Inelastic neutron scattering studies of76Ge and76Se: relevance to elevance to neutrinoless double-β decay

B P Crider,F M Prados-Estévez,M T Mcellistrem,J M Allmond,E E Peters,T J Ross,J R Vanhoy,S W Yates

doi:10.1051/epjconf/20159305001

Abstract

Inelastic neutron scattering measurements were performed at the University of Kentucky Accelerator Laboratory on enriched 76 Ge and 76 Se scattering samples. From measurements at incident neutron energies from 2.0 to 4.0 MeV, many new levels were identified and characterized in each nucleus; level lifetimes, transition probabilities, multipole mixing ratios, and other properties were determined. In addition, γ-ray cross sections for the 76 Ge(n,n′γ) reaction were measured at neutron energies up to 5.0 MeV, with the goal of determining the cross sections of γ rays in 2040-keV region, which corresponds to the region of interest in the neutrinoless double β decay of 76 Ge. Gamma rays from the three strongest branches from the 3952-keV level were observed, but the previously reported 2041-keV γ ray was not. Population cross sections across the range of incident neutron energies were determined for the 3952-keV level, resulting in a cross section of ~0.1 mb for the 2041-keV branch using the previously determined branching ratios. Beyond this, the data from these experiments indicate that previously unreported γ rays from levels in 76 Ge can be found in the 2039-keV region.

Highlights

1.1 Nuclear structure of the stable A = 76 nuclei1.2 Neutrinoless double-β decayThe observation of neutrino oscillations has revealed that neutrino flavors mix and neutrinos have mass; oscillation experiments yield only information on (Δm)2, and the absolute mass scale remains unknown [6]
The observation of 0νββ provides the best method for obtaining the mass of the neutrino, and it is the only practical way to establish if neutrinos are Majorana particles
At the University of Kentucky Accelerator Laboratory (UKAL), we have studied the nuclear structure of 76Ge and 76Se, the double-β decay daughter, with the (n,ncJ) reaction

Summary

Neutrinoless double-β decay

The observation of neutrino oscillations has revealed that neutrino flavors mix and neutrinos have mass; oscillation experiments yield only information on (Δm), and the absolute mass scale remains unknown [6]. Neutrinoless double-β decay, 0νββ, a leptonnumber-violating nuclear process that has not yet been observed, will occur only if the neutrinos have mass and are Majorana particles, i.e., they are their own antiparticles. EPJ Web of Conferences where G0ν(Qββ,Z) is the known phase-space factor for the emission of the two electrons, ¢mββ is the effective Majorana mass of the electron neutrino, and M0ν is a nuclear matrix element which must be calculated from nuclear structure models. A focus of many of our recent measurements has been to provide the detailed nuclear structure data for guiding these model calculations. Data constraining the nuclear model calculations for double-β decay become of particular importance as experimental searches for 0νββ, e.g., the MAJORANA and GERDA collaborations, are coming online or are pushing to increasing sensitivities. With colleagues from Yale University, Triangle Universities Nuclear Laboratory (TUNL), the Technische Universität Darmstadt, and the Australian National University, the University of Kentucky group has initiated a diverse program for obtaining detailed spectroscopic data relevant to 0νββ decay of 76Ge

Background Issues in double-β decay

Nuclear structure measurements

Cross section measurements

Data analysis

Results and discussion