Mass measurements of neutron-rich gallium isotopes refine production of nuclei of the first $r$-process abundance peak in neutron-star merger calculations

We report mass measurements of neutron-rich Ga isotopes ${}^{80-85}\mathrm{Ga}$ with TRIUMF's Ion Trap for Atomic and Nuclear science. The measurements determine the masses of ${}^{80-83}\mathrm{Ga}$ in good agreement with previous measurements. The masses of ${}^{84}\mathrm{Ga}$ and ${}^{85}\mathrm{Ga}$ were measured for the first time. Uncertainties between 25 and 48 keV were reached. The new mass values reduce the nuclear uncertainties associated with the production of $A\approx 84$ isotopes by the $r$-process for astrophysical conditions that might be consistent with a binary neutron star (BNS) merger producing a blue kilonova. Our nucleosynthesis simulations confirm that BNS merger may contribute to the first abundance peak under moderate neutron-rich conditions with electron fractions $Y_e=0.35-0.38$.

Figure 1

Time-of-flight spectra obtained with the MR-TOF-MS around ${}^{84}\mathrm{Ga}^{+}$ confirming the identification of ${}^{84}\mathrm{Ga}^{+}$ by blocking the resonant IG-LIS laser. The lower spectra shows $\approx 1200 {}^{84}{\mathrm{Ga}}^{+}$ ions from which the mass of ${}^{84}\mathrm{Ga}$ has been determined. The red lines are fits to the data using Lorentzian peak shapes.

Figure 2

Experimental two-neutron separation energies $S_{2\text{n}}$ for $Z=30-33$ (Zn to As) as a function of neutron number taken from AME2016 [62], including [71]. For comparison, $S_{2\text{n}}$ values based on the new TITAN masses (red) and based on commonly used mass models (FRDM [72], Duflo-Zucker [73], ETFSI-Q [74], and HFB-21 [75]) are shown.

Figure 3

Solar $r$-process abundance, with uncertainty shown as gray band, in comparison to the abundance resulting from neutron star merger network calculations for different $Y_e$ using GSINet. The individual abundance curves are shown with equal weights. The $r$-process abundance [7] has been scaled to match the average production of ${}^{82}\mathrm{Se}$. The arrow indicates the $A=84$ abundance maximum of the first $r$-process abundance peak.

Figure 4

(a) Final abundances averaged over calculations with $Y_e=0.35-0.38$ compared to the solar $r$-process abundance [7], with uncertainty shown as gray band. The colored bands show the 1-, 2-, and $3\sigma$ change in calculated production, as well as the maximum and minimum abundance from the Monte Carlo variation of the nuclear masses of ${}^{84,85}\mathrm{Ga}$ following a Gaussian distribution with $\sigma$ of 200 and 300 keV, respectively. For the new mass values only the maximum and minimum abundance band from the variation within their uncertainty is shown. (b) Change, in percentage, of the abundance pattern as a result of using the mass values from this work compared to the extrapolations given in the AME2016.

Figure 5

Time-integrated reaction flows at $Y_e=0.35$ from the freeze-out of the $r$-process at a neutron-to-seed ratio of unity until the final abundances are established, relevant for the nucleosynthesis in the mass regions $A=78-86$. Most abundant nuclei at freeze-out are marked with solid circles. Gray filled squares indicate stable nuclei. Red squares indicate nuclei for which the reaction rates have been affected by the uncertainties of the mass value of ${}^{84}\mathrm{Ga}$ and ${}^{85}\mathrm{Ga}$, while black ones indicate the abundance peaks at $A=80$ and $A=84$, corresponding to ${}^{80}\mathrm{Se}$ and ${}^{84}\mathrm{Kr}$. Arrows indicate the intensity of the flow.