Institut de Recherche sur les Lois Fondamentales de l'Univers

Position sensitivity of the AGATA prototype crystal analyzed using a database of calculated pulse shapes

The concept of the AGATA spectrometer is based on a shell of closely packed tapered hexagonal HPGe detectors that allow the tracking of individual gamma rays, i.e. the reconstruction of the paths of interactions of each gamma ray. The needed position sensitivity is achieved by an electrical segmentation of the outer contact into 36 individual segments. This is accomplished with six longitudinal boundaries lying on the flat surface of the taper and five transverse boundaries. While all charge generated in a single interaction will be finally collected at a single electrode, the drift of electrons and holes through the crystal can induce charge signals on multiple electrodes. At the end of the charge collecting process the neighboring electrodes that see transient induced signals will not have collected any net charge. The position of an interaction can be determined by means of a pulse shape analysis (PSA). The radial position is mainly reflected by the rise time of the net charge signal, whereas the azimuthal position and the depth can be determined by analyzing the size and polarity of the transient signals in adjacent segments. Two databases of simulated pulse shapes have been generated for crystals that differ slightly in the depth of the transverse segmentation. This was aimed at finding the segmentation depth that results in the best overall position sensitivity. The following geometry was used for the crystal: a regular hexagonal shape of 90 mm length and 40 mm radius at the back with an tapering angle of 10 deg. The radius of the central contact was 5 mm with a depth of 74 mm. The two segmentations in depth used were (from front to back) segmentation 1: 6, 10, 18.5, 18.5, 18.5 18.5 mm, and segmentation 2: 10, 10, 17.5, 17.5, 17.5, 17.5 mm. The databases are available to everyone in the gamma-ray tracking community. If you are interested in working with the database, please contact A. Gorgen. The relative position sensitivity in the different parts of the crystal has been analyzed by comparing differences in the pulse shapes for interactions taking place 1 mm apart. The results of this study are summarized in a report. The relative position sensitivity in the crystal is visualized there in two-dimensional maps slicing the crystal along various axes. An example of such a sensitivity map is shown above. Red areas indicate regions of excellent sensitivity, while the regions with poorer sensitivity appear blue. The best position sensitivity is achieved near the segment borders, which are clearly visible and differ from the geometrical segment borders because the crystal is not truly coaxial.

Many thanks to Thorsten Kröll, TU München, for his help in the simulations of the pulse shapes. More information on Pulse Shape Analysis can be found on the TU München PSA page.