| Cadmium Zinc Telluride(Cd1-xZnxTe) crystal is an attractive room-temperature semiconductor X/γ ray detecting material once appeared in the end of last century. In recent years, CdZnTe detectors have been widely used in a number of areas concerning about the detection of energy spectra as well as radiation imaging. For example, Chinese largest scientific equipment SSRF（Shanghai Synchrotron Radiation Facility）, “ShenGuang” prototypes in inertial confinement fusion（ICF）, positron emission tomography（PET） and single photon emission computed tomography（SPECT） in medical physics, energetic X-ray imaging survey telescope（EXIST） in astronomy, intelligent personal radiation location system（IPRL） in environment detection, and so on. So, it is of great importance to make a research on the improvement of the energy and spatial resolution of CdZnTe detectors.Among the CdZnTe detectors with different electrode structures, the unipolarity of Coplanar-grid（CPG） CdZnTe detector is the best, which reveals its potential of obtaining the better spectral response. However, the energy resolution of CPG CdZnTe detectors are always degraded by the fluctuation of the induced signal, and hence deviated from the theoretical value, which can be mainly attributed to the non-uniformity of the weighting potential distribution and the trapping of the charge carriers. What’s more, the applied range of CPG detectors are usually limited to the detection of the energy spectra for a long time. In some X/γ ray imaging applications such as PET and SPECT, however, pixellated CdZnTe detectors are always utilized, rather than the CPG detectors, because there is only one CPG pair on the anode surface, hence no spatial resolution can obtained. To apply the CPG detectors to some X/γ ray imaging applications, one may encounter some obstacles. Firstly, an array of CPG pairs, instead of a single CPG pair, should be presented on the anode surface to obtain the spatial resolution. Since the size of each CPG pairs is affected by a series of physical processes, such as charge collection, charge sharing and the charge loss on the surface, to name a few, the determination of the appropriate size of each CPG pairs remains a question. Secondly, the readout electronics will be rather complicated if the number of CPG pairs on the anode surface is becoming larger, since two preamplifiers are needed for each CPG pairs in the conventional readout format. The larger complexity of the readout electronics not only increase the cost, but also brought in more noise, which degrade the spectral response of the detector to a large extent.To solve these problems, sponsored by a mutual fund project of National Natural Science fundation and China Academy of Engineering Physics（No.10876044）, National Natural Science Foundation of China（No.61274048）, and Basic and Advanced Technology Research Project in Chongqing（cstc2014jcyjA90010）,the research we did are mainly as follows:（1） To optimize the spectral performance of the CPG detectors, we proposed an evaluation model to characterize the non-uniformity of the response of the CPG CdZnTe detectors resulting from the non-uniformity of the weighting potential and the trapping of the charge carriers. Figure of merit（FOM） was utilized to characterize the lateral uniformity of the detector response, while charge induction deficiency（CIE） was utilized to characterize the lateral uniformity of the detector response. Quantitative analyses were made of the width of the steering grid wsg, quasi peripheral collecting electrode woc and non-collecting electrode wonc as an effect of FOM. What’s more, computational formula of CIE through the integral of the weighting potential was deduced. Quantitative analyses were also made of the mobility-lifetime products（μeτe and μhτh）, bias voltage Vb and the pitch between the neighboring collecting electrodes p as an effect of FOM.（2） To extend the applied range of the CPG detectors, we have explored and verified the feasibility of applying the CPG CdZnTe detectors to X/γ ray imaging applications. After the analysis of charge collection with the detector volume, the charge loss occurred on the surface, and the transport of the electron cloud, we designed and manufactured a CPG CdZnTe detector with not only good energy resolution, but also good spatial resolution at the same time, which made it suitable for some X/γ ray imaging applications. The dimension of the CPG array CdZnTe detector is 7×7×5 mm3, and the size of each CPG pairs is 2.3×2.3 mm2. Eventually the spatial resolution of the detector is 3.3 mm, comparable with that of the pixellated CdZnTe detectors used in the early PET system. A digital energy spectra detection system was established, and the signals of the 4 CPG pairs were readout. The energy resolution of the CPG array CdZnTe detector under the irradiation of 662 keV gamma-rays is 2.7%, 3.6%, 3.1% and 3.9%, respectively.（3） To improve the expandability of the CPG pairs as well as the practicability of X/γ ray imaging applications of CPG detectors, the single electrode readout method was optimized, and then utilized in the design and manufacture of a CPG array CdZnTe detector in which the signals from only the collecting electrodes are needed to readout, so that the numbers of the preamplifiers previously needed can be halved. The relationship between the optimum gain factor G and the width of the collecting electrode wc was discovered, hence the optimum electrode structure can be predetermined by the mean drift length of the carriers（λe and λh）. A simulated energy spectra detection system was established, and the signals of the 4 CPG pairs were readout. The obtained energy resolution of the CPG array CdZnTe detector under the irradiation of 662 keV gamma-rays is 3.7%, 4.1%, 4.4% and 4.7%, respectively, on the premise of maintaining the intrinsic spatial resolution of 3.3 mm. |
