Plumbing Neutron Stars to New Depths with the Binding Energy of the Exotic Nuclide ${}^{82}\mathrm{Zn}$

Modeling the composition of neutron-star crusts depends strongly on binding energies of neutron-rich nuclides near the $N=50$ and $N=82$ shell closures. Using a recent development of time-of-flight mass spectrometry for on-line purification of radioactive ion beams to access more exotic species, we have determined for the first time the mass of ${}^{82}\mathrm{Zn}$ with the ISOLTRAP setup at the ISOLDE-CERN facility. With a robust neutron-star model based on nuclear energy-density-functional theory, we solve the general relativistic Tolman-Oppenheimer-Volkoff equations and calculate the neutron-star crust composition based on the new experimental mass. The composition profile is not only altered but now constrained by experimental data deeper into the crust than before.

Figure 1

The depth profile of a neutron star of 1.4 solar mass and $10\mathrm{km}$ radius. The scale on the right indicates the nuclide composition in the outer crust as predicted by the HFB-19 and HFB-21 mass models. Experimentally known nuclides are printed in bold. Including the new mass value for ${}^{82}\mathrm{Zn}$, the position of ${}^{80}\mathrm{Zn}$ has changed and ${}^{78}\mathrm{Ni}$ replaces ${}^{82}\mathrm{Zn}$ (changes marked red).

Figure 2

The nuclear chart based on AME2012 [38], highlighting the present ISOLTRAP results and the nuclides that compose neutron-star outer crusts according to experiment and the HFB-19 and HFB-21 models.

Figure 3

Schematic ISOLTRAP overview and time-of-flight resonance of ${}^{82}\mathrm{Zn}^{+}$. The main components relevant for this study are the incoming ISOLDE beam (1), reference ion source (2), RFQ cooler and buncher (3), MR-ToF mass separator (4), preparation Penning trap (5), precision Penning trap (6), and ToF detector (7). Top-left inset: Time-of-flight resonance of ${}^{82}\mathrm{Zn}^{+}$ (see text).