Among all the nuclides comprising protons and neutrons, 260 isotopes are stable. Based on mass models, 7000 to 10000 nuclides are predicted to exist; however, most of them are radioactive isotopes (RIs) with extremely short half-lives. To confirm their existence, they should be artificially produced and identified. Currently, approximately 3000 nuclides have been confirmed, which implies that more than half of all RIs is yet to be discovered.

RI Beam Factory (RIBF) at RIKEN Nishina Center, Japan, is one of the most advanced RI-beam facility in the world. The BigRIPS is a large-acceptance two-stage separator that supplies diverse RI beams, including drip-line nuclei. To produce RI beams, primary beams of stable isotopes from light mass to ²³⁸U are accelerated to 70% of the light speed; subsequently, they are impinged on ⁹Be production targets. The produced fragments are separated and identified in the BigRIPS. RIs farther from the stability line on the nuclear chart are more difficult to produce because the protons and neutrons are lost under a greater imbalance. Thus, the search for new isotopes necessitates high-intensity primary beams. Moreover, the BigRIPS is designed to exhibit large acceptances of angles and momentum to efficiently accumulate RI beams scattered from the target. Since the operation of the BigRIPS in 2007, 173 new isotopes have been discovered therein. Most of the known-limit RIs on the nuclear chart have been discovered at the RIBF.

In this study, we focus on the new isotopes in proton-rich Z = 50 (Z; atomic number) regions. High-intensity ¹²⁴Xe beam (700 billion particles/s) is impinged on a 3-mm ⁹Be target for four days, and four ⁹⁸Sn nuclei are produced via the projectile-fragmentation reaction. Two-quadrillion collisions between the ¹²⁴Xe and ⁹Be nuclei are required to produce a single ⁹⁸Sn ion. This is approximately one order of magnitude larger than that required to produce ²⁷⁸Nh (Nihonium) via the fusion reaction of ⁷⁰Zn + Bi. This level of collision is necessitated to produce new isotopes via the abovementioned fragmentation.

The region near ¹⁰⁰Sn, which corresponds to the heaviest double-magic self-conjugate (N = Z. N; neutron number) isotope, has garnered significant interest for studying the following physics topics: 1. The nuclear shell evolution of 0g_9/2, 1d_5/2, and 0g_7/2 orbitals as a function of N; 2. the isospin symmetry and its violation; 3. various decay modes, such as proton radioactivity, two-proton emission, α decay, and cluster decay; and 4. the end point of the rp-process in nucleosynthesis. The search for new isotopes is important not only for the expansion of the nuclear chart but also as an indispensable step for investigating various physics using RI beams.

(Written by H. Suzuki on behalf of all authors.)

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