Mass Measurements With The Rare-RI Ring For R-process Nuclei Around A=123

The atomic nucleus is a quantum many-body system composed of nucleons(i.e.protons and neutrons).Mass is one of the fundamental properties of the atomic nucleus and reflects the sum interaction,including the strong,weak,and electromagnetic interactions between the nucleons.The mass values of the atomic nuclei play an essential role in understanding of the nuclear structure,nuclear force,and the origin of the elements in the cosmos.The rapid neutron capture process(r-process)is considered to be responsible for the production of about one half of the elements heavier than iron up to bismuth and all of thorium and uranium.The experimental masses of the nuclei are not only needed for the r-process models but also essential for the improvement of theory since most of the nuclei relevant for r-process are not experimentally reachable today.However,precision mass measurements of such exotic nuclei far from the stability line are strongly restricted by their low yields and short half-lives.Isochronous mass spectrometry based on the storage ring is one of the useful tools which can conduct mass measurements for exotic nuclei with very short half-lives.The Rare RI Ring(R3)at Radioactive Ion Beam Factory(RIBF)in RIKEN,Japan is a recently commissioned cyclotron-like storage ring mass spectrometer specifically dedicated for exotic nuclei far from stability after the Experimental Storage Ring(ESR)at the GSI Helmholtzzentrum für Schwerionenforschung in Germany and the experimental Cooler Storage Ring(CSRe)at the Institute of Modern Physics,Chinese Academy of Sciences(IMP,CAS)in China.R3 is the only isochronous mass spectrometer coupled with a cyclotron.The pre-identification for the secondary particle can be performed before injection.In the experiment,the secondary beam was produced by in-flight-fission of the 345 MeV/nucleon 238U beam provided by the Superconducting Ring Cyclotron(SRC)impinged on the 6 mm thick beryllium target.The target was placed at the entrance of the BigRIPS separator.The secondary fragments were separated by the first stage of the BigRIPS with B?-?E-B? method.For this purpose,a 5 mm wedge-shaped degrader was installed at the F1 focal plane of the BigRIPS.The magnetic rigidity and the transmission efficiency were optimized for the isochronous reference particle.To inject the pre-identified nuclei of interest into R3,the individual self-triggered injection technique and ?E-TOF gate for trigger selection system were developed.Each nucleus had been stored in the R3 for about 0.7 ms and then extracted from it.The velocity,magnetic rigidity,and the revolution time of each event could be measured by the detectors installed at the focal planes of beamline and R3.To measure the mass of 123Pd,5 isotones,127Sn,126In,125Cd,124Ag,and 123Pd,were produced and extracted from the R3 successfully.The mass values determined by three methods,Bp-C method,Yano equation ? correction method and Yano equation Bp correction method,were consistent with each other and also agreed with the values from the AME2020.The relative mass uncertainty achieved for 123Pd was 2.3×10-6,which was comparable to the mass precision achieved in the ESR and in the earlier work of CSR isochronous mass spectrometry.To estimate the impact of the 123Pd mass measured with R3 in the r-process,r-process calculations were performed by employing the Portable Routines for Integrated nucleoSynthesis Modeling(PRISM)reaction network.We simulated nucleosynthesis for a set of 20 parameterized r-process trajectories to investigate the A=122 to A=123 abundance ratios.Compared to the ratio results obtained based on the finite-range droplet model FRDM2012 nuclear mass data,the obtained ratios results based on the new mass values from this experiment were more consistent with the observed solar r-process abundance.To measure the masses of 125 Ag and 124Pd,5 isotones,128Sn,127In,126Cd,125 Ag,and 124Pd,were produced and extracted from the R3 successfully.The isomeric state of 128Sn whose excitation energy is 2091.5 keV was observed and resolved.As the optics of the beamline was not tuned correctly,the Yano equation Bp correction method can not be used to determine the mass.The mass values of 125 Ag and 124Pd determined by the Bp-C method agreed well with the mass values determined by Yano equation ? correction method.While the two neutron separation energy determined with the new experimental values for the 125 Ag and 124Pd deviates a lot from the tendency predicted by the AME2020 indicating that the experimental mass values may not be reliable.One possible explanation is that the trajectory of the ions along the beamline may have a strong dependence on the emittance and momentum due to the incorrect optical setting of the beamline.Remeasurement of the masses of 125 Ag and 124Pd is highly recommended after the update of the kicker.