The rare-RI ring

We describe the rare-RI (radioactive isotope) ring at the RI Beam Factory (RIBF). The main purpose of the rare-RI ring is to measure the mass of very neutron-rich nuclei, the production rates of which are very small (hence 'rare RI') and the lifetimes of which are predicted to be very short. In the rare-RI ring, there are two innovative pieces of apparatus: individual injection, which can realize the injection of 200 A MeV rare RIs one by one, and a cyclotron-like storage ring, which allows high isochronous magnetic fields with large angular and momentum acceptances. With these devices, we will achieve a 10^-6 mass resolution, and will be able to access rare RIs, the production rate of which is down to 1 event/day/pnA. Construction of the rare-RI ring started in fiscal year 2012. (author)