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Title: Monte Carlo simulation of electron thermalization in scintillator materials: Implications for scintillator nonproportionality

Abstract

The lack of reliable quantitative estimates of the length and time scales associated with hot electron thermalization after a gamma-ray induced energy cascade obscures the interplay of various microscopic processes controlling scintillator performance and hampers the search for improved detector materials. We apply a detailed microscopic kinetic Monte Carlo model of the creation and subsequent thermalization of hot electrons produced by gamma irradiation of six important scintillating crystals to determine the spatial extent of the cloud of excitations produced by gamma rays and the time required for the cloud to thermalize with the host lattice. The main ingredients of the model are ensembles of microscopic track structures produced upon gamma excitation (including the energy distribution of the excited carriers), numerical estimates of electron-phonon scattering rates, and a calculated particle dispersion to relate the speed and energy of excited carriers. All these ingredients are based on first-principles density functional theory calculations of the electronic and phonon band structures of the materials. The details of the Monte Carlo model are presented along with the results for thermalization time and distance distributions. Here, these results are discussed in light of previous work. It is found that among the studied materials, calculated thermalization distancesmore » are positively correlated with measured nonproportionality. In the important class of halide scintillators, the particle dispersion is found to be more influential than the largest phonon energy in determining the thermalization distance.« less

Authors:
ORCiD logo [1]; ORCiD logo [1];  [1];  [2];  [1]
  1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
  2. Univ. of Michigan, Ann Arbor, MI (United States)
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1415294
Report Number(s):
PNNL-SA-128275
Journal ID: ISSN 0021-8979
Grant/Contract Number:
AC05-76RL01830
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Applied Physics
Additional Journal Information:
Journal Volume: 122; Journal Issue: 23; Journal ID: ISSN 0021-8979
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS

Citation Formats

Prange, Micah P., Xie, YuLong, Campbell, Luke W., Gao, Fei, and Kerisit, Sebastien. Monte Carlo simulation of electron thermalization in scintillator materials: Implications for scintillator nonproportionality. United States: N. p., 2017. Web. doi:10.1063/1.4998966.
Prange, Micah P., Xie, YuLong, Campbell, Luke W., Gao, Fei, & Kerisit, Sebastien. Monte Carlo simulation of electron thermalization in scintillator materials: Implications for scintillator nonproportionality. United States. doi:10.1063/1.4998966.
Prange, Micah P., Xie, YuLong, Campbell, Luke W., Gao, Fei, and Kerisit, Sebastien. 2017. "Monte Carlo simulation of electron thermalization in scintillator materials: Implications for scintillator nonproportionality". United States. doi:10.1063/1.4998966.
@article{osti_1415294,
title = {Monte Carlo simulation of electron thermalization in scintillator materials: Implications for scintillator nonproportionality},
author = {Prange, Micah P. and Xie, YuLong and Campbell, Luke W. and Gao, Fei and Kerisit, Sebastien},
abstractNote = {The lack of reliable quantitative estimates of the length and time scales associated with hot electron thermalization after a gamma-ray induced energy cascade obscures the interplay of various microscopic processes controlling scintillator performance and hampers the search for improved detector materials. We apply a detailed microscopic kinetic Monte Carlo model of the creation and subsequent thermalization of hot electrons produced by gamma irradiation of six important scintillating crystals to determine the spatial extent of the cloud of excitations produced by gamma rays and the time required for the cloud to thermalize with the host lattice. The main ingredients of the model are ensembles of microscopic track structures produced upon gamma excitation (including the energy distribution of the excited carriers), numerical estimates of electron-phonon scattering rates, and a calculated particle dispersion to relate the speed and energy of excited carriers. All these ingredients are based on first-principles density functional theory calculations of the electronic and phonon band structures of the materials. The details of the Monte Carlo model are presented along with the results for thermalization time and distance distributions. Here, these results are discussed in light of previous work. It is found that among the studied materials, calculated thermalization distances are positively correlated with measured nonproportionality. In the important class of halide scintillators, the particle dispersion is found to be more influential than the largest phonon energy in determining the thermalization distance.},
doi = {10.1063/1.4998966},
journal = {Journal of Applied Physics},
number = 23,
volume = 122,
place = {United States},
year = 2017,
month =
}

