title: Evaluation of Thallium Bromide as a Room Temperature Gamma-ray Detector
Creators: name: Voyce, Olivia
Publisher: name: University of Liverpool Repository
Year: 2023

description: Gamma-ray spectroscopy is a common non-destructive assay method used in the identification and characterisation of radioactive samples at nuclear sites. Room temperature semiconductor detectors (RTSD) are often deployed for portable gamma-ray detection during nuclear decommissioning. Thallium bromide (TlBr) is a novel RTSD which offers enhanced sensitivity over competing materials, such as cadmium zinc telluride (CZT), due to its increased atomic number and density. However, the longevity of TlBr devices is limited by ionic polarization of the crystal when an external bias is applied, which leads to the corrosion of the metal electrodes. The work reported here aimed to develop TlBr devices with metal oxide contacts to suppress the polarization effect and extend device lifetime. Monte Carlo simulations, using the Geant4 toolkit, were used to compare the sensi- tivity of TlBr and CZT devices to gamma-rays with energies of interest to nuclear assay (0 to 1.4 MeV). It was found that gamma-rays within this energy range typically penetrate much further into the CZT crystal whilst undergoing a larger fraction of multiple scatter- ing events. It was also shown that TlBr sensors with dimensions of 1.0 × 1.0 × 0.4 cm3 were as efficient at detecting 662.0 keV gamma-rays compared to 1.0 × 1.0 × 1.0 cm3 of CZT. These results were used to inform the design of devices used in experimental studies. A suite of thin TlBr samples were fabricated by the author with In, indium tin oxide (ITO) and SnO2 electrodes. The samples underwent X-ray photoelectron spectroscopy (XPS) and it was established, using the Kraut method of band alignment, that a type II staggered heterojunction formed at the metal oxide/TlBr interfaces. Current-voltage measurements determined that these contacts acted Ohmically in the ranges ± 100 V and that they had very low leakage currents. Due to the fact that metal oxides are less reactive than pure metals, and because of their comparable electronic behaviour, metal oxides may be used in place of metal as contacts on TlBr radiation detectors. Finally, In and ITO contacted TlBr devices (with dimensions 1.0 × 1.0 × 0.4 cm3) were fabricated by scientists at CapeSym, Inc. and underwent radiometric testing by the author at the University of Liverpool. The noise sensitivity of both detectors was increased by a voltage dependent electric field breakdown, which may have been caused by the polarisation phenomena. Both detectors displayed a range of signal amplitudes after being exposed to Cs-137 radiation, suggesting that charge had not been completely collected. This is most likely because the saturation voltage could not be reached before the electric field breakdown, as well as the presence of trapping centers within the crystal bulk. Energy spectra was collected for the In/TlBr device during irradiation from Cs-137 and Na-22 source. The full energy photopeak could not be resolved, but the shifting Compton edge illustrated the energy sensitivity of the device. These findings demonstrate that this thesis’ technique for developing energy sensitive TlBr detectors was successful. However, further work needs to be taken to improve charge collection within TlBr detectors and to compare the spectral response of metal and metal oxide contacted devices.