GRETA: The proof-of-principle for gamma-ray tracking

Gamma-ray tracking is a new concept for the detection of gamma radiation. O ne proposed implementation of this concept, called GRETA for Gamma Ray Ener gy Tracking Array, aims at an improvement in nuclear physics. It is based o n an array of highly segmented HPGe detectors. We have developed new techni ques to determine three-dimensional positions and energies of interactions based on pulse-shape analysis in a two-dimensionally segmented Ge detector and algorithms which use these informations to reconstruct the scattering s equence of gamma rays, even if many gamma rays hit the array at the same ti me, Such a detector will have a high efficiency and a good peak-to-backgrou nd ratio, an excellent Doppler-shift correction and high count rate capabil ity, as well as a high polarization sensitivity. However, the concept will not only improve the sensitivity for gamma rays in nuclear physics but larg e potential gain is also possible in other areas, such as gamma -ray imagin g used in strophysics of medicine. Only recently we have shown the proof-of -principle of the proposed concept based on the measured position resolutio n of better than 1 mm in three dimensions in a 36-fold segmented Ge detecto r at a gamma -ray energy of 374 keV.