of electron mean free path in solids” that reaches a minimum of about 0.3 nm near 100 eV in almost all materials. In the scintillator context a statement of the same phenomenon is that near a t
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