Excitation density, diffusion‐drift, and proportionality in scintillators

Abstract

Phone: þ1 336 758 5132, Fax: þ1 336 758 6142.Stopping of an energetic electron produces a track of high excitation density, especially near its end, and consequent high radial concentration gradient. The effect of high excitation density in promoting nonlinear quenching is generally understood to be a root cause of nonproportionality in scintillators. However, quantitative data on the kinetic rates of nonlinear quenching processes in scintillators are scarce. We report experimental measurements of second-order dipole-dipole rate constants governing the main nonlinear quenching channel in CsI, CsI:Tl, NaI, and NaI:Tl. We also show that the second of the extreme conditions in a track, i.e., radial concentration gradient, gives rise to fast ( picoseconds) diffusion phenomena which act both as a competitor in reducing excitation density during the relevant time of nonlinear quenching, and as a determiner of branching between independent and paired carriers, where the branching ratio changes with dE/dx along the primary electron track. To investigate the interplay of these phenomena in determining nonproportionality of light yield, we use experimentally measured rate constants and mobilities in CsI and NaI to carry out quantitative modeling of diffusion, drift, and nonlinear quenching evaluated spatially and temporally within an electron track which is assumed cylindrical Gaussian in this version of the model. 1 Introduction When an energetic electron is slowed and stopped in a scintillator (or any solid material with a band gap), the track of excitations left behind is characterized by at least two remarkably extreme numbers: (i) very high excitation density and (ii) very large concentration gradient of carriers and/or excitations. The first condition of high excitation density has been recognized for some time as crucial for setting rates of second-order (dipole-dipole) and third-order (Auger) quenching of local light yield in the vicinity of the track. However, with very few exceptions, those rate constants have not been determined. Therefore we present measurements of dipole-dipole quenching rate constants at high excitation density for three halide scintillators in the present work.In addition to the promotion of nonlinear quenching by high excitation density in a track, the high concentration gradient promotes radial diffusion of electrons and holes, which can limit the nonlinear quenching rates by rapidly diluting carrier concentration, and may affect linear rates by setting up charge separation. We show by numerical modeling of the diffusion and quenching processes together

Improvement of γ-ray energy resolution of LaBr3:Ce3+ scintillation detectors by Sr2+ and Ca2+ co-doping

“…That means close (<5 nm) to the track, and since the energy loss dE/dx of the ionizing primary electron increases with decreasing electron velocity (or energy E), the recombination losses are highest at the end of the main track. 19 It is also high in the side tracks of secondary electrons created by "head-on collisions" of the primary electron with electrons of the scintillating medium. The stochastic nature of side track formation is the origin of R intr .…”

“…The charge carrier dynamics taking place on the nm length scale around the track and in the ps time scale are extremely complicated 19,22 and at this stage not fully understood. The current idea is that the nonradiative recombination rate C nr of free electrons and holes competes with the trapping rate C tr of those charge carriers by Ce 3þ .…”

Energy resolution and related charge carrier mobility in LaBr3:Ce scintillators

“…2,6,21,29,35,36 It is attributed to radiationless electron-hole pair recombination in the regions of a high concentration n(x) of charge carriers along the ionization track as shown in Fig. 1.…”

“…2,6,[21][22][23][24][25] This process together with an ionization density that changes along an electron track and with primary electron energy causes the deterioration of the energy resolution. To avoid the recombination losses, charge carriers should be effectively transferred from the primary track to luminescence centers.…”

“…An important factor determining the rate at which carriers leave this volume is the carrier diffusion coefficient. 2,6,23 A high diffusion coefficient contributes to a more rapid transport of electrons, holes and excitons to regions further from the track where the radiationless recombination rate does not depend on ionization density.…”