Tracking $ \gamma$ rays in highly segmented HPGe detectors: A review of AGATA and GRETINA

International audienceThe AGATA and GRETINA/GRETA tracking spectrometers allow for the highest sensitivity $ \gamma$ -ray spectroscopy achievable today. Central to these instruments is the task of tracking $ \gamma$ rays, the process by which the interaction points observed in a tracking array are assembled into actual $ \gamma$ rays. Furthermore, the process evaluates the confidence with which individual photons are assembled, i.e., whether the track reconstruction results in a full-energy $ \gamma$ ray or rather in an event associated with Compton scattering, absorption in dead layers or other effects requiring rejection in order to produce spectra with a good peak-to-total ratio. The physics capabilities provided by these spectrometers as well as the different methods of tracking that have been developed are reviewed. The methods used to determine the interaction points from the charge collection signals obtained from the central contact and the segments of HPGe detectors, are presented as well. The performances of the existing tracking arrays, together with possible improvements, are summarized and discussed