Studies of the two-step scheme for new RI-beam production in the medium-heavy very-neutron-rich region

 Two studies about the “two-step scheme” performed at BigRIPS in RI Beam Factory will be presented.

 The first one is an investigation of usefulness of the two-step scheme [1], which was proposed [2] for an efficient production of medium-heavy very-neutron-rich radioactive isotopes (RI) as an alternative method to the in-flight fission of ²³⁸U (one-step scheme). The one-step scheme is expected to be limited soon: Production cross sections for the very neutron-rich RIs have been drastically decrease with neutron numbers (N). The two-step method may produce more exotic RIs by the following two steps: In the first step, a long-lived neutron-rich RI (e.g. ¹³²Sn) is produced by ISOL and post-accelerated, then, more exotic RI (e.g. ¹²⁸Pd) is produced by fragmentation in the second step. To evaluate its usefulness, we have measured the cross sections of very neutron-rich Pd isotopes produced from a ¹³²Sn beam and compared their estimated yields by the two- and one-step schemes under an assumption of 1-MW beam facilities. This comparison suggests that the two-step scheme with ¹³²Sn provides yields >40-times higher than those by the one-step scheme for very neutron-rich N = 82 region. Moreover, we examined which scheme is better to produce RIs at each region in the nuclear chart by using various long-lived RI beams supplied from ISOL in the two-step scheme.

The other one is about a new RI-beam production/separation method in BigRIPS, “in-separator two-step method” [3], which could have an advantage for obtaining yields of very neutron-rich RIs compared to the in-flight fission of ²³⁸U. In the proposed new method, less-exotic intermediate RI is produced in the 1^st stage and more-exotic objective RI is produced and separated in the 2^nd stage. The experimental results with ¹³³Sn as the intermediate RI will be shown.

References

[1] H. Suzuki et al., Phys. Rev. C 102 (2020) 064615.

[2] K. Helariutta et al., Eur. Phys. J. A 17 (2003) 181.

[3] H. Suzuki et al., RIKEN Acc. Prog. Rep. 55 (2021) S7.