Developing Larger TlBr Detectors—Detector Performance

Thallium bromide (TlBr) is a high atomic number (81, 35), dense (7.56 g/cm³) wide band gap (2.68 eV) semiconductor. In addition, TlBr has a cubic crystal structure and melts congruently at a relatively low temperature (~460 C). Recently, mobility-lifetime product of electrons in TlBr has been reported to be greater than 0.001 cm²/V. These properties make TlBr a promising material for room temperature gamma radiation detection. Employing device designs such as small pixel arrays that depend primarily on the motion of a single carrier type allows fabrication of thicker devices with better energy resolution than planar devices of the same thickness. We report on our recent progress in developing larger TlBr detectors. Over the past several months we have increased the electron mobility-lifetime product of our TlBr by more than one order of magnitude. Electron mobility-lifetime values as high as 3.0 times 10^-3 cm²/V have been measured. Devices with small pixel design have been built with 3, 5, and 10 mm thickness and pixel pitch of 1 mm, 1.5, and 2.0 mm respectively. Pulse height spectra have been recorded over a range of energies from 60 keV to 662 keV. Energy resolution (FWHM) as high as approximately 5% at 122 keV and 1.7% at 662 keV has been obtained without any 3-D corrections. Such arrays are well suited for 3-D correction techniques similar to those applied to CZT devices, indicating that further improvement in energy resolution should be achievable. These latest results demonstrate promise for TlBr as a room temperature semiconductor gamma ray detector.