Authors: S. R. Bishop and H. L. Tuller
Sponsor: Radiation Monitoring Devices, Inc.

High energy radiation (e.g. γ-rays) is detected by wide-band gap semiconductors operating under ambient conditions.  When the detector intercepts a γ-ray, electron – hole pairs are formed resulting in an increase in electrical conductivity.  A high dark conductivity is required to maximize detector response.  Current semiconductor technologies require cooling to very low temperatures which adds to cost and reduces portability.  TlBr is an attractive detector material because given its low dark conductivity at room temperature and its high mass density leading to higher radiation absorption.

In this project, we characterize the dominant ionic conduction properties in TlBr by impedance spectroscopy.  TlBr is primarily a Schottky type ionic conductor, meaning that Tl and Br move through the material by vacancy motion.  By doping the material with both donors and acceptors, and measuring the bulk conductivity over a wide range of temperatures, we have been able to develop a detailed defect model and, on this basis, a doping strategy to minimize the dark conductivity.