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Register a new userPhotoexcitation of $^{76}$Ge
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R. Schwengner
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The dipole strength of the nuclide $^{76}$Ge was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.8 and 12.3 MeV at the $gamma$ELBE facility. We identified 210 levels up to an excitation energy of 9.4 MeV and assigned spin $J$ = 1 to most of them. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical $gamma$-ray cascades. The photoabsorption cross section up to the neutron-separation energy was determined and is compared with predictions of the statistical reaction model. The derived photon strength function is compared with results of experiments using other reactions.
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The Majorana Demonstrator is a neutrinoless double-beta decay search consisting of a low-background modular array of high-purity germanium detectors, $sim2/3$ of which are enriched to 88% in $^{76}$Ge. The experiment is also searching for double-beta decay of $^{76}$Ge to excited states (e.s.) in $^{76}$Se. $^{76}$Ge can decay into three daughter states of $^{76}$Se, with clear event signatures consisting of a $betabeta$-decay followed by the prompt emission of one or two $gamma$-rays. This results with high probability in multi-detector coincidences. The granularity of the Demonstrator detector array enables powerful discrimination of this event signature from backgrounds. Using 41.9~kg-y of isotopic exposure, the Demonstrator has set world leading limits for each e.s. decay of $^{76}$Ge, with 90% CL lower half-life limits in the range of $(0.75-4.0)times10^{24}$~y. In particular, for the $2 u$ transition to the first $0^+$ e.s. of $^{76}$Se, a lower half-life limit of $7.5times10^{23}$~y at 90% CL was achieved.
The MAJORANA DEMONSTRATOR is searching for double-beta decay of $^{76}$Ge to excited states (E.S.) in $^{76}$Se using a modular array of high purity Germanium detectors. $^{76}$Ge can decay into three E.S.s of $^{76}$Se. The E.S. decays have a clear event signature consisting of a $betabeta$-decay with the prompt emission of one or two $gamma$-rays, resulting in with high probability in a multi-site event. The granularity of the DEMONSTRATOR detector array enables powerful discrimination of this event signature from backgrounds. Using 21.3 kg-y of isotopic exposure, the DEMONSTRATOR has set world leading limits for each E.S. decay, with 90% CL lower half-life limits in the range of $(0.56-2.1)cdot10^{24}$ y. In particular, for the $2 u$ transition to the first $0^+$ E.S. of $^{76}$Se, a lower half-life limit of $0.68cdot10^{24}$ at 90% CL was achieved.
The GERDA experiment searches for the lepton number violating neutrinoless double beta decay of $^{76}$Ge ($^{76}$Ge $rightarrow$ $^{76}$Se + 2e$^-$) operating bare Ge diodes with an enriched $^{76}$Ge fraction in liquid argon. The exposure for BEGe-type detectors is increased threefold with respect to our previous data release. The BEGe detectors feature an excellent background suppression from the analysis of the time profile of the detector signals. In the analysis window a background level of $1.0_{-0.4}^{+0.6}cdot10^{-3}$ cts/(keV$cdot$kg$cdot$yr) has been achieved; if normalized to the energy resolution this is the lowest ever achieved in any 0$ ubetabeta$ experiment. No signal is observed and a new 90 % C.L. lower limit for the half-life of $8.0cdot10^{25}$ yr is placed when combining with our previous data. The median expected sensitivity assuming no signal is $5.8cdot10^{25}$ yr.
An extensive, model-independent analysis of the nature of triaxial deformation in $^{76}$Ge, a candidate for neutrinoless double-beta ($0 ubetabeta$) decay, was carried out following multi-step Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and $gamma$ bands. Both sequences were determined to be characterized by the same $beta$ and $gamma$ deformation parameter values. In addition, compelling evidence for low-spin, rigid triaxial deformation in $^{76}$Ge was obtained for the first time from the analysis of the statistical fluctuations of the quadrupole asymmetry deduced from the measured $E2$ matrix elements. These newly determined shape parameters are important input and constraints for calculations aimed at providing, with suitable accuracy, the nuclear matrix elements relevant to $0 ubetabeta$.
Neutrinoless double-beta decay($0 ubetabeta$) decay is a hypothetical process that violates lepton number, and whose observation would unambiguously indicate that neutrinos are Majorana fermions. In the standard inverted-ordering neutrino mass scenario, the minimum possible value of m$_{betabeta}$ corresponds to a half-life around 10$^{28}$ yr for $0 ubetabeta$ decay in $^{76}$Ge, which is the target of the next generation of experiments. The current limits of GERDA and textsc{Majorana Demonstrator} indicate a half-life higher than 10$^{26}$ yr. These experiments use high-purity germanium (HPGe) detectors that are highly-enriched in $^{76}$Ge. They have achieved the best intrinsic energy resolution and the lowest background rate in the signal search region among all $0 ubetabeta$ experiments. Taking advantage of these successes, a new international collaboration - the Large Enriched Germanium Experiment for Neutrinoless $betabeta$ Decay (LEGEND) - has been formed to build a ton-scale experiment with discovery potential covering the inverse-ordering neutrino mass range in a decade, following a phased approach. This first part of LEGEND proceedings describes GERDA and textsc{Majorana Demonstrator} capabilities and the general plan of LEGEND to reach the goal, while the second part is focused in the status of the first stage of LEGEND, LEGEND-200.
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