Brugnara, Daniele (2018) Perspectives on the measurement of competitive double gamma decay with the AGATA tracking array. [Magistrali biennali]
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In this thesis, the capability of AGATA to measure the rare competitive double-gamma decay has been studied with a detailed simulation. This consists in a rare process, in which an excited state of a nucleus decays emitting two gamma rays despite the emission of a single photon is allowed. From a theoretical standpoint, the decay is correctly described as a second order process in perturbative QED. The measurement, would not only return more detailed physical information, namely on the energy and angular distributions of the photons, but also test the capabilities of the advanced gamma-ray spectrometer AGATA. The main challenge that the measurement faces is the suppression of Compton events without relying on timing properties due to the resolution limitations of germanium detectors. The thesis is focused on assessing whether the good energy resolution and spatial reconstruction properties of the detector, together with a strict event selection and optimization, can overcome the difficulties of such measurement. The emission of single and double-gamma photons has been simulated and optimal values of the algorithms were found for this specific measurement. The performance of the tracking was characterized and a further event selection was performed. While the measurement of the competitive double-gamma decay has proved itself challenging from an experimental standpoint, the current work only represents a starting point and a statement of the current capabilities. In fact, while the tracking algorithm showed a good performance with its optimal parameters, the need for a specific algorithm is argued in this thesis. In that sense, the community is working towards new tracking algorithms based on a different statistical approach. Since the array is in continuous development and improvement by the AGATA community, many progresses are expected as the research proceeds and many of them will likely be of crucial importance for the measurement of the competitive double-gamma decay. At last, a preliminary analysis of data taking with AGATA and a 137Cs source was performed. The background activity as well as the neutron damage correction and the stability of the calibration over time during the time span of the experiment have been discussed. While the analysis is only preliminary, a final comparison between the experimental data and the simulation has been performed.
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