Wed·Nov 1, 2017Wednesday·November 1, 2017

1. 01*

   Is Seniority a Partial Dynamic Symmetry in the First $\nu g_{9/2}$ Shell?

   A.I. Morales🇮🇹 · G. Benzoni🇮🇹 · H. Watanabe🇨🇳 · G. de Angelis🇮🇹 · S. Nishimura🇯🇵 · L. Coraggio🇮🇹 · A. Gargano🇮🇹 · N. Itaco🇮🇹 · T. Otsuka🇯🇵 · Y. Tsunoda🇯🇵 · P. Van Isacker🇫🇷 · F. Browne🇬🇧 and 51 other authors

   The low-lying structures of the midshell $\nu g_{9/2}$ Ni isotopes ${}^{72}$Ni and ${}^{74}$Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following $\beta$ decay of the mother nuclei ${}^{72}$Co and ${}^{74}$Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first $(8^{+})$ levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron $g_{9/2}$ shell. We find that the disappearance of the seniority isomerism in the $(8_1^{+})$ states can be explained by a lowering of the seniority-four $(6^{+})$ levels as predicted years ago. For ${}^{74}$Ni, the internal de-excitation pattern of the newly observed $(6_2^{+})$ state supports a restoration of the normal seniority ordering up to spin $J=4$. This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near ${}^{78}$Ni.

   nucl-exnucl-thPLB(2018)·25 citations
2. 02*

   Direct detection of the 229Th nuclear clock transition

   Lars von der Wense · Benedict Seiferle · Mustapha Laatiaoui🇩🇪 · Jürgen B. Neumayr · Hans-Jörg Maier · Hans-Friedrich Wirth · Christoph Mokry · Jörg Runke · Klaus Eberhardt🇩🇪 · Christoph E. Düllmann · Norbert G. Trautmann · Peter G. Thirolf

   Today's most precise time and frequency measurements are performed with optical atomic clocks. However, it has been proposed that they could potentially be outperformed by a nuclear clock, which employs a nuclear transition instead of the atomic shell transitions used so far. By today there is only one nuclear state known which could serve for a nuclear clock using currently available technology, which is the isomeric first excited state in ${}^{229}$Th. Here we report the direct detection of this nuclear state, which is a further confirmation of the isomer's existence and lays the foundation for precise studies of the isomer's decay parameters. Based on this direct detection the isomeric energy is constrained to lie between 6.3 and 18.3 eV, and the half-life is found to be longer than 60 s for ${}^{229\mathrm{m}}$Th${}^{2+}$. More precise determinations appear in reach and will pave the way for the development of a nuclear frequency standard.

   nucl-exphysics.ins-detNature(2016)·150 citations
3. 03*

   Search for Zero-Neutrino Double Beta Decay in 76Ge with the Majorana Demonstrator

   C.E. Aalseth🇺🇸 · N. Abgrall🇺🇸 · E. Aguayo🇺🇸 · S.I. Alvis🇺🇸 · M. Amman🇺🇸 · I.J. Arnquist🇺🇸 · F.T. Avignone III🇺🇸 · H.O. Back🇺🇸 · A.S. Barabash🇷🇺 · P.S. Barbeau🇺🇸 · C.J. Barton · P.J. Barton and 117 other authors

   The \MJ\ Collaboration is operating an array of high purity Ge detectors to search for neutrinoless double-beta decay in ${}^{76}$Ge. The \MJ\ \DEM\ comprises 44.1~kg of Ge detectors (29.7 kg enriched in ${}^{76}$Ge) split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. Here we present results from data taken during construction, commissioning, and the start of full operations. We achieve unprecedented energy resolution of 2.5 keV FWHM at \qval\ and a very low background with no observed candidate events in 10 kg yr of enriched Ge exposure, resulting in a lower limit on the half-life of $1.9\times 10^{25}$ yr (90\% CL). This result constrains the effective Majorana neutrino mass to below 240 to 520 meV, depending on the matrix elements used. In our experimental configuration with the lowest background, the background is $4.0_{-2.5}^{+3.1}$ counts/(FWHM t yr).

   nucl-exphysics.ins-detPRL(2018)·238 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.