Signal compensation in CZT detectors grown by the Vertical Bridgman method using a twin-shaping filter technique

CdTe/CZT is now a material consolidated for the detectors realization operating at room temperature, which find a large variety of applications in astrophysics, medical imaging and security. An Italian collaboration, involving the CNR/IMEM and INAF/IASF institutes, was born several years ago with the aim to develop a national capability to produce CZT detectors starting from the material growth to the final detection device. The collection efficiency of the charge carriers affects some important features of these detectors, such as the pulse height, energy resolution, photopeak efficiency. In fact the low mobility of the charge carriers (particularly the holes) and trapping/detrapping phenomena can degrade the CdTe/CZT detector response, depending on the distance between the charge formation position and the collecting electrodes. Two kinds of techniques can be used to improve both the collection efficiency and the energy resolution, based on the optimization of the electrode geometry and/or signal compensation methods. We have implemented a biparametric method that uses a twin pulse shaping active filter to analyze the same signal from the detector: one “slow”, which is proportional to the energy of the incident photon, and one “fast”, which depends on the position of the interaction with respect to the collecting electrode. We present this biparametric technique applied on planar CZT detectors grown by the Vertical Bridgman method at CNR/IMEM (Parma), the experimental results obtained as a function of the bias voltage, photon energy, shaping time pairs and the compensated spectra.