Impact of selenium addition to the cadmium-zinc-telluride matrix for producing high energy resolution X-and gamma-ray detectors

Abstract

Both material quality and detector performance have been steadily improving over the past few years for the leading room temperature radiation detector material cadmium-zinc-telluride (CdZnTe). However, although tremendous progress being made, CdZnTe still suffers from high concentrations of performance-limiting defects, such as Te inclusions, networks of sub-grain boundaries and compositional inhomogeneity due to the higher segregation coefficient of Zn. Adding as low as 2% (atomic) Se into CdZnTe matrix was found to successfully mitigate many performance-limiting defects and provide improved compositional homogeneity. Here we report record-high performance of Virtual Frisch Grid (VFG) detector fabricated from as-grown Cd_0.9Zn_0.1Te_0.98Se_0.02 ingot grown by the Traveling Heater Method (THM). Benefiting from superior material quality, we achieved superb energy resolution of 0.77% at 662 keV (as-measured without charge-loss correction algorithms) registered at room temperature. The absence of residual thermal stress in the detector was revealed from white beam X-ray topographic images, which was also confirmed by Infra-Red (IR) transmission imaging under cross polarizers. Furthermore, neither sub-grain boundaries nor their networks were observed from the X-ray topographic image. However, large concentrations of extrinsic impurities were revealed in as-grown materials, suggesting a high likelihood for further reduction in the energy resolution after improved purification of the starting material.

Figure 1

Optical photograph of the VFG detector sample with a gold contact on each end face. The sample dimensions are 3.5 × 3.5 × 9.15 mm³. The ruler on the same graph paper is displayed in the inset of Fig. 5b.

Figure 2

Cross polarized IR transmission images for an as-grown Cd_0.9Zn_0.1Te_0.98Se_0.02 sample: under (a) zero bias and (b) under the applied bias. Cathode is placed at the bottom surface and the anode on the top surface of the sample.

Figure 3

(a) X-ray topographic image and (b) IR transmission image of the as-grown Cd_0.9Zn_0.1Te_0.98Se_0.02 detector sample. Slight bending on the top left side of the sample is highlighted by the ellipse.

Figure 4

High magnification IR transmission images with higher magnifications from left to right. Length of the Te-inclusion indicated in (c) is 16.27 µm. The scale bars indicated on bottom right corners correspond to 500 µm, 100 µm and 50 µm for figure (a–c) respectively.

Figure 5

(a) Dark I–V characteristics of the FG detector at room temperature and (b) pulse height spectrum of ¹³⁷Cs source of the Frisch-grid detector (shown in the inset) fabricated from as-grown Cd_0.9Zn_0.1Te_0.98Se_0.02 THM-grown ingot. Here, P/V is commonly known as Peak-to-Valley. It is the ratio of the peak count at 662 keV to the minimum count in the valley region (in the range of ~ 480–600 channel number. Detector dimensions: 3.5 × 3.5 × 9.15 mm³.

Figure 6

(a) Pulse height spectrum of ¹³³Ba source of the Frisch-grid detector fabricated from as-grown Cd_0.9Zn_0.1Te_0.98Se_0.02 THM-grown ingot and (b) magnified version of data in (a). Detector dimensions: 3.5 × 3.5 × 9.15 mm³.

Figure 7

Pulse height spectrum of (a) ²²Na source of the Frisch-grid detector fabricated from as-grown Cd_0.9Zn_0.1Te_0.98Se_0.02 THM-grown ingot, (b) magnified version of (a), and (c) ⁶⁰Co energy spectrum. Detector dimensions: 3.5 × 3.5 × 9.15 mm³.