Journal Article:
Free Publicly Available Full Text
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  • The thermalization of epithermal electrons due to the rotationally inelastic and elastic collisions in molecular nitrogen is studied with the Monte Carlo simulation. The elastic and rotational cross sections are taken as the experimental momentum-transfer cross section and the Gerjuoy--Stein formula with the effective quadrupole moment, respectively, which are consistent with the swarm data. The accuracy of the approximate theory based on the assumption of the local Maxwell electron velocity distribution (MD) is examined, where the initial electron velocity distribution is taken to be the MD. The electron velocity distribution deviates significantly from the MD; consequently the degradation of themore » effective electron temperature (reduced mean electron energy), which is dominated by the rotationally inelastic collisions, is slower than that for the MD and the pressure normalized thermalization time tau/sub th/ p is about 20% larger than that for the MD. The degradation and tau/sub th/ p are compared with those calculated from the experimental energy exchange rate coefficient, relaxation time, or relaxation rate.« less
  • The Monte Carlo simulation (MCS) of the thermalization of low-energy electrons (epsilon< or approx. =0.1 eV) due to the rotationally inelastic and elastic collisions in normal H/sub 2/ (J. Chem. Phys. 79, 3367 (1983), referred to as I) is extended to high-energy subexcitation electrons (epsilonapprox.1 eV) by taking into account the vibrationally inelastic collisions and using available experimental cross section data. The MCS is performed for the thermalization of subexcitation electrons with the initial Maxwell, delta function, or Platzman velocity distribution at the initial effective electron temperature 10/sup 3/< or =T/sub e/(0)< or =3 x 10/sup 4/ K in normalmore » H/sub 2/ at the gas temperature 77< or =T< or =10/sup 3/ K. The electron velocity distribution deviates significantly from the local Maxwell distribution (MD) even for the initial Maxwell distribution owing to the vibrationally and rotationally inelastic collisions. Consequently, the degradation of the effective electron temperature T/sub e/ (reduced mean electron energy) is slower than that obtained with the MD assumption. The thermalization time tau/sub th/ when T/sub e//T = 1.1 is insensitive to the initial electron velocity distribution and effective electron temperature. At T> or approx. =300 K, tau/sub th/ is about 70% larger than that for the MD, where tau/sub th/ is dominated by the rotationally inelastic collisions. At the low gas temperature T = 77 K, tau/sub th/ is about 160% larger than that for the MD, where tau/sub th/ is dominated by the elastic collisions.« less
  • We use an ensemble Monte Carlo technique to model the thermalization of electron-hole plasmas following a laser excitation. For concreteness, we concentrate on the results of two recent experiments. Our calculations quantitatively confirm the existence of separate effective electron and hole temperatures during the first 10 ps in Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As. The carrier cooling can be explained by invoking both nonequilibrium phonons and carrier degeneracy. Comparison with a band-edge luminescence experiment brings out features concerning the electron-hole and intervalley scattering contributions.
  • A direct comparison is made between the transient imaginary component of the microwave conductivity during the electron thermalization in Ar obtained by a Monte Carlo simulation (MCS) and a microwave frequency-shift measurement. Both the MCS and experimental conductivities indicate the peak pattern due to the existence of the Ramsauer minimum, and the overall agreement between theory and experiment shows considerable improvement as compared with the previous discrepancy found in the comparison of the electron temperature (energy) degradation in rare gases with Ramsauer minima. The agreement is better for the Margenau conductivity formula than for the alternative one, contrary to themore » case of He. There still exists, however, some disagreement; the MCS value of the thermalization time for the conductivity to reach within 1% of the thermal value is 24 ..mu..s, while the experimental value is 16 ..mu..s for the gas temperature T = 291 K, pressure p = 309 Torr, and the radian microwave frequency ..omega../2..pi.. = 9.461 GHz. Some possible causes of the disagreement are briefly discussed.« less