Ali Murteza Altıngün, Emrah Kalemci
Faculty of Engineering and Natural Sciences, Sabancı University, Orhanlı, Tuzla, Istanbul, 34956, Turkey
## Abstract
The geometry of electrodes is one of the most important factors in determining the performance of orthogonal-strip detectors. The aim of this work is to study the performance of a 5 mm thick cross-strip CdZnTe detector with different electrode widths. Our study consists of two main parts, simulations and experiments. We utilized four different anode sizes ranging from 0.1 mm to 0.6 mm. The anodes were interspersed with steering electrodes with varying sizes from 0.3 mm to 0.85 mm. The maximum gap size between the anodes and steering electrode strips was set to 0.3 mm, while the minimum gap size was 0.125 mm. The performance of the detector was investigated in terms of the steering electrode bias voltage, the energy resolution, and the charge sharing effect. For simulations, we developed a C++ based simulation program for charge transport inside the CdZnTe detector and charge collection at the electrodes. For photon interactions we used GEANT4 toolkit and for electric field and weighting potential simulations we used COMSOL software. The results demonstrated that -50 V is the optimal steering electrode bias for our detector when -500 V was applied to the cathodes and that the energy resolution performance drops with increasing steering electrode width. Also, the charge sharing effect becomes more dominant for larger steering electrode sizes. The experimental result are further compared with the simulations. The results are in a good agreement and the comparison validates our simulation model. Although, our simulation framework has need of better estimation for the intrinsic noise of CdZnTe. These results suggest that an optimization study between electrode widths and steering electrode bias is required to obtain the best performance in orthogonal-strip CdZnTe detectors.
## Introduction
In recent years, important advantages of
Cadmium Zinc Telluride (CdZnTe) based semiconductor detectors have attracted the attention of many researchers in the fields of medical imaging, astronomy, and homeland security. Due to its high stopping power, high internal resistivity, and large band gap providing a good energy resolution at room temperature [1], CdZnTe outperforms most other types of hard X-ray detectors. Many studies with different geometries have shown that CdZnTe detectors possess very good energy [2], [3], [4], spatial [4], [5] and time resolution characteristics [4], [6], [7]. Pixellated and cross-strip electrodes are the two prominent CdZnTe detector designs in use. In the pixel design, anodes are composed of small electrodes (pixels), while the cathode is a whole planar electrode. This configuration provides quite good spatial resolution, as well as energy resolution due to the small pixel effect. However, the readout electronics cost increases with the number of pixels since every pixel requires a single readout channel. In the cross-strip detector design, on the other hand, anodes and cathodes are perpendicular to each other, imitating N x N pixels with 2 x N electronic channels. This promises to be especially advantageous for applications that require less power and weight, such as for small satellites in space [3], [8]. The reduced complexity of the readout electronics and low cost make cross-strip configurations advantageous also for Compton cameras [9], [10], [11], [12] and positron emission tomography (PET) applications that utilize large number of detectors [13], [14], [15], [16]. While the cross-strip design decreases the number of required readout channels drastically, due to higher capacitance, the energy resolution is poorer compared to the pixellated designs.
A photon interaction in CdZnTe volume creates N=Ey/ɛ electron–hole pairs on average (analogous to electron–ion pairs in gaseous detectors), where Ey is the incident photon energy and ɛ = 4.6 eV is the pair creation energy, the energy required to create an electron–hole pair in the crystal. As bunch of electrons (holes) produced by a single photon, which is called an electron (a hole) cloud, drift toward the anodes (cathodes) by the applied electric field, its size will expand due to carrier diffusion and charge repulsion. Because of the expansion in the cloud sizes, the charge carriers may induce charges on several neighboring electrodes [1]. This phenomenon is called charge sharing effect which worsens the spectral performance of the detectors. However, there are methods available to mitigate the charge sharing effect to recover good spectral performance [17], [18].
Despite its promising properties, CdZnTe detectors suffer from hole collection inefficiency. Due to the low mobility of holes and the defects in CdZnTe crystals, holes are trapped. This results in a low energy tail in the pulse height spectrum. By utilizing the small pixel effect [19], [20], it is possible to make the detectors sensitive to only electron motion. The small size of a pixel or strip ensures the weighting potential of the anodes to have near-zero values in the vicinity of the cathode. Therefore the motion of holes will induce negligible charge on the anodes.
Some electrons might drift to regions between the anodes, which causes incomplete electron collection. To overcome this problem, anodes can be interspersed with steering electrodes (SE), which is used in CdZnTe based detectors successfully for almost two decades. By applying a potential between the anodes and the SE, electrons are steered toward the anode strips, resulting in an improvement in the electron collection efficiency and the spectral performance [12], [21], [22]. Also, steering electrodes create an electrostatic shielding so that the hole collection inefficiency does not affect the spectral performance of the detectors. The effect of different potentials between the anodes and the SE in strip detectors has been reported in [15], [16].
In the cross-strip design, the sizes of the strips and the gaps between them affect the detector performance significantly [15]. We performed experiments with a CdZnTe detector that has anode and SE strips with varying sizes in addition to uniform size cathode strips aligned orthogonal to the anodes. The results of the experimental work were then compared to simulations with the motivation of offering an optimization study for the cross-strip CdZnTe detectors that can be utilized for applications that require low cost, power, and complexity. Some of the earlier results of the optimization study presented here have already been used in the design of the XRD cross-strip CdZnTe detector on BeEagleSat [8].
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/optimization-study-of-the-electrode-design-of-a-5-mm-thick-orthogonal-strip-cdznte-detector-system.html