First application of mass measurement with the Rare-RI Ring reveals the solar r-process abundance trend at A=122 and A=123

Abstract

The Rare-RI Ring (R3) is a recently commissioned cyclotron-like storage ring mass spectrometer dedicated to mass measurements of exotic nuclei far from stability at Radioactive Isotope Beam Factory (RIBF) in RIKEN. The first application of mass measurement using the R3 mass spectrometer at RIBF is reported. Rare isotopes produced at RIBF, 127 Sn, 126 In, 125 Cd, 124 Ag, 123 Pd, were injected in R3. Masses of 126 In, 125 Cd, and 123 Pd were measured and the mass uncertainty of 123 Pd was improved. The impact of the new 123 Pd result on the solar r-process abundances in a neutron star merger event is investigated by performing reaction network calculations of 20 trajectories with varying electron fraction Ye. It is found that the neutron capture cross section on 123 Pd increases by a factor of 2.2 and β-delayed neutron emission probability, P1n, of 123 Rh increases by 14%. The neutron capture cross section on 122 Pd decreases by a factor of 2.6 leading to pileup of material at A = 122, thus reproducing the trend of the solar r-process abundances. Furthermore, the nuclear deformation predicted to reach its maximum before N = 82 in the Pd isotopic chain is examined. The new mass measurement shows no evidence of such large deformation, though, experimental uncertainty should be further improved to draw a definitive conclusion. This is the first reported measurement with a new storage ring mass spectrometery technique realized at a heavy-ion cyclotron and employing individual injection of the pre-identified rare nuclei. The latter is essential for the future mass measurements of the rarest isotopes produced at RIBF.

The R-Process Alliance: A Nearly Complete R-Process Abundance Template Derived from Ultraviolet Spectroscopy of the R-Process-Enhanced Metal-Poor Star HD 222925

“…These discrepancies could signal the need for further improvements in nuclear structure models, and nuclear masses in particular, for nuclei just below the A ∼ 130 peak (see Kratz et al 2014). For example, the Sn and Sb behavior is consistent with a recent measurement by Li et al (2021) that improved the uncertainty on the mass of the neutron-rich isotope 123 Pd. That measurement implies a slight decrease in the r-process abundance of the A = 123 isobar, which is linked to one of only two stable Sb isotopes.…”