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Study on modified point contact germanium detectors for low background applications


Salathe,  Marco
Division Prof. Dr. Manfred Lindner, MPI for Nuclear Physics, Max Planck Society;

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Salathe, M. (2015). Study on modified point contact germanium detectors for low background applications. PhD Thesis, Ruprecht-Karls-Universität, Heidelberg.

Point contact germanium (PCGe) detectors play a vital role in research such as, neutrinoless double beta decay, direct dark matter detection, the neutrino magnetic moment and coherent neutrino-nucleus scattering. This dissertation presents an experimental investigation of two commercially available, almost identical large volume PCGe detectors (~760 g). Their point contact size was stepwise modified according to specified simulations of the electric field in the detectors. The reduction of the point contact size improved both, the pulse shape discrimination efficiency of background signals at a few MeV and the noise from electronics. The lowest contribution of noise to the energy resolution was (299 ± 2) eV and (336 ±2) eV (FWHM) for the two detectors, respectively. The better value translates to an energy threshold of 1 keV for an experiment with a low background level. A sensitivity study concluded that a resolution of less than 150 - 180 eV is required to detect the coherent neutrino-nucleus scattering near nuclear reactors. This is a factor of two better than the currently obtained value. Additionally, two different approaches have been used to determine the Fano factor. They both yielded consistent results, the better result is 0.1076 ± 0.0005. Before the most optimal results could be obtained, an evaluation of three shaping filters (the rounded-top cusp, rounded-top trapezoidal and Gaussian filter) and a ballistic deficit correction was performed. Once completed the most ideal combinations of filters were used to procure the dissertation's findings.