Position-sensitive germanium detectors for gamma-ray imaging and spectroscopy

Presented at the SPIE Annual Meeting July 30 to August 4, 2000 Position-sensitive germanium detectors for gamma-ray imaging and spectroscopy M. Amman and P. N. Luke Ernest Orlando Lawrence Berkeley National Laboratory University of California, Berkeley, California 94720 ABSTRACT Gamma-ray imaging with position-sensitive germanium detectors offers the advantages of excellent energy resolution, high detection efficiency, and potentially good spatial resolution. The development of the amorphous-semiconductor electrical contact technology for germanium detectors has simplified the production of these position-sensitive detectors and has made possible the use of unique detection schemes and detector geometries. We have fabricated prototype orthogonal-strip detectors for gamma-ray imaging studies using this contact technology. With these detectors, we demonstrate that a gamma-ray interaction event in the detector can be located in three dimensions. This more accurate determination of the interaction event position should ultimately lead to better image resolution. We have also taken advantage of the bipolar blocking nature of the amorphous-semiconductor contacts in order to investigate the use of field- shaping electrodes. The addition of such electrodes is shown to improve the spectroscopic performance of the detectors by substantially eliminating charge collection to the inter-electrode surfaces. In addition, we demonstrate that this incomplete charge collection process can also be reduced by adjusting the properties of the amorphous-semiconductor layer. In this paper, we summarize the development of these position-sensitive detectors and present the results of our studies with the detectors. Keywords: gamma-ray imaging, gamma-ray spectroscopy, germanium detector, position sensing, orthogonal strip 1. INTRODUCTION Gamma-ray imaging combined with high-resolution spectroscopy together form a powerful tool that is needed for many applications including those in the areas of astronomy, environmental remediation, and nuclear safeguards monitoring. Detectors based on high-purity germanium (Ge) with their excellent energy resolution and efficiency are commonly used for spectroscopy applications. However, producing position-sensitive Ge-based detectors with the fine spatial sensitivity required for many imaging applications has been a significant technical challenge. Despite this, significant progress has been made in the development of these detectors. 1-11 The amorphous-semiconductor contact technology 10-13 in particular could potentially fulfill the needs in this area. This technology has the advantages of requiring only a single contact process for the entire detector, providing automatic passivation between detector electrodes, and allowing fine electrode segmentation for accurate position measurement. We have produced a number of orthogonal-strip detectors based on the amorphous-semiconductor contact technology. This effort was aimed to both better refine the fabrication technology and to develop techniques to improve the performance of these detectors. In planar-geometry position-sensitive Ge detectors, position detection is normally made in only the directions parallel to the detector plane and not in the depth direction. Since gamma rays will interact at random depths within the detector, this imprecise position measurement will lead to image degradation. With our detectors, we have studied the position measurement of the gamma-ray interaction events in all three dimensions rather than just two. 14,15 The depth of interaction in these detectors is obtained from the difference in the arrival time of the holes at an electrode on one side of the detector and that of the electrons at an electrode on the opposing detector surface. This separate detection of the hole collection and the electron collection is fundamental to the technique and is aided by the small-electrode charge collection properties 16-18 achieved with the strip electrodes. The accurate measurement of the energy deposited by an interaction event in the detector is another critical requirement of these detectors. Incomplete charge collection leading to degraded spectroscopic performance can result because of the segmented contact structure. We show that for some interaction events in our detectors, charge is not completely collected within the signal measurement time because of collection to the detector surfaces between electrode segments. Consequently, we have investigated two separate approaches to overcome this problem. In the first approach an electrode (field-shaping electrode) is added between each charge-sensing electrode. By applying the appropriate bias