55 research outputs found

    First observation of scissors mode states in an odd-mass nucleus

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    Nuclear resonance fluorescence experiments are reported to search for enhanced M1 scissors mode states in the deformed odd-mass nucleus Dy163. A concentration of dipole strengths near 3 MeV excitation energy is found, which fits nicely into the systematics observed for M1 excitations in the neighboring even-even Dy isotopes. The observed strength distribution and the decay branching ratios are discussed in the context of the interacting boson-fermion model.Dirección General de Investigación Científica y Técnica PB89-063

    Characterization of 30 76^{76}Ge enriched Broad Energy Ge detectors for GERDA Phase II

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    The GERmanium Detector Array (GERDA) is a low background experiment located at the Laboratori Nazionali del Gran Sasso in Italy, which searches for neutrinoless double beta decay of 76^{76}Ge into 76^{76}Se+2e^-. GERDA has been conceived in two phases. Phase II, which started in December 2015, features several novelties including 30 new Ge detectors. These were manufactured according to the Broad Energy Germanium (BEGe) detector design that has a better background discrimination capability and energy resolution compared to formerly widely-used types. Prior to their installation, the new BEGe detectors were mounted in vacuum cryostats and characterized in detail in the HADES underground laboratory in Belgium. This paper describes the properties and the overall performance of these detectors during operation in vacuum. The characterization campaign provided not only direct input for GERDA Phase II data collection and analyses, but also allowed to study detector phenomena, detector correlations as well as to test the strength of pulse shape simulation codes.Comment: 29 pages, 18 figure

    Background free search for neutrinoless double beta decay with GERDA Phase II

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    The Standard Model of particle physics cannot explain the dominance of matter over anti-matter in our Universe. In many model extensions this is a very natural consequence of neutrinos being their own anti-particles (Majorana particles) which implies that a lepton number violating radioactive decay named neutrinoless double beta (0νββ0\nu\beta\beta) decay should exist. The detection of this extremely rare hypothetical process requires utmost suppression of any kind of backgrounds. The GERDA collaboration searches for 0νββ0\nu\beta\beta decay of 76^{76}Ge (^{76}\rm{Ge} \rightarrow\,^{76}\rm{Se} + 2e^-) by operating bare detectors made from germanium with enriched 76^{76}Ge fraction in liquid argon. Here, we report on first data of GERDA Phase II. A background level of 103\approx10^{-3} cts/(keV\cdotkg\cdotyr) has been achieved which is the world-best if weighted by the narrow energy-signal region of germanium detectors. Combining Phase I and II data we find no signal and deduce a new lower limit for the half-life of 5.310255.3\cdot10^{25} yr at 90 % C.L. Our sensitivity of 4.010254.0\cdot10^{25} yr is competitive with the one of experiments with significantly larger isotope mass. GERDA is the first 0νββ0\nu\beta\beta experiment that will be background-free up to its design exposure. This progress relies on a novel active veto system, the superior germanium detector energy resolution and the improved background recognition of our new detectors. The unique discovery potential of an essentially background-free search for 0νββ0\nu\beta\beta decay motivates a larger germanium experiment with higher sensitivity.Comment: 14 pages, 9 figures, 1 table; ; data, figures and images available at http://www.mpi-hd.mpg/gerda/publi

    The first search for bosonic super-WIMPs with masses up to 1 MeV/c2^2 with GERDA

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    We present the first search for bosonic super-WIMPs as keV-scale dark matter candidates performed with the GERDA experiment. GERDA is a neutrinoless double-beta decay experiment which operates high-purity germanium detectors enriched in 76^{76}Ge in an ultra-low background environment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN in Italy. Searches were performed for pseudoscalar and vector particles in the mass region from 60 keV/c2^2 to 1 MeV/c2^2. No evidence for a dark matter signal was observed, and the most stringent constraints on the couplings of super-WIMPs with masses above 120 keV/c2^2 have been set. As an example, at a mass of 150 keV/c2^2 the most stringent direct limits on the dimensionless couplings of axion-like particles and dark photons to electrons of gae<31012g_{ae} < 3 \cdot 10^{-12} and α/α<6.51024{\alpha'}/{\alpha} < 6.5 \cdot 10^{-24} at 90% credible interval, respectively, were obtained.Comment: 6 pages, 3 figures, submitted to Physical Review Letters, added list of authors, updated ref. [21

    Limits on uranium and thorium bulk content in GERDA Phase I detectors

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    Internal contaminations of 238^{238}U, 235^{235}U and 232^{232}Th in the bulk of high purity germanium detectors are potential backgrounds for experiments searching for neutrinoless double beta decay of 76^{76}Ge. The data from GERDA Phase~I have been analyzed for alpha events from the decay chain of these contaminations by looking for full decay chains and for time correlations between successive decays in the same detector. No candidate events for a full chain have been found. Upper limits on the activities in the range of a few nBq/kg for 226^{226}Ra, 227^{227}Ac and 228^{228}Th, the long-lived daughter nuclides of 238^{238}U, 235^{235}U and 232^{232}Th, respectively, have been derived. With these upper limits a background index in the energy region of interest from 226^{226}Ra and 228^{228}Th contamination is estimated which satisfies the prerequisites of a future ton scale germanium double beta decay experiment.Comment: 2 figures, 7 page

    Final Results of GERDA on the Search for Neutrinoless Double-β\beta Decay

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    The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-β\beta (0νββ0\nu\beta\beta) decay of 76^{76}Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in 76^{76}Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of 5.2×1045.2\times10^{-4} counts/(keV\cdotkg\cdotyr) in the signal region and met the design goal to collect an exposure of 100 kg\cdotyr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg\cdotyr of total exposure. A limit on the half-life of 0νββ0\nu\beta\beta decay in 76^{76}Ge is set at T1/2>1.8×1026T_{1/2}>1.8\times10^{26} yr at 90% C.L., which coincides with the sensitivity assuming no signal.Comment: 7 pages, 3 figures, submitted to Physical Review Letter

    Upgrade for Phase II of the Gerda experiment

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    The GERDA collaboration is performing a sensitive search for neutrinoless double beta decay of 76^{76}Ge at the INFN Laboratori Nazionali del Gran Sasso, Italy. The upgrade of the GERDA experiment from Phase I to Phase II has been concluded in December 2015. The first Phase II data release shows that the goal to suppress the background by one order of magnitude compared to Phase I has been achieved. GERDA is thus the first experiment that will remain background-free up to its design exposure (100 kg yr). It will reach thereby a half-life sensitivity of more than 1026^{26} yr within 3 years of data collection. This paper describes in detail the modifications and improvements of the experimental setup for Phase II and discusses the performance of individual detector components

    Final Results of GERDA on the Search for Neutrinoless Double-β Decay

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    The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-β (0νββ) decay of ^{76}Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in ^{76}Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of 5.2×10^{-4} counts/(keV kg yr) in the signal region and met the design goal to collect an exposure of 100 kg yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg yr of total exposure. A limit on the half-life of 0νββ decay in ^{76}Ge is set at T_{1/2}>1.8×10^{26}  yr at 90% C.L., which coincides with the sensitivity assuming no signal
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