<title>Position-sensitive germanium detectors for gamma-ray imaging and spectroscopy</title>

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 fieldshaping 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.

Three-Dimensional Position-Sensitive Germanium Detectors

“…However, Ge detectors with the position sensitivity required for imaging applications have not been widely used primarily because of the difficulties in fabricating such detectors. Progress in the development of these detectors has however been made over the years [13,[16][17][18][19][20][21][22][23][24][25][26]. In particular, the amorphous-semiconductor contact technology developed by our group has the potential to satisfy the needs of imaging applications [24][25][26].…”

“…The LBNL-developed amorphous-semiconductor contact technology can potentially fulfill the needs in this area. Through this project, this fabrication technology has been further refined for the specific requirements of imaging applications [25,26]. The basic structure of a prototype orthogonal-strip detector made using amorphous-semiconductor contacts is illustrated in Figure 5.…”

“…This uncertainty will lead to a loss of image resolution. To mitigate this problem, we have developed a method to measure the depth of the gamma-ray interaction (z position) thereby producing a three-dimensional position-sensitive detector [26,31]. The technique, as depicted in Figure 7, relies on measuring the time difference between the electron arrival at the x strips and the hole arrival at the y strips.…”

New contact development for Si(Li) orthogonal-strip detectors

“…This is due to the fact that the Fermi level of α-Si lies somewhere within the band gap of crystalline silicon. Previous experiments showed that layers of α-Ge and α-Si worked well as contacts for germanium detectors and the segmentation was relatively simple [2].…”

“…The additional requirement for a high operating temperature (200 K to 240 K) makes it necessary to use silicon detectors instead of germanium since germanium requires liquid nitrogen cooling. The orthogonal strips on the front and back sides of the individual detector will be used to determine the x-and y-coordinates of the Compton interaction, whereas the z-coordinate will be determined based on a timing method ( [2] and [3]). …”