Unipolar CdZnTe Nuclear Radiation Detector And Its Infrared Stimulation Effect

In today's world, application and detection of nuclear radiation attract more and more attentions. As the key technology of space science, nuclear science and biomedicine, etc., X and gamma rays' detection has important applications in nuclear power utilization, medical imaging, security check, environmental monitoring, astronomy detection, and so on. Especially in recent years, many factors make the high resolution gamma ray detector become hotspot in nuclear radiation detection field, such as the completion of “Shen Guang-III” prototype and the development of inertial confinement fusion(ICF), progress in solar probe and cosmic background radiation detection, urgent need for non-destructive inspection of large spacecraft and advanced medical imaging.Cadmium zinc telluride(CdZnTe) has properties of high average atomic number, high resistivity(?1010 ?·cm), wide band gap(1.57 eV) and absent of polarization phenomenon. All of above advantages make CdZnTe nuclear radiation detector have better energy resolution and imaging performance than the traditional scintillator detector. Meanwhile, CdZnTe makes up the shortages of other semiconductor materials, such as the low detection efficiency of silicon(Si) for high energy radiation and the high purity germanium(HPGe) cannot work at room temperature, this make Cd ZnTe detector become the most promising room temperature semiconductor nuclear radiation detector. While the poor transport properties of charge carriers in CdZnTe material, especially those for holes resulting in the reduction of charge collection efficiency and the deterioration of spectra performance. Many efforts have been invested to overcome this drawback from the aspects of crystal growth, pulse signal processing and detector design.At present, the investigations of CdZnTe detector abroad are mainly focused on development of integrated readout circuit, signal processing and pulse amplitude correction, and so on. However the domestic studies are still in their infancy and have a certain gap compared with overseas research, which mainly focuse on the large size and high quality CdZnTe crystal growth. While investigations about the enhancement of charge collection efficiency and improvement of spectra performance though detector structural design or external stimulation approach are relatively inadequate. In order to further improve the spectra performance of CdZnTe detector and meet the diagnostic requirement of X and gamma rays in “Shen Guang-III” large scale science facilities, nuclear medicine, astrophysics and other fields, we have conducted research on the unipolar CdZnTe nuclear radiation detector with the support of National Natural Science Foundation of China(NSAF.10876044, No.62174048). Based on the principles of unipolar CdZnTe detector, two ways are used to improve the spectra performance of CdZn Te detector in this thesis, including simulation of weighting potential, electric field and charge collection efficiency and experiment of spectra measurement. The main contents of this research are as follows:(1) The disadvantages of conventional planar CdZnTe detector caused by poor transport properties of holes are analyzed based on the charge induction and collection theory in nuclear radiation detectors. A variety of single charge carrier collecting techniques and principles of unipolar CdZnTe detectors are introduced in detail. Finite element simulation software COMSOL Multiphysics is used to simulate the weighting potential and electric field distributions of various unipolar CdZnTe detectors such as Frisch grid detector, hemispherical detector, coplanar grid detector and pixelated detector. The structure optimization of these unipolar CdZnTe detectors is fully discussed based on Shockley-Ramo theory, which provides a basis for design and fabrication of unipolar CdZnTe detector.(2) The effect of structure parameters, material parameters and applied voltage on charge collection efficiency of the capacitive Frisch grid CdZnTe detector has been analyzed through the finite element simulation of weighting potential and weighting field distributions. Basing on the simulation optimization, a 5×5×10 mm3 capacitive Frisch grid CdZnTe detector is fabricated. The improvement of unipolar property in this detector through increasing Frisch ring heights has been verified by the spectra measurement. An optimal energy resolution FWHM of 2.8% at 662 keV(137Cs) and peak-to-valley ratio of 15.3 is obtained from the detector with 10 mm high Frisch ring.(3) The 940 nm IR stimulation experiment is applied on the capacitive Frisch grid CdZnTe detector to investigate the enhancement of charge carrier transport properties, and then the improvement of the spectrometric characteristics with IR illumination. Improvements are obtained in the energy resolution FWHM from 3.3% to 2.3% at 662 keV(137Cs) and in the peak-to-valley ratio from 14.0 to 21.0 through IR stimulation. Simultaneously, relatively good energy resolution could also be obtained even under low operating voltage and poor unipolar conditions. Without using any pulse shape correction and decreasing the detection efficiency, such a simple method provides an external approach for charge collection efficiency enhancement and spectrometric characteristics improvement of the CdZn Te detector(4) A 4×4 common-grid pixelated CdZnTe detector with different pixel sizes has been designed and fabricated using a piece of CdZnTe crystal. This design allowes us to study different array geometries on the same CdZnTe crystal, and the disturbances from using different materials could be eliminated. By the spectra measurement combined with weighting potential and electric field simulation, the small pixel effect, guide effect of the steering grid and edge effect in common-grid pixelated CdZnTe detector are investigated. An optimal energy resolution FWHM of 3.80% at 662 keV(137Cs) and peak-to-valley ratio of 5.65 is obtained from the 0.8 mm wide center pixel with the optimal bias voltage of-60 V. Optimization design of detector structure and the discussion about edge effect provide a theoretical and experimental basis for the design and fabrication of large array high resolution pixelated CdZnTe detector.