Proceedings Article | 9 September 2019
KEYWORDS: Sensors, Imaging spectroscopy, Gamma radiation, Spectroscopy, Calibration, Gamma ray imaging, Crystals, Spectral resolution, Image resolution, Prototyping
Recently we evaluated spectral performance of virtual Frisch-grid (VFG) CdZnTe (CZT) detectors which offer an economical and robust design for high-efficiency detectors for gamma spectroscopy and imaging. We developed and tested several arrays prototypes coupled to a front-end ASIC. Each detector in the array module operates as a mini time-projection chamber offering high energy and position resolution. Previously we reported high-energy resolution of <0.9% FWHM at 662 keV achieved with the individual detectors and the whole arrays. We employ learning algorithms based on extensive detector calibrations to reconstruct positional information and use this information to correct detectors’ response non-uniformities due to the presence of crystal defects and enhance the spectral resolution of the array. In this work, we evaluated the position resolution achievable with virtual Frisch-grid CZT detectors, which is important for making high-resolution imaging instruments. Based on our measurements, we conclude that <100-micron resolution is achievable with these detectors on a small scale. However, it would require very accurate calibrations to ensure such resolution for the entire detector’s volume due to local variations of the electric field. High spectral and position resolution of position-sensitive virtual Frisch-grid CZT detectors make them attractive for a variety of applications including gamma-ray astronomy, medical and industrial imaging, environmental cleanup, nuclear safeguards and security. The arrays consisted of VFG detectors fabricated from 6x6x20 or 5x7x25 mm3 CZT crystals acquired from Redlen, Inc. The detectors have a simple design. Each crystal, furnished with two gold contacts on the top and bottom surfaces (the anode and the cathode) is encapsulated inside the ultra-thin polyester shell for electrical insulation and mechanical protection of the detector as we previously described. The shell tightly envelops the crystal and holds in place two CuBe flat-spring contacts on the cathode and the anode faces. 4.5-mm wide pads, cut from copper adhesive foil, are attached over the shell near the anode side.
The detectors were placed vertically on the detector board and gently pressed from the top using the cathode board having the decoupling capacitors and resistors. The anode spring contacts touch the designated anode pads on the board, while the charge-sensing pads were soldered to the board contacts. The signals generated by the incident photons on the anodes, cathodes and 4 position-sensing pads were routed to the corresponding front ASIC inputs (charge-sensitive preamplifiers). The decoupling circuitries are required for reading the signals from cathodes, which were biased at 2500-3000 V. The ASIC used in these measurements allowed us to capture the signals from individual cathodes, anodes and pads. To calculate the normalized X and Y coordinates, we used the center of gravity method.
For the Z coordinate we used the C/A ratio. As we described it previously, this approximation is sufficient to for correcting the response non-uniformity. The measured XYZ values constitute a configuration space, which correlates to the spatial variations in the measured anode signals. Thus, by segmenting the XYZ space into small voxels we sort out the signals corresponding to each of them and apply correction accordingly using a 3-dimensional correction matrix (CM) generated during calibration. For comparison purposes, we also apply drift-time corrections, along the Z direction, which are called 1D correction. The detectors calibration was done for several temperatures and using several gamma ray lines. For calibration we collected the pulse-height spectra from known gamma ray sources and use them to evaluate the channels’ baselines, gains and the 3D correction matrixes for each detector.
