F. Nowacki

Disassembling the nuclear matrix elements of the neutrinoless ββ decay

Abstract In this article we analyze the nuclear matrix elements (NME) of the neutrinoless double beta decays ( 0 ν β β ) of the nuclei 48 Ca, 76 Ge, 82 Se, 124 Sn, 128 Te, 130 Te and 136 Xe in the framework of the interacting shell model (ISM). We study the relative value of the different contributions to the NMEs, such as higher order terms in the nuclear current, finite nuclear size effects and short range correlations, as well as their evolution with the maximum seniority permitted in the wave functions. We discuss also the build-up of the NMEs as a function of the distance between the decaying neutrons. We calculate the decays to 0 1 + final states and find that these decays are at least 25 times more suppressed with respect to the ground state to ground state transition.

First results of the search for neutrinoless double-beta decay with the NEMO 3 detector.

The NEMO 3 detector, which has been operating in the Frejus underground laboratory since February 2003, is devoted to the search for neutrinoless double beta decay (bb0nu). Half-lives of the two neutrino double beta decays (bb2nu) have been measured for 100Mo and 82Se. After 389 effective days of data collection from February 2003 until September 2004 (Phase I), no evidence for neutrinoless double beta decay was found from ~7kg of 100Mo and ~1 kg of 82Se. The corresponding lower limits for the half-lives are 4.6 x 10^23 years for 100Mo and 1.0 x10^23 years for 82Se (90% C.L.). Depending on the nuclear matrix elements calculation, limits for the effective Majorana neutrino mass are<0.7-2.8 eV for 100Mo and<1.7-4.9 eV for 82Se

Shell-model calculations of stellar weak interaction rates. I. Gamow-Teller distributions and spectra of nuclei in the mass range A = 45–65

Abstract Electron capture and beta-decay rates on nuclei in the mass range A = 45–65 play an important role in many astrophysical environments. The determination of these rates by large-scale shell-model calculations is desirable, but it requires to reproduce the Gamow-Teller strength distributions and spectra of the pf shell nuclei. We show in this paper that large-scale shell-model calculations, employing a slightly monopole-corrected version of the well-known KB3 interaction, fulfill these necessary requirements. In particular, our calculations reproduce the experimentally available GT + and GT − strength distributions and the nuclear halflives, and describe the nuclear spectra appropriately.

Collapse of the N=28 shell closure in (42)Si.

The energies of the excited states in very neutron-rich (42)Si and (41,43)P have been measured using in-beam gamma-ray spectroscopy from the fragmentation of secondary beams of (42,44)S at 39A MeV. The low 2(+) energy of (42)Si, 770(19) keV, together with the level schemes of (41,43)P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that (42)Si is best described as a well-deformed oblate rotor.

Shell Model Studies of the Double Beta Decays of {sup 76}Ge, {sup 82}Se, and {sup 136}Xe

The lifetimes for the double beta decays of

Shell model study of the neutron-rich nuclei around N=28

~{76}

Search for 28O and study of neutron-rich nuclei near the N = 20 shell closure

Ge,

Limits on different Majoron decay modes of Mo-100 and Se-82 for neutrinoless double beta decays in the NEMO-3 experiment

~{82}

Shell Model description of the ββ decay of 136Xe

Se and

How magic is the magic68Ninucleus

~{136}