TANGO - A New Tracking Algorithm for Gamma-Rays

The rapidly advancing field of the nuclear spectroscopy into a large extend owes its success to the progress in the development of gamma-ray detectors. A new generation of germanium spectrometers employing principles of gamma-ray tracking like AGATA [1], GRETA [2] and DESPEC [3] are expected to make a significant improvement in the efficiency and the Peak/Total ratio over the presently existing detector clusters such as Euroball and Gammasphere. Despite the obvious advantages, there are still some limitations of the novel gamma-ray tracking technique which is the topic of the present investigations. The Compton scattering process dominates the photonmatter interaction cross section in the energy region starting from 160 keV till 8 MeV. The principles of gammaray tracking employ the energy and momentum conservation relations for Compton scatterings inside the germanium detector. For this purpose a 3D position information for each interaction point is required. It is obtained by the application of the well established technique of the pulse shape analysis (PSA) [4–7]. Despite of the large volumes of the modern germanium detectors, their efficiency is still much lower than 100%. The incomplete absorption of the initial photon energy due to the (multiple) Compton scatterings degrades the efficiency and increases the spectral background (Compton tail). The later causes the low Peak/Total ratio which limits the sensitivity of the detectors to individual gamma-lines. The tracking algorithms which were developed for AGATA and GRETA arrays were not able to improve the Peak/Total ratio despite having all the information on the Compton scatterings in hand [8,9]. In order to overcome this limitation a new gamma-ray tracking algorithm “TANGO” was developed. The algorithm considers the cases of the full-energy absorption and the escape events separately finding the most appropriate fit for the observed photon track. The fit quality is described in terms of a “Figure of Merit”, two of which are calculated assuming these two kinds of events. The algorithms which are based on a single “Figure of Merit” (like MGT [8]) can not properly distinguish between the two possible reasons for a “bad” fit: an incomplete energy deposition (escape) and deviations of the PSA reconstructed positions from the real ones which are larger than the average. In contrast, the TANGO algorithm allows for a much better separation between photo peak and escape events even if the PSA makes errors. Figure 1 shows the performance of the TANGO algorithm in comparison with the MGT code which was the previous best result. In the case of the low photon multiplicities (e.g. single events) TANGO shows clearly much better performance. With a very little compromise of the efficiency the Peak/Total can be increased till 0.97 comparing to 0.8 for MGT. This corresponds to the decrease of the Compton background by a factor of 7. Such high values for the Peak/Total are unprecedented in germanium gamma-ray spectroscopy and should in near future significantly improve the quality of the spectroscopic information. Further development of the algorithm for the case of large photon multiplicities (e.g. 30) is currently in progress.