Precision Mass Measurements of Neutron-Rich Neodymium and Samarium Isotopes and Their Role in Understanding Rare-Earth Peak Formation

The Canadian Penning Trap mass spectrometer at the Californium Rare Isotope Breeder Upgrade (CARIBU) facility was used to measure the masses of eight neutron-rich isotopes of Nd and Sm. These measurements are the first to push into the region of nuclear masses relevant to the formation of the rare-earth abundance peak at $A\sim 165$ by the rapid neutron-capture process. We compare our results with theoretical predictions obtained from “reverse engineering” the mass surface that best reproduces the observed solar abundances in this region through a Markov chain Monte Carlo technique. Our measured masses are consistent with the reverse-engineering predictions for a neutron star merger wind scenario.

Figure 1

A typical histogram of a final phase measurement showing the positions of ions on the detector plane. Each cluster corresponds to a different ion species present in the beam. Here we show a $t_{\mathrm{acc}}=55.001\mathrm{ms}$ measurement of ${{}^{160}\mathrm{Nd}}^{2+}$ (circled). In this example, of the 440 total ions detected in 3 hr, we observed 40 counts of ${{}^{160}\mathrm{Nd}}^{2+}$. Also present in this spectrum are clusters of ${{}^{160}\mathrm{Pm}}^{2+}$, three contaminant species, and some unexcited ions near the trap center.

Figure 2

Comparison between experimental values and theoretical predictions (red band) of the nuclear masses relative to the Duflo-Zuker mass model for neodymium and samarium isotopes in a merger accretion disk wind scenario ($s/k_B=30$, $\tau =70\mathrm{ms}$, and $Y_e=0.2$).

Figure 3

Rare-earth peak abundances using Duflo-Zuker masses (black dashed line) as compared to the result for this same astrophysical trajectory after the algorithm finds the mass predictions in Fig. 2 (solid red band). Pink and blue curves serve to show the change in the abundance pattern obtained from using other disk wind parameters but with the same mass surface.