Reverse Engineering the r Process*

A nuclear mass survey of rare-earth isotopes has been conducted with the Canadian Penning Trap mass spectrometer using the most neutron-rich nuclei thus far extracted from the CARIBU facility. With the phase-imaging ion-cyclotron-resonance measurement technique, several masses along Ce-Eu isotopic chains near N=100 have been measured with sub-10 keV/c² precision for the first time. Independently, a detailed study exploring the role of nuclear masses in the formation of the r-process rare-earth abundance peak has been performed. Employing a Markov chain Monte Carlo (MCMC) technique, mass predictions of lanthanide isotopes have been made which uniquely reproduce the observed solar abundances near A = 164 under three distinct astrophysical outflow conditions. We compare our mass measurements to each of the predicted mass surface trends and find strong consistency with the ‘hot’ r-process environment where the rare-earth peak forms during an extended period of (n,γ) ↔ (γ,n) equilibrium. Here, I will give an overview of the mass measurement campaigns, briefly describe the MCMC calculations, and conclude with future measurement prospects at next-generation rare isotope beam facilities.