Examining the nuclear mass surface of Rb and Sr isotopes in the A≈104 region via precision mass measurements

Abstract

Background: The neutron-rich $A\ensuremath{\approx}100, N\ensuremath{\approx}62$ 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 $^{103}\mathrm{Rb}$ and $^{103--105}\mathrm{Sr}$ with uncertainties of less than 45 keV/${c}^{2}$. The uncertainties in the mass excess value for $^{102}\mathrm{Rb}$ and $^{102}\mathrm{Sr}$ 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.