Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
The spatial response of CdZnTe gamma-ray detectors as measured by gamma-ray mapping
Introduction
Over the last few years several detector designs have been demonstrated which attempt to compensate for the poor average performance of commercially available cadmium zinc telluride (CZT) material. Aside from material uniformity issues, even the best spectroscopic-grade CZT suffers from a low photopeak efficiency [1] when operated as a planar detector for moderate- to high-energy photons (>300 keV). The low performance of CZT in a planar configuration can be attributed to the large difference in the electron and hole charge transport properties, where the hole drift length is several orders of magnitude lower than the electron drift length. This leads to a dependence of the measured output signal on the interaction position within the detector. Photons with an energy above about 600 keV are absorbed uniformly throughout typical detector thicknesses (0.2–1.0 cm). This causes photons with the same energy to produce detector signal values which vary significantly.
The common solution to this problem has been the design of devices which suppress the dependence of the measured signal on interaction position [2]. This is accomplished by reducing the volume of the detector which produces detector signal (sensing volume) while maximizing the volume of the detector available for photon interactions (absorption volume). In all designs this is accomplished by localizing the weighting potential to a small region very near the anode. This can be achieved by using a comparatively small anode electrode which causes the electric field generated from this electrode to fall rapidly as a function of distance from the electrode. Designs of this type usually require additional electrodes (noncollecting or focusing) to increase the overall electric field in the device so that low-field regions do not exist.
A few of the detector designs which operate on this principle are the coplanar grid [3], [4], small pixel [5], [6], [7], SpectrumPlus [8], and lateral contact [9] designs. We have explored the spatial response of several of these designs with our spatial gamma mapping system. We must emphasize that the samples we have examined were chosen due to abnormal detector response. Thus, many of the results presented here are from suboptimal detectors which are not a fair representation of the capability of the best devices. In most instances, material nonuniformity is the cause of the performance loss and such effects will be mentioned where appropriate. We will attempt to differentiate between performance effects caused by device design and those caused by material nonuniformity throughout this paper.
Section snippets
Hardware
The details of our gamma-mapping system have been discussed in a previous publication [10] and therefore, only a brief summary will be given here. The system consists of a computer-controlled translation stage, a finely collimated photon source, and a nuclear spectroscopic electronics chain. The translation stage is capable of submicron resolution which is more than adequate for our needs. We utilize an isotopic photon source (57Co) collimated with a Pb–Bi pin-hole collimator which gives a 200
Device performance
A device has essentially two failure modes which are not necessarily exclusive. One particular region in the detector may exhibit the same overall spectral response with lower sensitivity (same energy resolution, FWHM, etc., but fewer counts in the photopeak), or this region may maintain its sensitivity while the spectral response is altered (same overall counts but the distribution within the spectrum is altered). It has been our observation that loss of detector sensitivity over some portion
Conclusions
We have successfully applied our gamma mapping system to characterize various CZT device designs. The information gained with the system is useful for both characterization of the effects of device design and the effects of material uniformity. We have attempted to focus on the device design aspect in this publication, but it is difficult to find devices that do not exhibit some type of performance degradation due to material nonuniformity.
The lateral contact device is simple to fabricate but
Acknowledgements
The authors would like to acknowledge the DOE Office of Research and Development, of the Office of Nonproliferation and National Security (NN-20) for their financial support. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Department of Energy under contract number: DE-AC04-94AL85000.
References (12)
Nucl. Instr. and Meth. A
(1996)- et al.
Nucl. Instr. and Meth. A
(1997) - et al.
Nucl. Instr. and Meth. A
(1999) - et al.
Nucl. Instr. and Meth. A
(1999) - et al.
Mater. Res. Soc. Symp. Proc.
(1998) IEEE Trans. Nucl. Sci.
(1995)