Thallium bromide and thallium bromoiodide gamma ray spectrometer development

Thallium bromide (TlBr) is of interest as a material for room temperature gamma ray spectroscopy due to its high density, high Z and wide bandgap. In addition, its cubic crystal structure and relatively low melting point facilitate crystal growth by melt techniques. Recent advances in material purification, crystal growth and device processing have led to mobility-lifetime products of electrons in the mid 10^-3 cm²/V range enabling working detectors up to 15-mm thick to be fabricated. Although performance of TlBr devices is promising, long term detector stability at room temperature is an issue. We are investigating various compositions of the ternary compound, thallium bromoiodide (TlBr_xI_1-x) to vary the band gap and determine the effect of added thallium iodide (TlI) on detector performance. In this paper we report on our recent progress in TlBr gamma-ray spectrometer development as well as our initial results from TlBr_xI_1-x devices. Results from TlBr detectors up to 15-mm thick will be presented including depth corrected pulse height spectra. Detector stability will also be discussed. This work is being supported by the Domestic Nuclear Detection Office (DNDO).