Study of Mass-measurement Method for N=Z Nuclei with Isochronous Mass Spectrometry

Nuclear mass data of N=Z nuclei is crucial for the investigation of the rp-and the νp-processes. In addition, access to the nuclei on the N=Z line will help us solve key questions relating to many open questions of nuclear structure. Isochronous mass spectrometry (IMS) combined with a fragment separator is a very fast, efficient and high resolution mass measurement tool. Since the m/q values of the N=Z nuclei are very close, the current storage ring mass spectrometers CSRe/IMP and ESR/GSI cannot realize the identification of the N=Z ions of different species via revolution time spectrum, so it is impossible to realize mass measurements on them. An IMS using a newly constructed storage ring named the ‘Rare-RI Ring’ (R3) has been implemented at the RIKEN Nishina Center to determine the masses of short-lived rare nuclei with a relative precision of the order of 10^-6. With the R3 operated as an IMS coupled to the high-resolution beam-line BigRIPS employed as a fragment separator, high-resolution particle identification of the N=Z nuclei on an event-by-event basis with the beam-line before their injection to R3 makes the mass measurements possible. Monte Carlo simulation studies of beam tracking, high-resolution particle identification and selection of the secondary beams have been carried out with a dedicated ion-optics design. The results show that the revolution time of all the N=Z nuclei are independent of momentum dispersion in the storage ring when we set one species of N=Z nucleus in an isochronous condition. The mass calibration method for the N=Z nuclei has also been discussed in this report based on the simulation.