Collection of holes in thick TlBr detectors at low temperature

A 3.5×3.5×4.6mm3 thick TlBr detector with pixellated Au/Cr anodes made by Radiation Monitoring Devices Inc. was studied. The detector has a planar cathode and nine anode pixels surrounded by a guard ring. The pixel pitch is 1.0mm. Digital pulse waveforms of preamplifier outputs were recorded using a multi-channel GaGe PCI digitizer board. Several experiments were carried out at −20°C, with the detector under bias for over a month. An energy resolution of 1.7% FWHM at 662keV was measured without any correction at −2400V bias. Holes generated at all depths can be collected by the cathode at −2400V bias which made depth correction using the cathode-to-anode ratio technique difficult since both charge carriers contribute to the signal. An energy resolution of 5.1% FWHM at 662keV was obtained from the best pixel electrode without depth correction at +1000V bias. In this positive bias case, the pixel electrode was actually collecting holes. A hole mobility-lifetime of 0.95×10−4cm2/V has been estimated from measurement data.