| In recent years, the implement of national space station program and theestablishment of the national science facilities, which include the prototype of“ShenGuang III” and the Large Sky Area Multi-Object Fiber Spectroscopy Telescope(LAMOST), are milestone of aerospace technology, high energy physics and astronomytechnology. And hence, the corresponding diagnostic technique for high energyradiation source also has important significance as the unique technology support ofthese major research plans. The pixellated Cadium Zinc Telluride (CdZnTe) irradiationdetector, which has excellent energy resolution and high detection efficiency, hasexperienced a rather rapid development due to the development of the semiconductorgrown technology in decades. And the pixellated CdZnTe detector are now investigatedin a large variety of fields, including nuclear physics, X-ray and Gamma ray astronomyand nuclear medicine.As for now, the researches of grown technique to produce CdZnTe crystals with therequired good performance and the crystal surface process are the majority part of thedomestic investigation. The overseas researchers dedicate to the design of novelstructure electrode, the improvement of weighting potential theory of CdZnTe detectorand the implement of electrical signal process technique for the detector pulse. Thepurpose of all these works is the spectrum resolution improvement, which is carried outaccording to the improvement of the collection efficiency of the photon generatedcarriers in the CdZnTe crystal. Nevertheless, there are several realistic problems. Forexample, how to rise the detector resolution, improve the edge effect based on thedigital pulse processing method instead of the material grown method? How toevaluation the imaging quality of the large area pixellated CdZnTe with the modulationfunction theory? And during the high energy radiation imaging detection, how tocombine the induced signal theory, the experimental and theoretical results of the carrierdrift characteristic with the imaging signals? Thus, the evaluation of detector imagingperformance according to the carrier drift characteristic hasn’t been discussed.Moreover, the stability of CdZnTe detector under extreme condition hasn’t beeninvestigated. In this thesis, the corresponding researches for the problems mentionabove are investigated based on the support of National Natural Science Foundation ofChina (No.10876044) and the fundamental research funds for the central universities (CDJXS11122219). The main work of this thesis is provided as follows:1. The basic principles of interaction theory between radiation particles andmaterials and the transmit attenuation theory of radiation particles are introduced. Theinteraction and transfers of different energy particles in CdZnTe material are discussed.The collection efficiency of carrier charge and weighting potential distribution fordifferent electrode CdZnTe detectors are analyzed based on the principle of CdZnTedetector. These pre-works establish the essential theoretical foundation for thepenetrated investigation of pixellated CdZnTe system, and also provide some researchideas.2. The surface leakage current of2×2,4×4pixel array electrodes are tested andanalyzed based on the leakage theory of pixellated CdZnTe detector. The containmentmethod of noise signal resulted from the crystal leakage has been discussed. After theargumentation of signal processing principle for the preamplifier readout system and theshaping amplifier system, we fabricated the corresponding pulse shaping circuit whichintroduced the pole-zero cancellation circuit and Sallen-Key filter. In order to reduce theinfluence of noise signal which resulted from leakage current, the acquirement systemand corrected method for the digitization of probed CdZnTe detector pulse areinvestigated in detail. At first, according to the application of peak finding and spectrumstatic programs for digital pulse signal, the spectrum of Gamma source137Cs is obtainedin our experiment. Moreover, the implement of pulse amplitude corrected algorithmbased on the digital signal is presented for the purpose of spectroscopy responseimproving. The average energy resolution of corner pixels has been improved1.6%,and the best energy resolution of pixels achieved2.5%. The experimental resultsindicate that the energy resolution of4×4pixellated CdZnTe detector system isexcellent. The influence of edge pixel effect is substantially reduced, the resolution ofedge pixels get improved efficiently.3. Based on the discussion of pinhole imaging theory, a pinhole imaging systemwith pixellated CdZnTe detector are constructed for the Gamma source. The energyresolution and peak efficiency of CdZnTe pixellated detector are tested and analyzed.During our preliminary experiments, modulation transfer function (MTF) and additionalnoise characteristic of pinhole imaging system were investigated and discussed. Wepresented and evaluated test image of5mm diameter137Cs gamma source. Themeasured pinhole images have been restored with the Lucy-Richardson algorithmsimultaneously. As shown through the experiment results, there was lateral spread information during source images and degraded image can be restored appropriatelywith the application of Lucy-Richardson algorithm. Inner area spatial resolution getsimproved through the restoration method and estimated source diameter has error0.5mm. Experiments indicated that pinhole imaging CdZnTe detector system could beapplied to the acquisition of gamma source intensity distribution while the dimensioninformation of small gamma source can be measured effectively.4. With the discussion of induced charge signal distribution that caused by thetrapped carriers near the anode electrode for the CdZnTe detector, a novel analyticmodel which mainly considers the trapped carrier effect in the pixellated CdZnTedetector is obtained. The constructed numerical model carries out the combinationbetween the modulation transfer function of detector and the physical parameters of theCdZnTe material. Based on the simulated results and carrier drift theory, the pixellatedCdZnTe detector imaging performance, which influenced by the carrier drift and crystalcharacteristics, has been estimated for different carrier mobility-lifetime values andphoton energies. Moreover, a40×40mm2CdZnTe imaging system has beenestablished. The comparison of the presampling MTF shows that the simulation result iswell consistent with the experimental data.5. Based on the established CdZnTe imaging system, the radiation imagingexperiment is accomplished with the Rh target X-ray source and the137Cs Gammasource. For the first time, the imaging polarization effect has been observed.Furthermore, the detector imaging characteristic that under extreme experimentalconditions are investigated according to the change of the physical parameters like tubevoltage, tube current, et.al. For the penetrated study of the polarization phenomenon, weestablish two kinds of models with analytic method and finite element method based onthe Poisson equation. Thus, the inner electric potential, the electric field distribution andthe space charge density of pixellated CdZnTe detector are simulated. The simulationresults reveal that the accumulation of the hole carriers, which resulted from theextremely low drift ability of the hole carrier, leads to a relatively highspace-charge-density area in the CdZnTe bulk when the increase of irradiated photonflux. And hence, the induced signal screen effect of the anode pixels in the centerirradiated area is mainly attributed to the distorted electric field which makes electroncarriers drifting toward the high potential area in the CdZnTe crystal instead of the pixelanodes. Particularly, the finite model predicts the fluctuant change of the polarizationeffect, which has been approved during our experiments. The detailed discussion has been presented with the increase of the incident photon energy in the finite elementmodel. |
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