First Application of Mass Measurements 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 cyclotronlike 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 - Sn127, In126, Cd125, Ag124, Pd123 - were injected in R3. Masses of In126, Cd125, and Pd123 were measured whereby the mass uncertainty of Pd123 was improved. This is the first reported measurement with a new storage ring mass spectrometry technique realized at a heavy-ion cyclotron and employing individual injection of the preidentified rare nuclei. The latter is essential for the future mass measurements of the rarest isotopes produced at RIBF. The impact of the new Pd123 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 Pd123 increases by a factor of 2.2 and β-delayed neutron emission probability, P1 n, of Rh123 increases by 14%. The neutron capture cross section on Pd122 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. The trend of the two-neutron separation energies (S2n) was investigated for the Pd isotopic chain. The new mass measurement with improved uncertainty excludes large changes of the S2n value at N=77. Such large increase of the S2n values before N=82 was proposed as an alternative to the quenching of the N=82 shell gap to reproduce r-process abundances in the mass region of A=112-124.