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Charge carrier mobility and nonproportionality of LaBr$_3$:Ce scintillators

The nonproportional response and related energy resolution of LaBr$_3$:Ce$^3+$ scintillation crystals doped with different concentrations of cerium were studied between 80K and 450K. For Ce$^3+$ concentration of 5% and 30%, LaBr$_3$ showed best proportionality and energy resolution at 80K. For LaBr$_3$:0.2%Ce$^3+$ the best energy resolution and the lowest degree of nonproportional response were instead observed around room temperature. The experimental results were analyzed in terms of charge carrier mobility and using theory of carrier transport in wide band gap semiconductors. We found that scattering of carriers by both lattice and impurity are the key processes determining the particular temperature dependence of carrier mobility and ultimately the scintillation nonproportionality. The calculated maximum of the LaBr$_3$:0.2%Ce$^3+$ carrier mobility corresponds well with the experimentally observed minima of its degree of nonproportionality, when assuming about 100ppm ionized impurity concentration.