Mass measurements of neutron-rich Rb and Sr isotopes

We report on the mass measurements of several neutron-rich $\mathrm{Rb}$ and $\mathrm{Sr}$ isotopes in the $A\approx 100$ 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 $3\sigma$ 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 $80\mathrm{keV}$ 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 $A\approx 100$ region of the nuclear chart.

Figure 1

Typical time-of-flight (TOF) spectrum for ions extracted from the EBIT recorded with a microchannel plate detector (MCP) just upstream of MPET. Charge state $q=10+$ (central peak) was used for mass determination of all the isotopes to minimize the amount of ionized residual gas (colored peaks) entering the Penning trap.

Figure 2

Time-of-flight (TOF) resonance of ${}^{101}\mathrm{Sr}[t_{1/2}=118\left(3\right)\mathrm{ms}]$. Black dots are the average time-of-flight and associated standard deviation. Continuous red line is a fit [23] to the data.

Figure 3

Black points show all TOF-ICR data for ${}^{98}\mathrm{Rb}^{g,m;q=15+}$ from Ref. [12] added. Dashed lines show individual contributions of ${}^{98}\mathrm{Rb}^{m;q=15+}$ and ${}^{98}\mathrm{Rb}^{g;q=15+}$, respectively, as well as a fit to both states together (solid line). The reduced ${\chi }^2$ for a two-state model is ${\chi }_{r,g+m}^2=1.7$ as compared to ${\chi }_r^2=2$ for a one-state model.

Figure 4

(a) Fractional $r$-process abundances, each relative to the most abundant isotope, by using the waiting-point approximation for Rb, Sr isotopic chains with temperature $T=1.4\times {10}^9$ K and neutron density $n_n={10}^{20}{\mathrm{cm}}^{-3}$ for $S_n$ from the AME 2012 [33, 39] (open symbols) and $S_n$ from this work (full symbols). (b) The ratio $Y_{\text{TITAN}}/Y_{\text{AME}2012}$ corresponding to the fractional abundances shown in panel (a).

Figure 5

Two-neutron separation energies for $Z=36–39$ (Kr to Y) versus neutron number. For comparison with AME 2012, $S_{2n}$ based on new TITAN masses are plotted in red circles. $S_{2n}$ graphed with open circles or squares are based on estimated mass values [33, 39].