The stability of TlBr detectors at low temperature

Thallium bromide (TlBr) is a promising semiconductor detector material due to its high atomic number (Tl: 81, Br: 35), high density (7.56 g/cm 3) and wide band gap (2.68 eV). Current TlBr detectors suffer from polarization, which causes performance degradation over time when high voltage is applied. A 4.6-mm thick TlBr detector with pixellated anodes made by Radiation Monitoring Devices Inc. was used in the experiments. The detector has a planar cathode and nine anode pixels surrounded by a guard ring. The pixel pitch is 1.0-mm. Digital pulse waveforms of preamplifier outputs were recorded using a multi-channel GaGe PCI digitizer board for pulse shaping. Several experiments were carried out at À 20 1C while the detector was under bias for over a month. No polarization effect was observed and the detector's spectroscopic performance improved over time. Energy resolution of 1.5% FWHM at 662 keV has been measured without depth correction at À 2000 V cathode bias. Average electron mobility-lifetime of (5.7 7 0.8) Â 10 À 3 cm 2 /V has been measured from four anode pixels. Thallium bromide (TlBr) is a promising new semiconductor detector material for X-and gamma-ray applications for homeland security, astrophysics and medical imaging. It has a high gamma-ray stopping power due to its high density (7.56 g/cm 3) and high atomic number (Tl: 81, Br: 35). Also, its wide band gap (2.68 eV) makes it very suitable for room-temperature operation. However, current TlBr detectors suffer from polarization effects which degrade spectroscopic performance over time when bias voltage is applied. It has been suggested in Ref. [1] that ionic conductivity causes polarization in TlBr detectors. By applying bias voltage, Tl + and Br À ions are accumulated under the cathode and anode electrodes, respectively [2]. This causes a degradation in the electric-field intensity inside the detector; i.e. the polarization effect, and therefore signal amplitude reduces over time until complete breakdown of the electric field. It is shown that the polarization effect in TlBr detectors can be suppressed with Tl electrodes coated by Au contacts [2]. The polarization effect is also partially mitigated by reversing the polarity of the high bias applied to the detector [3]. It is shown in Ref. [4] that no polarization effect is observed with TlBr detectors at À 20 1C when high bias is applied for over 5 days. In this work, a 4.6-mm thick TlBr detector was kept under bias for …