A germanium gamma ray imager with 3D position sensitivity

A germanium based gamma ray imager with 3D position sensitivity is being developed. The imager will detect gamma rays in the range of ∼10 keV to 1.5 MeV and locate their interactions in the germanium to within 2 mm in X, Y and Z. The instrument is segmented into 39 orthogonal strips in both the X and Y directions with a 2 mm pitch. Each channel is separately instrumented with a miniature low-noise charge-sensitive preamplifier followed by shaping and timing electronics. Germanium based imagers with 2-D position sensitivity have recently been developed and fully instrumented by our group. The current instrument adds the capability for determining the Z position (depth of interaction). The Z coordinate is found by looking at the relative timing of the signals induced by the drifting electrons and holes. A relatively simple analog approach for determining the depth of interaction was implemented. Some of the intended applications for this type of instrument include balloon based imaging experiments and field portable devices for environmental remediation. Therefore, much of the effort was concentrated on developing a detector and electronics scheme that was simple, compact, and low power. The instrument is described with emphasis on the technique used for measuring the Z coordinate. Results obtained from spectroscopy as well as Z position measurements are shown.