Mass measurements of neutron-rich Rb and Sr isotopes

Abstract

We report on the mass measurements of several neutron-rich $\mathrm{Rb}$ and $\mathrm{Sr}$ isotopes in the $A\ensuremath{\approx}100$ region with the TITAN Penning-trap mass spectrometer. By using highly charged ions in the charge state $q=10+$, the masses of $^{98,99}\mathrm{Rb}$ and $^{98--100}\mathrm{Sr}$ have been determined with a precision of 6--12 keV, making their uncertainty negligible for $r$-process nucleosynthesis network calculations. The mass of $^{101}\mathrm{Sr}$ has been determined directly for the first time with a precision eight times higher than the previous indirect measurement and a deviation of $3\ensuremath{\sigma}$ when compared to the Atomic Mass Evaluation. We also confirm the mass of $^{100}\mathrm{Rb}$ from a previous measurement. Furthermore, our data indicate the existence of a low-lying isomer with $80\phantom{\rule{4pt}{0ex}}\mathrm{keV}$ excitation energy in $^{98}\mathrm{Rb}$. We show that our updated mass values lead to minor changes in the $r$ process by calculating fractional abundances in the $A\ensuremath{\approx}100$ region of the nuclear chart.