Beomjun Park a b c, Juyoung Ko a, Jangwon Byun a, Byungdo Park d, Man-Jong Lee a b, Jeongho Kim d
a Department of Chemistry, Konkuk University, Seoul, 143-701, Republic of Korea
b Advanced Crystal Material/Device Research Center, Konkuk University, Seoul, 143-701, Republic of Korea
c Interdisciplinary Program in Precision Public Health, Korea University, Seoul, 02841, Republic of Korea
d Department of Radiation Oncology, Samsung Changwon Hospital, Sungkyunkwan University School of Medicine, Changwon, 51353, Republic of Korea
## Abstract
CdZnTeSe (CZTS) has attracted attention for applications in X- and gamma-ray detectors owing to its improved properties compared to those of CdZnTe (CZT). In this study, we grew and processed single crystals of CZT and CZTS using the Bridgeman method to confirm the feasibility of using a dosimeter for high-energy X-rays in radiotherapy. We evaluated their linearity and precision using the coefficient of determination (R2) and relative standard deviation (RSD). CZTS showed sufficient RSD values lower than 1.5% of the standard for X-ray dosimetry, whereas CZT's RSD values increased dramatically under some conditions. CZTS exhibited an R2 value of 0.9968 at 500 V/cm, whereas CZT has an R2 value of 0.9373 under the same conditions. The X-ray response of CZTS maintains its pulse shape at various dose rates, and its properties are improved by adding selenium to the CdTe matrix to lower the defect density and sub-grain boundaries. Thus, we validated that CZTS shows a better response than CZT to high-energy X-rays used for radiotherapy. Further, the applicability of an onboard imager, a high-energy X-ray (>6 MV) image, is presented. The proposed methodology and results can guide future advances in X-ray dose detection.
## 1. Introduction
The objective of radiotherapy is to kill tumors and minimize side effects [1]. To achieve these goals, treatment precision needs to be enhanced the through equipment quality assurance (QA), patient-specific QA, and in vivo dosimetry. Various radiation detectors have been used for QA and in vivo dosimetry during radiation therapy. Metal oxide semiconductor field-effect transistor (MOSFET) dosimeters have been widely used as in vivo dosimeters because of their small size, room-temperature operation, and real-time measurement capabilities. However, MOSFET has the disadvantage of a finite lifetime [2].
For decades, CdTe-based semiconductors have been the leading materials for X- and gamma-ray detectors operating at room temperature [3,4]. Particularly, CdZnTe (CZT) is the most promising material and is widely used in various fields such as high-energy physics, aerospace, and diagnosis [[4], [5], [6], [7]]. Moreover, CdTe-based semiconductors can be used as dosimeters [8]. CdTe-based semiconductors exhibit high effective atomic numbers and detection efficiencies. Additionally, CdTe-based detectors exhibit high charge-collection efficiency, low leakage current, and sufficient electron carrier transport characteristics [4]. Thus, they are suitable dose detectors for radiotherapy. Besides, there are no reported finite-lifetime characteristics of CdTe-based semiconductors, which is a limitation of MOSFET dosimeters.
Recently, bandgap engineering, including optimized zinc (Zn) content and selenium (Se) doping, has been attempted to improve the performance of CZTs [[8], [9], [10], [11], [12]]. Increasing the Zn content in the CZT composition almost linearly increases the band gap, reducing the shot noise and leakage current [9,13]. Moreover, the addition of Se to the CdZnTe matrix reduces the density of subgrain boundaries, Zn segregation, and other defect densities [ [[9], [10], [11], [12],14,15]]. CdZnTeSe (CZTS) exhibits better spectroscopic properties than CZT [16,17].
In this study, Cd0.9Zn0.1Te (CZT) and Cd0.9Zn0.1Te0.98Se0.02 (CZTS) ingots were grown by the Bridgman technique and exposed to high energy X-rays generated by a linear accelerator (Linac) after synthesizing the X-ray detector by standard fabricating procedures. We have evaluated the feasibility of CZTS in terms of linearity and precision in the field of radiotherapy for the first time. Furthermore, we proposed the possibility of applying CZTS to an onboard imager, which is the image sensor of a linear accelerator.
## 2. Materials and methods
2.1. Crystal growth and detector fabrication
Both the CZT and CZTS ingots were grown using the Bridgman method with a stoichiometric composition of Cd0.9Zn0.1Te1 and Cd0.9Zn0.1Te0.98Se0.02, and then sliced into flat wafers. These planar wafers were polished with sandpaper/polishing cloth using alumina abrasive, chemically etched with 2% bromine in MeOH, electrode deposited, and passivated. Details of the growth and fabrication are similar to those in previous studies [11,18], except for the stoichiometry. Electrode deposition was performed using an electroless method based on AuCl3 solution. Passivation was achieved by immersing the CZT and CZTS samples in a hydroperoxide solution for 180 s [19]. The physical dimensions of the processed detectors were 5.0 × 5.0 × 2.5 for CZTS and 6.1 × 5.6 × 3.1 for CZT.
The detectors were placed in contact with a custom-made PCB board with a gold wire and conductive epoxy (Circuitworks conductive epoxy, Chemtronics) to separate the anode and cathode signals, as shown in Fig. 1(a). Conductive epoxy was used instead of tin soldering to minimize thermal damage. The PCB was then soldered to the wires and connected to the data acquisition system, as shown in Fig. 1(b).
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/feasibility-study-of-cdznte-and-cdzntese-based-high-energy-x-ray-detector-using-linear-accelerator.html