A Pulsed Bias Voltage Shutdown Circuit for a Matrix Readout System of a CdTe Radiation Detector

A pulsed bias voltage shutdown circuit to suppress the polarization effect in a CdTe radiation detector equipped with a matrix readout system was investigated. The matrix readout system is suitable for radiation detection systems that have many pixels and that are used at a relatively low counting rate, such as single photon emission computed tomography (SPECT). CdTe detectors have a time-dependent polarization effect that must be suppressed. One method to suppress this effect is pulsed bias voltage shutdown. However, on the bias voltage supply side of the matrix readout system, the speed of the voltage change depends on a time constant consisting of a bias resistor and a coupling capacitor. Therefore, the voltage cannot be changed within the time constant for the matrix readout system. To overcome this limitation, three circuits have been added: a clamp circuit to the noise filter resistor of the bias voltage supply, a clamp circuit to the bias resistor, and protection circuits at the charge amplifier inputs. These circuits make it possible to change the bias voltage faster than the time constant of the bias resistor and the coupling capacitor. Although a noise signal is observed after the bias voltage has recovered, the noise decays below 20 keV about 30 ms after the bias voltage recovery. Our results demonstrated that the proposed pulsed bias voltage shutdown method is very efficient to suppress the polarization effect within 100 ms for the CdTe detector with the matrix readout system.