Abstract
Pulse shape discrimination (PSD) is widely used in many inorganic scintillators, which have different decay components, such as BaF2 and CsI. Conversely, LaBr 3 :Ce is considered not having different components but has been proved to have the capability of discriminating gamma and alpha events using fast digitizers. The physical mechanism of such PSD capability of LaBr 3 :Ce still remains unclear. This paper describes the origin of very small pulse shape difference for incident particles with different linear energy transfer (dE/dx) or ionization density. We attribute the pulse shape difference to the second-order dipole-dipole Forster transfer of self-trapped excitons (STEs) and Ce3+* excited states, which also contributes to the non-proportionality. Coupled rate and transport equations, including the thermalization of electrons, nonlinear quenching of STEs, energy transfer from STE to Ce and the radiative decay of Ce concentration, are established. This model can explain the non-proportionality and the origin of the α/γ pulse shape difference.
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