A Scintillation Photon-Counting X-ray Detector For Security Inspection Imaging

The 14 th Five-Year Plan Period and Long-Range Objectives of China emphasizes the importance of improving public security because it is the foundation of both national security and social stability.Due to the growth of e-commerce,there is an enormous threat to China’s public safety from the unlawful distribution of dangerous commodities including drugs,explosives,and illegal medicines through express delivery parcels.Existing perspective security inspection machines employ dual-energy X-ray detectors,and it identifies harmful items mostly based on the morphological characteristics and density information of objects in the image,which is limited by its poor effective atomic number resolution.However,the density of dangerous organic substances such as drugs,explosives,and prohibited medicines is very near and lacks specific morphological characteristics,making screening of such dangerous commodities difficult.To solve the aforementioned problems,there is an urgent demand to develop photoncounting X-ray detectors to improve the accuracy of energy spectrum data acquisition and to achieve the identification of dangerous organic substances by using energy spectrum imaging technology.Semiconductor photon-counting X-ray detectors based on Cd Te/CZT materials have been used in the medical field because of their excellent energy performance,but physical defects such as polarization effect and charge sharing effect can lead to degradation of performance and unstable operating conditions under high counting.In addition,the mass production of such materials also has the problems of low yield rate and complex production process and has not been used in the security industry on a large scale despite more than 60 years of development.In recent years,due to the rapid development of crystal materials and optoelectronic device technology,scintillator photon counting Xray detectors using fast scintillation crystal and silicon photomultiplier(SiPM)has great potential in the energy spectrum imaging.Optimizing the design of scintillator X-ray detectors and developing high-performance,low-cost scintillation pulse digital acquisition,and energy acquisition methods are key to the development of such detectors.Therefore,this thesis addresses the need for energy spectral imaging applications for perspective security screening equipment and focuses on the design of the core components,key technologies and methods in scintillator photon counting X-ray detectors.Firstly,by analyzing the effects of physical factors such as SiPM operating voltage,and yttrium silicate(YSO)crystal size on the energy resolution performance,the designed YSO/SiPM scintillator X-ray detector was optimized to achieve 59.4% @30 ke V,37.9%@59.5 ke V and 37.3% @80.9 ke V.The count rate performance of the designed YSO/SiPM scintillator detector was also characterized by single-threshold comparison and counting of scintillation pulse signals with linear count rates up to 1.5 Mcps/pixel and maximum saturation count rates up to 2.0 Mcps/pixel.These results preliminary validated the feasibility of the designed YSO/SiPM scintillator detector for application in perspective security screening equipment.Secondly,to solve the problem of digital sampling and energy acquisition of scintillation pulses in photon-counting X-ray detectors with low cost,high density,and high accuracy,the Multi-Voltage Threshold(MVT)sampling method using a function threshold(named function threshold MVT)was proposed.This method uses a single comparator to compare a function threshold signal(typically a sine signal)with a scintillation pulse and records the time at which the scintillation pulse crosses the threshold signal,while the voltage across the threshold of the pulse signal is obtained by inverting the recorded time with the set function threshold signal characteristics.This method is characterized by low cost and low power consumption.A digital scintillation pulse sampling platform was established to study the factors affecting the accuracy of energy information acquisition under this digital method,and the influence of the accuracy of the time-to-digital converter and the frequency,amplitude,and phase of the function threshold signal on the energy acquisition accuracy of the function threshold MVT sampling method was investigated through simulation.The design,optimization,and performance calibration of the functional threshold MVT sampling circuit were then carried out based on the above findings.Finally,based on the above work,the integrated development of a scintillator photon counting X-ray detector was completed using the designed YSO/SiPM scintillator detector and functional threshold MVT sampling circuit,and the acquisition and preliminary analysis of images of phantoms of different substances as well as energy spectrum data were carried out on the perspective security imaging platform.The results showed that the perspective security equipment using scintillator photon counting X-ray detector can achieve the identification of organic substances such as alcohol,water and sugar water,flour and laundry detergent.This thesis has completed the design of a scintillator photon counting X-ray detector and its application verification in the perspective security screening imaging equipment.It is preliminarily proved that the perspective security inspection equipment using a scintillator photon counting detector has the ability to achieve the identification of organic substance segmentation,and the obtained research results are expected to provide key technology and core components for the development of intelligent security inspection equipment with the ability to identify dangerous organic substance segmentation.