Yang Kang a, Rui Wu a, Sen Wu a, Tingting Tan a, Yingrui Li a b, Gangqiang Zha a b
a State Key Laboratory of Solidiļ¬cation Processing, MIIT Key Laboratory of Radiation Detection Materials and Devices, Northwestern Polytechnical University, Xi'an, 710072, China
b Shenzhen Research Institute of Northwestern Polytechnical University, Shenzhen, 518063, China
## Abstract
During X-ray scanning imaging, the X-ray dose received by CdZnTe photon counting detectors varies dynamically because of the different absorption coefficients that occur at different locations in the object being imaged. This paper explores the phenomenon of count rate relaxation in CdZnTe detectors during X-ray dose mutations and studies the intrinsic relationship of this phenomenon with the polarization effect. The characterization of the count rate relaxation processes of CdZnTe detectors at three typical X-ray doses are studied in detail. It is concluded that the count rate relaxation process of the CdZnTe detectors correspond to the polarization process, which can be divided into three stages according to the accumulation rate of space charges, and the relaxation time depends on the settling time of the polarizing electric field. In addition, the effects of temperature and bias voltage on the relaxation phenomenon of CdZnTe detectors are studied. The results show the relaxation process of the count rate can be regulated by adjusting the temperature and bias voltage of the CdZnTe detectors. Changes in temperature and bias mainly act on the trapping and de-trapping processes of deep-level traps, thereby affecting the establishment of the polarization electric field, which further proves the close relationship between the count rate relaxation phenomenon and the polarization effect. Finally, the effects of count rate relaxation on DR image quality are discussed based on self-built DR imaging system.
## Introduction
Recently, photon counting detectors with spectral resolution capabilities have been gathering significant interest from increasing numbers of researchers [1]. In X-ray imaging applications, such detectors can offer the advantages of dramatically improved image quality, increased signal-to-noise ratio and dose efficiency, and extremely high spatial resolution [[2], [3], [4]]. The compound semiconductor CdZnTe is considered one of the most promising photon counting detector materials because of its high effective atomic number, high resistivity, and suitable band gap width [[4], [5], [6]]. In addition, the CdZnTe detectors' energy detection range is from 10 keV to 3 MeV, which basically covers the range required for all X-ray detection applications [[7], [8], [9]]. However, under high flux X-ray irradiation, due to the limitations of poor hole transport performance of CdZnTe detectors and application specific integrated circuits (ASICs), CdZnTe detectors may exhibit polarization behavior which could degrade their detection performance. The essence of the polarization effect is that high incident X-ray flux causes the space charge near the cathode to accumulate and form an internal electric field, thus causing the detector to fail. Some studies have also noted that the applied bias voltage and the operating temperature can also affect the polarization effect [10,11]. The polarization effect has become one of the most important factors limiting the application of CdZnTe photon counting [[12], [13], [14], [15], [16], [17]]. In practical application, CdZnTe photon counting detectors are typically used in DR scanning imaging. During the dynamic scanning process, the X-ray dose received by the CdZnTe photon counting detectors varies from time to time due to differences in the absorption of X-rays in different regions of the imaged object. It is found that there is a stable process in the output count value of the detectors, which is defined as the count rate relaxation phenomenon, and stable time is defined as the relaxation time. The relaxation process may affect the accuracy of the output counts, thereby causing serious interference to the subsequently applied algorithms and the quality of DR imaging. However, few studies have deeply explored the relaxation phenomenon of count rate, and the research on the quality of DR imaging is also rarely reported. A systematic study of the count rate relaxation phenomenon in CdZnTe detectors will be highly significant and can provide guidance with regard to the actual application.
This paper presents a study of the relaxation phenomenon in CdZnTe photon counting detectors. Two typical CdZnTe detectors, which show different counting characteristics under high-flux X-ray exposure, are adopted for comparison study. The relaxation phenomena of the two types of detectors under different X-ray doses are completely recorded to analyze the nature of the relaxation effect. The deep correlation with the polarization effect is revealed by systematically analyzing the relaxation process and relaxation time. Furthermore, by changing the temperature and bias voltage of the CdZnTe detectors, the regulation of the relaxation process was realized, and the results further confirmed the correlation between the relaxation phenomenon and the polarization effect. Finally, we explore the impact of count rate relaxation of CdZnTe detectors on DR imaging quality.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/study-on-relaxation-phenomenon-of-cdznte-photon-counting-detectors-in-x-ray-imaging.html