Examining the nuclear mass surface of Rb and Sr isotopes in the A≈104 region via precision mass measurements
Background: The neutron-rich A≈100, N≈62 mass region is important for both nuclear structure and nuclear astrophysics. The neutron-rich segment of this region has been widely studied to investigate shape coexistence and sudden nuclear deformation. However, the absence of experimental data of more neutron-rich nuclei poses a challenge to further structure studies. The derivatives of the mass surface, namely, the two-neutron separation energy and neutron pairing gap, are sensitive to nuclear deformation and shed light on the stability against deformation in this region. This region also lies along the astrophysical r-process path, and hence precise mass values provide experimental input for improving the accuracy of the r-process models and the elemental abundances. Purpose: (a) Changes in deformation are searched for via the mass surface in the A=104 mass region at the N=66 mid-shell crossover. (b) The sensitivity of the astrophysical r-process abundances to the mass of Rb and Sr isotopic chains is studied. Methods: Masses of radioactive Rb and Sr isotopes are precisely measured using a Multiple-Reflection Time-of-Flight Mass Separator (MR-TOF-MS) at the TITAN facility. These mass values are used to calculate two-neutron separation energies, two-neutron shell gaps and neutron pairing gaps for nuclear structure physics, and one-neutron separation energies for fractional abundances and astrophysical findings. Results: We report the first mass measurements of Rb103 and Sr103-105 with uncertainties of less than 45 keV/c2. The uncertainties in the mass excess value for Rb102 and Sr102 have been reduced by a factor of 2 relative to a previous measurement. The deviations from the AME extrapolated mass values by more the 0.5 MeV have been found. Conclusions: The metrics obtained from the derivatives of the mass surface demonstrate no existence of a subshell gap or onset of deformation in the N=66 region in Rb and Sr isotopes. The neutron pairing gaps studied in this work are lower than the predictions by several mass models. The abundances calculated using the waiting-point approximation for the r process are affected by these new masses in comparison with AME2016 mass values.
