First Results from the CARIBU Facility: Mass Measurements on the $r$-Process Path

The Canadian Penning Trap mass spectrometer has made mass measurements of 33 neutron-rich nuclides provided by the new Californium Rare Isotope Breeder Upgrade facility at Argonne National Laboratory. The studied region includes the ${}^{132}\mathrm{Sn}$ double shell closure and ranges in $Z$ from In to Cs, with Sn isotopes measured out to $A=135$, and the typical measurement precision is at the 100 ppb level or better. The region encompasses a possible major waiting point of the astrophysical $r$ process, and the impact of the masses on the $r$ process is shown through a series of simulations. These first-ever simulations with direct mass information on this waiting point show significant increases in waiting time at Sn and Sb in comparison with commonly used mass models, demonstrating the inadequacy of existing models for accurate $r$-process calculations.

Figure 1

Locations of the nuclides for which masses have been measured by the CPT at CARIBU. The shell closures at $N=82$ and $Z=50$ are highlighted.

Figure 2

Example data, a 200-ms excitation of ${}^{141}\mathrm{I}^{+}$. The points represent the time-of-flight (TOF) data and the curve a fit to the theoretical response.

Figure 3

Results of simulations performed at 1.5 GK. Top: The effective half-life of Sn versus neutron density. Each mass model tested predicts a drop in time at lower density than in reality. Bottom: The mean mass number of Sb versus neutron density. The mass models all predict a breakaway from the neutron shell closure earlier than in reality, with HFB-21 [37] showing an unnaturally wide expanse in which ${}^{137}\mathrm{Sb}$ dominates. See the text for additional details.