An alternative pulse height correction method for pole zero cancellation circuitry

Abstract

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 40ms shutdown, we have to wait for 130ms (net 170ms dead time) due to the saturation of the amplifier when equipped with a PZC stage while only 10ms (net 50ms 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 122keV photo peak at a count rate of 20kcps 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 662keV photo peak at a count rate of 21kcps 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.