Precision Mass Measurements on Neutron-Rich Rare-Earth Isotopes at JYFLTRAP: Reduced Neutron Pairing and Implications for $r$-Process Calculations

The rare-earth peak in the $r$-process abundance pattern depends sensitively on both the astrophysical conditions and subtle changes in nuclear structure in the region. This work takes an important step towards elucidating the nuclear structure and reducing the uncertainties in $r$-process calculations via precise atomic mass measurements at the JYFLTRAP double Penning trap. ${}^{158}\mathrm{Nd}$, ${}^{160}\mathrm{Pm}$, ${}^{162}\mathrm{Sm}$, and ${}^{164–166}\mathrm{Gd}$ have been measured for the first time, and the precisions for ${}^{156}\mathrm{Nd}$, ${}^{158}\mathrm{Pm}$, ${}^{162,163}\mathrm{Eu}$, ${}^{163}\mathrm{Gd}$, and ${}^{164}\mathrm{Tb}$ have been improved considerably. Nuclear structure has been probed via two-neutron separation energies $S_{2n}$ and neutron pairing energy metrics $D_n$. The data do not support the existence of a subshell closure at $N=100$. Neutron pairing has been found to be weaker than predicted by theoretical mass models. The impact on the calculated $r$-process abundances has been studied. Substantial changes resulting in a smoother abundance distribution and a better agreement with the solar $r$-process abundances are observed.

Figure 1

Time-of-flight spectrum for ${}^{163}{\mathrm{Eu}}^{+}$. Background shading indicates the total number of ions, where darker shading indicates more ions.

Figure 2

Two-neutron separation energies $S_{2n}$ from this work (red lines) together with the experimental (solid black circles) values and an extrapolated value for ${}^{165}\mathrm{Tb}$ (open black circle) from AME16 [64]. The dashed blue lines indicate the values assuming the ground state of ${}^{163}\mathrm{Gd}$ was measured in this work.

Figure 3

Neutron pairing energies from this work (red circles) and AME16 (blue line) in comparison with various theoretical predictions for the Gd isotopes.

Figure 4

Solar $r$-process abundances [3] (black circles) in comparison with the calculations using the experimental AME16 [64] $+$ FRDM12 masses [33] (blue and purple lines, respectively) and the new masses from this work (orange and red lines) for representative trajectories (a) with fission cycling and (b) without fission cycling. (c) Change, in percent, of the abundance pattern as a result of using the masses from this work. (d) Residuals for scenario (a) based on the mass values from this work (red) and the baseline (purple), where the bands represent the solar abundance uncertainties.