An alternative pulse height correction method for pole zero cancellation circuitry

CdTe diode radiation detectors have a problem called the “polarization effect”, which causes CdTe performance to degrade with time. The pulsed bias voltage shutdown technique, in which a bias voltage is turned off and on quickly to recombine the accumulated charges, is used to suppress the effect. When this technique is used with a charge sensitive amplifier equipped with a pole zero cancellation (PZC) circuit, an extra dead time is observed. After a 40 ms shutdown, we have to wait for 130 ms (net 170 ms dead time) due to the saturation of the amplifier when equipped with a PZC stage while only 10 ms (net 50 ms dead time) is necessary without a PZC. However, PZC is essential to reduce the degradation of the energy resolution in high count rate conditions. Therefore, an alternative technique to avoid degrading the energy resolution in high count rate conditions without a PZC circuit was investigated. A present pulse height was corrected by using both the pulse heights and the elapsed times from the previous events. As a result, in a readout circuit having no PZC circuitry, an energy resolution of 4.7% for a 122 keV photo peak at a count rate of 20 kcps was obtained when using the correction, and a 9.1% one was obtained without the correction. In addition, an energy resolution of 2.7% for a 662 keV photo peak at a count rate of 21 kcps was obtained when using the correction, and a 5.3% one was obtained without the correction. It is shown that this new correction method has almost the same effect as PZC circuitry.