The role of hole mobility in scintillator proportionality

Time-dependent radial diffusion and drift are modeled in the high carrier concentration gradient characteristic of electron tracks in scintillators and other radiation detector materials. As expected, the lower mobility carrier (typically the hole) controls the ambipolar diffusion. Carrier separation when electron and hole mobilities are unequal produces a built-in radial electric field near the track analogous to an n-intrinsic semiconductor junction. The diffusion is shown to have significant effects on both the low dE/dx and high dE/dx ends of electron light-yield curves and their respective contributions to nonproportionality. In CsI:Tl, it is shown that electron confinement toward the end of the track accentuates high-order quenching such as Auger recombination or dipole–dipole transfer, while in HPGe extremely rapid (<1 fs) dilution of carrier concentration by radial diffusion renders Auger quenching negligible. Separation of geminate carriers is accentuated in the beginning of the track if electron and hole mobilities are widely unequal as in CsI:Tl, leading to bimolecular recombination of trapped carriers by slower thermal hopping routes as the favored channel at low dE/dx.