Nuclear Structure Relevant to Neutrinoless Double Beta Decay: 76Ge and 76Se

TL;DR

This study measures neutron valence orbit occupations in 76Ge and 76Se to improve nuclear matrix element calculations relevant for neutrinoless double beta decay, revealing discrepancies with theoretical models.

Contribution

It provides experimental data on valence neutron occupations in 76Ge and 76Se, highlighting differences from QRPA calculations to aid decay rate predictions.

Findings

Fermi surface is more diffuse than predicted by QRPA

Significant changes in three orbit populations between ground states

Discrepancies suggest need for refined nuclear models

Abstract

The possibility of observing neutrinoless double beta decay offers the opportunity of determining the neutrino mass IF the nuclear matrix element were known. Theoretical calculations are uncertain and measurements of the occupations of valence orbits by nucleons active in the decay can be important. The occupation of valence neutron orbits in the ground states of 76Ge and 76Se were determined by precisely measuring cross sections for both neutron-adding and removing transfer reactions. Our results indicate that the Fermi surface is much more diffuse than in theoretical (QRPA) calculations. We find that the populations of at least three orbits change significantly between these two ground states while in the calculations the changes are confined primarily to one orbit.