WFU Physics Colloquium

TITLE: Understanding nonproportionality in radiation detectors: Experiment and numerical modeling together yield a breakthrough!

SPEAKERS: Qi Li and Joel Grim

TIME: Wednesday March 16, 2011 at 4:00 PM

PLACE: Room 101 Olin Physical Laboratory

Recently, experiment and numerical modeling at Wake Forest have produced a breakthrough in understanding the 50 year old problem of what material parameters control the ultimate resolution of scintillator radiation detectors, and physically how that comes about. Experimentally, measurements of nonlinear quenching with picosecond resolution focus attention on the first 10 picoseconds of a scintillation pulse. It is shown that even though the scintillation pulse may last for 10 microseconds or more, the first 10 picoseconds encompass nonlinear quenching and the important physical interactions that control it. Theoretically, it is shown that the ability of free carriers and excitons to escape the densely-excited particle track within this 10-ps nonlinear quenching window is the factor governing nonproportionality. Thus it is the effective diffusion coefficient of the mixture of excitons and free carriers in the track that is the important material parameter. The model includes a method for determining branching ratio between excitons and free carriers as a function of position along the track, and that branching depends only on the electron and hole masses. The only fitting parameter is an exciton scattering time, and it can be assumed constant for all materials in the fitted set of non-halide insulators and semiconductors. This model produces a remarkably accurate description for many oxide and semiconductor scintillators.

In the first part of the colloquium Qi Li will discuss the transport model that explains nonproportionality. In the second part, Joel Grim will discuss the quantitative measurement of the dipole-dipole nonlinear quenching rate constant on which much of the modeling rests and in addition will discuss the so-called Onsager criterion for exciton-free carrier branching in the appropriate context of the electrochemical potential landscape around a dense track, as well as kinetic Monte Carlo modeling of electron-hole pairing and transport. These new results build on WFU colloquia presented last fall by W. Setyawan from Duke on the band structures of the materials and K. B. Ucer from WFU on the foundations in the time-resolved measurements. Both Qi Li and Joel Grim will present their work at the 2011 Spring MRS meeting where Joel is a Graduate Student Award finalist.