Scientists at the Institute of Modern Physics (lMP) of the Chinese Academy of Sciences (CAS) and their collaborators have demonstrated the effectiveness of an innovative nuclear mass measurement method, using the rare-RI ring (R3) facility at Japan’s Radioactive Isotope Beam Factory, and applied this method to the mass measurement of a neutron-rich palladium isotope. The study was published in Physical Review Letters on April 15. 

The origin of elements in the universe is one of the most important questions in nuclear astrophysics. The rapid neutron capture process (r-process) is considered to be responsible for the production of about half of the elements heavier than iron. However, it is still unclear where r-processes take place. Nevertheless, neutron star mergers are some of the most likely sites. 

Simulations of the nucleosynthesis process provide important information for interpreting relevant observations and locating astrophysical sites for the r-process. Experimental mass values of nuclides not only provide reliable data for calculations, but also help improve mass models. However, since the r-process involves a large number of neutron-rich nuclei with low yields and short lives, it is extremely difficult to produce and measure these nuclei in laboratories. Therefore, a large amount of nuclear data needed in r-process calculations relies on theoretical models. 

According to previous studies, palladium-123 has a significant impact on the abundance of A=122 and A=123 nuclides in the r-process. To provide reliable data for research, scientists measured the mass of palladium-123 at the R3 facility. 

This is the first application of mass measurement using the R3 facility at the RIKEN Radioactive Isotope Beam Factory in Japan, which can produce the world’s highest intensity radioactive isotope beam. The R3 facility is a recently commissioned cyclotron-like storage ring mass spectrometer. Based on an isotope-selectable, self-triggered injection technique, pre-identified ions can be selected and injected into R3 event by event. Therefore, the R3 facility has a unique advantage in the mass measurement of short-lived nuclides with extremely low production yields. 

In the experiment, scientists produced the nuclei of interest by fission fragmentation of the uranium-238 beam impinged on a beryllium target. They measured a total of 166 palladium-123 ions with a relative mass uncertainty of 2.3 x 10^-6. 

Based on the new mass data, scientists calculated A=122 and A=123 element abundance ratios in 20 r-process trajectories of a neutron star merger using the reaction network model. Compared with the ratio results obtained based on the theoretical mass model, the ratio results obtained based on this experiment’s new mass results are more consistent with observed solar r-process abundance. 

The new experimental results improve understanding of the fine structure of element abundance patterns created in the r-process.