Accurate Determination of the Ionization Energy in Pixelated TlBr Correcting for Charge Collection Efficiency

Thallium-bromide (TlBr) is currently under investigation for use as an alternative room-temperature semiconductor gamma-ray spectrometer. Performance of better than 1% full-width at half maximum at 662 keV can be achieved when TlBr detectors are cooled to −20 °C. The theoretical limit of energy resolution is determined by the ionization energy in semiconductor detectors, and accurately measuring it is important for determining the best possible performance. One method to determine the ionization energy of semiconductor detectors compares pulse heights obtained from the semiconductor to pulse heights from a silicon detector. Due to their higher trapping, the charge collection efficiency (CCE) of TlBr is significantly lower than it is in silicon, therefore a correction to the ionization energy must be made. In this paper, we present the theory and measurement for accurately determining the CCE using the Shockley-Ramo theorem and apply it to measurement of ionization energy in pixelated TlBr detectors. The ionization energy of two TlBr detectors is measured and found to be 4.83(8) and 5.49(10) eV for the two samples at room temperature and −20 °C, respectively.