Abstract
The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double-beta decay in ^{76}Ge with an array of about 40 high-purity isotopically-enriched germanium detectors. The experimental signature of the decay is a monoenergetic signal at Q_{beta beta }=2039.061(7) keV in the measured summed energy spectrum of the two emitted electrons. Both the energy reconstruction and resolution of the germanium detectors are crucial to separate a potential signal from various backgrounds, such as neutrino-accompanied double-beta decays allowed by the Standard Model. The energy resolution and stability were determined and monitored as a function of time using data from regular ^{228}Th calibrations. In this work, we describe the calibration process and associated data analysis of the full Gerda dataset, tailored to preserve the excellent resolution of the individual germanium detectors when combining data over several years.
Highlights
Neutrinoless double-β (0νββ) decay is a hypothetical, second-order weak interaction process in which a nucleus changes its charge number by two units with the emission of two electrons but without accompanying anti-neutrinos
The GERmanium Detector Array (Gerda) collaboration searched for the 0νββ decay of the isotope 76Ge with a Q-value of Qββ = 2039.061(7) keV [2] by operating high-purity germanium (HPGe) detectors isotopically enriched to >86% in 76Ge, making them the potential source of 0νββ decay
The broad energy germanium (BEGe) detectors are smaller but offer superior energy resolution and pulse shape discrimination (PSD) properties compared to the coaxial detectors [3], while the inverted coaxial (IC) detectors provide energy resolution and PSD properties similar to the BEGe detectors [4] but with a larger mass comparable to that of the coaxial detectors, allowing for the easier design of larger germanium arrays
Summary
Neutrinoless double-β (0νββ) decay is a hypothetical, second-order weak interaction process in which a nucleus changes its charge number by two units with the emission of two electrons but without accompanying anti-neutrinos This lepton-number violating process is only permitted if neutrinos are massive Majorana fermions, i.e. if there is a Majorana mass term in the Lagrangian of the underlying theory. One strength of HPGe detectors is their unparalleled energy resolution (typically σ /E∼0.1% at Qββ ) It permits the almost complete rejection of background events from regular two-neutrinoaccompanied double-β decays [10], an otherwise irreducible background in 0νββ decay searches [11,12]. 3 we discuss the analysis of the calibration data and the energy scale determination, including the procedures employed to monitor and maintain the stability of the HPGe detectors over time.
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