Position Sensitive Germanium Detectors for the Advanced Compton Telescope

The nuclear line region of the gamma ray spectrum remains one of the most challenging and elusive goals of high energy astrophysics. The scientific objectives are well defined, but require well over a factor of 10 increase in sensitivity compared to present day instruments to be achieved. The most promising approach to achieve this sensitivity and a broad range of scientific objectives is offered by an Advanced Compton Telescope (ACT) that would function in the 0.5 to 30 MeV energy range. The ACT builds on the successful COMPTEL instrument on NASA’s Gamma Ray Observatory by substituting modern detectors with over an order of magnitude better energy and spatial resolution. Germanium detectors are a natural choice, as they are available in large volumes, provide the best possible energy resolution, and are capable of fine spatial resolution. These improvements alone provide the required gain in sensitivity. Further optimization and the use of more sophisticated techniques promise even greater improvements. We discuss the current status of the germanium detector technology. New results from the characterization of a crossed strip detector using amorphous-germanium contacts and a demonstration of three-dimensional position resolution are presented.