Use of high-granularity CdZnTe pixelated detectors to correct response non-uniformities caused by defects in crystals
Introduction
3D position-sensitive CdZnTe (CZT) pixelated detectors, developed by the team from the University of Michigan, offer significant enhancements in CZT detector spectral performance and the capability for gamma-ray imaging [1], [2], [3], [4]. Further improvements of their position resolution will allow for more accurate corrections of detector-response non-uniformities, which will further increase the spectral and spatial resolution of CZT detectors and increase their acceptance for practical applications. Recently, we demonstrated the feasibility of correcting the small-scale response inhomogeneity and enhancing the performance of position-sensitive virtual Frisch-grid CZT detectors fabricated from unselected off-the-shelf CZT crystals [5]. By reading the signals from 4 strips placed on the device’s side surfaces and using this information to measure the coordinates of the interaction points with an accuracy of ~100 μm, we achieved high-granularity segmentation of these detectors, up to 60×60×150 voxels. The measured signals generated in each of these voxels by interaction events were corrected before adding the events to the pulse-height spectrum. The three-dimensional response matrix, used to apply the in-fly corrections, was obtained by calibrating each of the voxels before measurements. As a result, we improved the energy resolution of different 6×6×15 mm3 detectors from 1.5–2.5% to 0.6–1.1% FWHM at 662 keV.
The goal of the work presented here was to demonstrate that high-granularity position sensing could also be applied to enhance the performance of large-volume pixelated detectors fabricated from unselected off-the-shelf CZT crystals.
Signals measured in CZT detectors are always affected by carrier trapping in crystals. In high mu-tau product material, >10−2 V/cm2, the trapping centers have low concentrations and should not be a problem provided their spatial distributions are uniform inside the crystals. Unfortunately, the dislocations and subgrain boundaries, commonly present in commercial CZT material, cause non-uniformities in the trapping centers distributions and, thus, fluctuations of the collected-charge signals. We note that variations of the collected-charge signals are solely attributed to random distributions of the interaction points and therefore can be corrected by making high-granularity detectors.
Two approaches can be considered to enhance the spatial resolution in pixelated detectors by using the collected- and transient-charge signals. The first approach [6], [7], [8], [9], [10] is applied when the whole charge from the electron cloud is collected on a single pixel. In such cases, the electron cloud induces transient signals on neighboring pixels, and the X–Y coordinates of interaction points can be obtained from the amplitudes of the transient signals. Theoretically, this approach should provide a sub-pixel resolution, but only within a geometrical area limited by the size of the electron cloud. The second approach is applied when the total charge from the electron cloud is shared between two or more neighboring pixels. As in the previous approach, the signal amplitudes corresponding to these pixels can be used to refine the positions of the interaction points with accuracy better than the pixel size, but, as in the previous case, within a certain geometrical area limited by the size of the electron cloud. In reality, both types of events occur in pixelated detectors with the relative number of shared events increasing with a decreasing pixel size. The total charge from the electron cloud can be collected on a single pixel (single-pixel events), or it can be shared among several pixels (charge-sharing events). This means that both approaches should be combined to evaluate the coordinates of the interaction points in pixelated detectors. Recently, Montemont et al. [11] demonstrated a novel algorithm for processing waveforms captured after charge-sensitive preamplifiers to refine the positions of the interaction points. Its key feature is that it uses time-correlated (synchronized) sampling amplitudes from several pixels, regardless of whether the signals generated on the pixels are collecting or transient. We will discuss this approach in detail in Section 2.3.
For this study, we employed pixelated detectors with conventional contact patterns similar to the ones used in 3D devices, but with smaller dimensions, and relied on charge sharing for attaining high-granularity position resolution. We undertook three runs of measurements using pixelated detectors fabricated with progressively smaller pixel sizes from the same set of CZT crystals used for each consecutive run of measurements. For the performance baseline, we measured the pulse-height spectra from the 1.4-mm pixel size detectors, for which most of the electron clouds generated by the interaction events are collected on a single pixel. In contrast, for 0.8- and 0.5-mm pixel detectors, the majority of the events are shared between several adjacent pixels, allowing the high-granularity segmentation to improve the overall performance of these detectors.
Section snippets
Experimental
We conducted three runs of measurements to evaluate the performance of twelve 15×15×10 mm3 3D position-sensitive pixelated detectors fabricated from the same set of CZT crystals, but with progressively smaller pixel sizes, viz., 1.4, 0.8, and 0.5 mm. To read the signals generated in the detectors, we employed the data-acquisition system based on the H3D front-end ASIC developed in collaboration between BNL's Instrumentation Division and the University of Michigan [12], [13], [14], [15]. By using
Results and discussion
The goal of this work was to demonstrate the feasibility of using high-granularity position sensing to correct non-uniformities in the response of CZT pixelated detectors. We did not consider here other important aspects related to pixelated detectors, many of which have been previously discussed in the literature [11], [12], [13], [14], [15], [16], [17], [18].
Conclusions
The basic concept behind high-granularity detectors is to measure the locations of interaction points with high spatial resolution and accordingly correct the measured charge signals generated from these locations. By using the high-resolution position sensing, we demonstrated the enhancement in the response of the detectors fabricated from unselected CZT crystals.
We investigated the spectral responses of 15×15×10 mm3 pixelated detectors fabricated with progressively smaller pixel sizes: 1.4,
Acknowledgments
This work was supported by U.S. Department of Energy, Office of Defense Nuclear Nonproliferation Research & Development (DNN R&D). The manuscript has been authored by Brookhaven Science Associates, LLC under Contract no. DE-AC02-98CH1-886 with the U. S. Department of Energy.
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