Development of a fast pulse-shape analysis for asymmetric AGATA germanium detectors

The Advanced GAmma Tracking Array AGATA is the first complete 4π γ-ray spectrometer constructed entirely from Germanium detectors. This detector array not only achieves a good energy resolution, but also offers the possibility to determine the position, to within a few millimetres accuracy, where the γ-ray interacts with the Germanium crystal, thereby enabling corrections, of the Doppler shift of the γ-quantum due to the velocity of the parent nucleus. Furthermore, it is possible to reconstruct the track of a Compton scattered γ-photon. The high granularity of such spectrometers is achieved by segmentation of the crystal as well as analysis of the time-profile of the pulses from the preamplifier's output. It was within the framework of the AGATA project that a novel algorithm was developed for calculating the position of interaction of γ-photons in the detector with the aim to facilitate a fast on-line preprocessing of the digitized signals from the detectors. The processing must be done in real-time because of the high data rates set by the experiment (∼ 30 μs per event). The physical limits of the granularity of the detection process were investigated with detailed simulations by means of analysis of the pulse shapes and the distribution of the energy in the segments. The algorithm is based on binary search during which the measured pulse form is reduced by wavelet transformation. The wavelet coefficients thus obtained are compared against a reference databank. The databank contains specially transformed, highly compressed pulse forms together with their positions of interaction. One can thus compare the measured and reference slope-patterns with a quantitative fit. Accuracies in the ascertained three spatial coordinates of a single γ-interaction are on average x∼±0.6 mm; y∼±1 mm and z∼±0.7 mm. Therefore average computation time of ∼100 μs on a 1.7 GHz CPU is necessary. The binary search method can resolve, in the case of a twofold interaction (Compton scattering), the positions of interaction from one another. The ability to distinguish the locations of two interactions depends on the location of the interactions and the distance between them. (orig.)