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

Background: The neutron-rich $A\approx 100, N\approx 62$ 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.

Figure 1

A time-of-flight spectrum of ${}^{103}\mathrm{Rb}^{+}$ and ${}^{103}\mathrm{Sr}^{+}$ ions after 386 turns inside TITAN's MR-TOF-MS. Inset: Zoomed area containing ${}^{103}\mathrm{Sr}^{+}$ and ${}^{103}\mathrm{Rb}^{+}$ ions on a logarithmic scale. ${}^{84}\mathrm{Sr}{}^{19}\mathrm{F}^{+}$ served as calibration species for conversion from time to mass. The spectrum contains data from a single file. Multiple files were recorded and analyzed for final masses.

Figure 2

Mass excess difference between the value measured in this work and the value reported in AME2016 [16], i.e., ${\mathrm{ME}}_{\text{TITAN}}-{\mathrm{ME}}_{\text{AME2016}}$ for (a) ${}_{37}\mathrm{Rb}$ and (b) ${}_{38}\mathrm{Sr}$ isotopes. The shaded band indicates the AME2016 uncertainties, and slanted lines in the shaded region denote values from extrapolation. ME difference is also plotted for a previous measurement from ISOLTRAP [30], published after the AME2016.

Figure 3

(a) Two-neutron separation energy $S_{2n}$ and (b) two-neutron shell gap ${\mathrm{\Delta }}_{2n}$ as a function of neutron number for isotopic chains in the neighborhood of Rb and Sr. ${\mathrm{\Delta }}_{2n}$ have been offset for clarity. The values measured in this work are shown by blue triangles (Rb) and red circles (Sr) connected by solid lines. AME2016 values are shown with different symbols and connected by the dashed line. Open symbols denote AME2016 extrapolations. Shell closure at $N=50$ (peak) and shape transition at $N=59$ (dip) are visible in both plots.

Figure 4

(a) $S_{2n}$ values for isotopes of ${}_{38}\mathrm{Sr}$ compared with values from different mass models, and (b) the differences between $S_{2n}$ values from this work and different mass models. Circles and squares represent experimental and AME2016 values, respectively, with open squares being extrapolated values.

Figure 5

(a) Two-neutron pairing gap $D_n$ calculated from this work and AME2016 for Sr (shifted by 1 MeV for clarity) and Rb isotopes. Extrapolated values from AME2016 are denoted by open squares and triangles. Shell closure at $N=50$ is clearly visible. (b) $D_n$ for ${}_{38}\mathrm{Sr}$ isotopes compared with $D_n$ values from different mass models.

Figure 6

(a) Fractional $r$-process abundance for Rb and Sr isotopes relative to most abundant isotopes using waiting point approximation. Open circles are values taken from AME2016. Filled circles denote a combination of values calculated from AME2016 values and new TITAN mass values from this work. (b) The ratio of fractional abundances ($Y_{\text{AME2016}}/Y_{\text{TITAN}}$) corresponding to values plotted in panel (a).