Joel Q. Grim

A transport-based model of material trends in nonproportionality of scintillators

Electron-hole pairs created by the passage of a high-energy electron in a scintillator radiation detector find themselves in a very high radial concentration gradient of the primary electron track. Since nonlinear quenching that is generally regarded to be at the root of nonproportional response depends on the fourth or sixth power of the track radius in a cylindrical track model, radial diffusion of charge carriers and excitons on the ∼10 picosecond duration typical of nonlinear quenching can compete with and thereby modify that quenching. We use a numerical model of transport and nonlinear quenching to examine trends affecting local light yield versus excitation density as a function of charge carrier and exciton diffusion coefficients. Four trends are found: (1) nonlinear quenching associated with the universal “roll-off” of local light yield versus dE/dx is a function of the lesser of mobilities μe and μh or of DEXC as appropriate, spanning a broad range of scintillators and semiconductor detectors; (...

The Origins of Scintillator Non-Proportionality

Recent years have seen significant advances in both theoretically understanding and mathematically modeling the underlying causes of scintillator non-proportionality. The core cause is that the interaction of radiation with matter invariably leads to a non-uniform ionization density in the scintillator, coupled with the fact that the light yield depends on the ionization density. The mechanisms that lead to the luminescence dependence on ionization density are incompletely understood, but several important features have been identified, notably Auger-like processes (where two carriers of excitation interact with each other, causing one to de-excite non-radiatively), the inability of excitation carriers to recombine (caused either by trapping or physical separation), and the carrier mobility. This paper reviews the present understanding of the fundamental origins of scintillator non-proportionality, specifically the various theories that have been used to explain non-proportionality.

Picosecond Studies of Transient Absorption Induced by BandGap Excitation of CsI and CsI:Tl at Room Temperature

Abstract-We report picosecond time-resolved measurements of optical absorption induced by a sub-picosecond pulse of light producing two-photon bandgap excitation of Csl and CsI:Tl at room temperature. The transient spectrum of undoped Csl reveals for the first time strong infrared absorption rising through the 0.8-eV limit of present measurements. We suggest that this infrared band is due to transitions of the bound electron in the off-center self-trapped exciton (STE), implying that there should be a band deeper in the infrared associated with the known on-center STE in Csl. Previously reported visible and ultraviolet transient absorption bands at 1.7, 2.5, and 3.4 eV are confirmed in these measurements as attributable to hole excitations of STE. In 0.3% thallium doped Csl, infrared absorption possibly attributable to STEs is observed for approximately the first 5 ps after excitation at room temperature, but decays quickly. The absorption bands of Tl0 (electron trapped at Ti+ activator) and of self-trapped holes are the main species seen at longer times after excitation, during which most of a scintillation pulse would occur. This is in accord with a recently published report of nanosecond induced absorption in CsI:Tl.

Kinetic Monte Carlo Simulations of Scintillation Processes in NaI(Tl)

Developing a comprehensive understanding of the processes that govern the scintillation behavior of inorganic scintillators provides a pathway to optimize current scintillators and allows for the science-driven search for new scintillator materials. Recent experimental data on the excitation density dependence of the light yield of inorganic scintillators presents an opportunity to incorporate parameterized interactions between excitations in scintillation models and thus enable more realistic simulations of the nonproportionality of inorganic scintillators. Therefore, a kinetic Monte Carlo (KMC) model of elementary scintillation processes in NaI(Tl) is developed in this paper to simulate the kinetics of scintillation for a range of temperatures and Tl concentrations as well as the scintillation efficiency as a function of excitation density. The ability of the KMC model to reproduce available experimental data allows for elucidating the elementary processes that give rise to the kinetics and efficiency of scintillation observed experimentally for a range of conditions.

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

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 ηeh 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/cm3) 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.

Role of carrier diffusion and picosecond exciton kinetics in nonproportionality of scintillator light yield

The effect of high excitation density in promoting nonlinear quenching that is 2nd or 3rd order in electron-hole density is generally understood to be a root cause of nonproportionality in scintillators. We report and discuss quantitative data on just how fast these nonlinear channels are in specific cases. Kinetic rate constants for the creation of excitons from electrons and holes and for their quenching by dipole-dipole transfer have been measured in CsI and NaI. We show in addition that the strong radial concentration gradient in an electron track gives rise to fast (~ picoseconds) diffusion phenomena that act both as a competitor in reducing excitation density during the relevant time of nonlinear quenching, and as a determiner of branching between independent carriers and pairs (excitons), where the branching ratio changes along the primary electron track. We use the experimentally measured nonlinear quenching rate constants and values of electron and hole carrier mobilities to carry out quantitative modeling of diffusion, drift, and nonlinear quenching evaluated spatially and temporally within an electron track which is assumed cylindrical in this version of the model. Magnitude and inequality of electron and hole mobilities has consequences for quenching and kinetic order that vary with dE/dx along the path of an electron and therefore affect nonproportionality. It will be demonstrated that in a material with high mobilities like high-purity germanium, Auger recombination is effectively turned off by diffusive carrier dilution within < 1 fs in all parts of the track. In alkali halide scintillators like CsI and CsI:Tl, electron confinement and high-order quenching are accentuated toward the end of a particle track because of hole self-trapping, while separation of geminate carriers is accentuated toward the beginning of the track, leading to 2nd order radiative recombination and opening additional opportunities for linear trapping.

Scintillation Detectors of Radiation: Excitations at High Densities and Strong Gradients

This chapter discusses the electron-hole recombination processes that occur in the high excitation densities and strong radial gradients of particle tracks in scintillator detectors of radiation. The particle tracks are commonly those of high-energy Compton- or photo-electrons produced in energy-resolving gamma-ray detectors, but could also include those of heavier charged particles such as those following interaction with neutrons. In energy-resolving radiation detectors, intrinsic proportionality of light yield to gamma ray energy or electron energy is an important concern. This chapter gives special emphasis to understanding the physical basis for nonproportionality, while reviewing recent results on fundamental physics of nonlinear quenching, cooling and capture of hot electrons, co-evolving free-carrier and exciton populations, and diffusion in the dense and highly structured excitation landscape of electron tracks. Particular attention is paid to short-pulse laser experiments at Wake Forest University giving data and insight on the above phenomena complementary to more traditional scintillator experiments using gamma-ray or electron excitation. Numerical modeling of diffusion, nonlinear quenching (NLQ), exciton formation, and linear capture processes serves to test and establish links between the laser excitation and particle excitation measurements.

Intraneuronal vesicular organelle transport changes with cell population density in vitro

Primary neuron cultures are widely used in research due to the ease and usefulness of observing individual cells. Therefore, it is vital to understand how variations in culture conditions may affect neuron physiology. One potential variation for cultured neurons is a change in intracellular transport. As transport is necessary for the normal delivery of organelles, proteins, nucleic acids, and lipids, it is a logical indicator of a cells physiology. We test the hypothesis that organelle transport may change with varying in vitro population densities, thus indicating a change in cellular physiology. Using a novel background subtraction imaging method we show that, at 5 days in vitro (DIV), transport of vesicular organelles in embryonic rat spinal cord neurons is positively correlated with cell density. When density increased 6.5-fold, the number of transported organelles increased 2.2+/-0.3-fold. Intriguingly, this effect was not observable at 3-4 DIV. These results show a significant change in cellular physiology with a relatively small change in plating procedure; this indicates that cells appearing to be morphologically similar, and at the same DIV, may still suffer from a great degree of variability.