Three-dimensional position sensing and field shaping in orthogonal-strip germanium gamma-ray detectors

We have fabricated a prototype orthogonal-strip germanium detector for gamma-ray imaging studies. With this detector we demonstrate that a gamma-ray interaction event in the detector can be located in three dimensions. In particular, we determine the interaction depth from the difference in time in the arrival of the holes at an electrode on one side of the detector and the arrival of the electrons at an electrode on the opposing detector surface. This depth of interaction sensing should lead to improved image resolution. A separate issue with such a detector is the loss of energy resolution and efficiency resulting from incomplete charge collection for interaction events taking place in the gap regions between the detector electrodes. We also demonstrate with our prototype detector that this problem can be substantially reduced by using field-shaping electrodes. A bias applied between the charge-sensing electrodes and the field-shaping electrodes improves the charge collection to the charge-sensing electrodes and consequently the detector performance.