Study On Characteristics Of Detector In Micro-nano CT System

CT system with micro-nanometer three-dimensional spatial resolution is called micro-nano CT system(Micro-nano CT). This project is a sub project of the development and the application of three-dimensional imaging analyzer for micro-nano structure which is financially supported by the national key foundation for exploring scientific instruments. This paper studies the large pixel array, ultrahigh resolution and high sensitivity detector which is the key core component of micro-nano CT system, and studies closely the characteristics of detector, such as the conversion efficiency and spatial resolution of low-energy X-ray conversion screen, optical coupling and transmission, and the high-resolution large matrix photoelectric conversion devices. The study of the characteristics of the detector is very important for the development of detectors of micro-nano CT.This paper analyzes and compares the characteristics and limitations of four commonly used flat panel detectors, and focuses on problems of the structure of the scintillator fiber-coupled CCD detector which is the requirements of the micro-nano CT system with ultrahigh spatial resolution, and features of three key components including X-ray detector conversion screen, optical light cone and high-resolution CCD, and spectral matching, size matching of components.The properties of scintillator material that micro-nano CT system commonly used are analyzed and the Cs I scintillator material is selected for the system. Through theoretical analysis, modeling and simulation, the relationship between X-ray conversion factor or the modulation transfer function of Cs I scintillator and the parameters including scintillator thickness, substrate reflection coefficient and X-rays energy separately. Accordingly, the scintillation screen parameters are designed. This paper analyzes the physical characteristics of the fiber, focuses on the relationship between the fiber structure and light transmission, and designs the project of fiber respectively coupling with scintillation screen and CCD.This paper studies the characteristics of KAF- 8300 CCD. What’s more, this paper also analyzes its structure, operating principle, performance parameters, output amplifier and so on. In addition, the overall scheme, specific hardware and software design of CCD drive system are completed. Key parts of the drive system include a timing pulse generator module and power drive module. Aiming to the type and characteristics of KAF-8300 CCD chip, the timing generator module adopts FPGA generate complex CCD drive timing. FPGA adopts EP2C70F896C6 of Altera Company. And the power drive module adopts discrete devices and integrated power devices. The timing generated by FPGA is converted to the clock of CCD. Hardware and program debugging are completed as well. Debugging results show that the drive system can make sure KAF-8300 works properly with the ultimate output rate of 28 MHz. It has good reliability and operability on micro-nano CT.Based on the analysis of the detector characteristics for micro-nano CT, this paper presents the preliminary design scheme of the thickness of the scintillation screen and the fiber coupling through theoretical analysis, modeling and simulation. The selection of the CCD and the design of readout drive system are accomplished. Furthermore, this paper presents the corresponding optimization and improvement measures, and achieves device feature matching. The research of the detector characteristics in this paper provides theoretical analysis, simulation and experimental validation. This research has important guidance for detector design, optimization and selection of key components for micro-nano CT.