Atomic nuclei, which are quantum many-body systems bound by the strong interaction, exhibit rich and complex diversity. While stable isotopes form the world around us, unstable radioactive isotopes (RIs) decay with finite lifetimes.　Although theoretical models predict the existence of over 7,000 nuclides, only approximately 3,500 nuclei have been experimentally confirmed, leaving more than half undiscovered. To address the fundamental question of what combinations of protons and neutrons can exist as nuclei, international RI facilities are competing fiercely. In the paper introduced here, we report the discovery of 22 new exotic isotopes, marking a critical milestone in mapping the known boundaries of nuclear existence.

The experiment was conducted at the RIKEN RI Beam Factory using the BigRIPS in-flight separator. The accelerator complex delivered a highly intense, 345-MeV/nucleon primary beam of ²⁰⁸Pb to induce projectile fragmentation on a beryllium target. An advantage of using a ²⁰⁸Pb primary beam over a conventional ²³⁸U beam is the absence of fission product contamination, allowing the production of high-purity secondary beams for rare isotopes of interest. Incorporating state-of-the-art detectors to measure time-of-flight, magnetic rigidity, and energy loss, we achieved clear particle identification for both atomic numbers and mass-to-charge ratios with greater than 3s. This robust analysis successfully confirmed 17 neutron-rich isotopes (¹⁷⁹Ho, ^181,182Er, ^184-186Tm, ^188,189Yb, ¹⁹¹Lu, ^193,194Hf, ^195,196Ta, ^198,199W, ^200,201Re) and 5 proton-rich isotopes (¹¹⁸Ce, ¹²⁰Pr, ^152,153Hf, ¹⁵⁴Ta).

These discoveries carry profound physical implications for both frontiers of the nuclear chart. On the neutron-rich side, the newly identified isotopes are located close to the predicted path of the r-process (rapid neutron-capture process), which is responsible for synthesizing elements heavier than iron in explosive stellar environments such as supernovae and/or neutron star mergers. Crucially, our successful synthesis of ²⁰¹Re and neighboring nuclei near the neutron magic number N=126 accelerates our quantitative understanding of how the third elemental abundance peak is formed in the universe. On the proton-rich side, the discovered isotopes lie near or beyond the proton drip line, where the valence protons are unbound. The ability to access these nuclei using the in-flight RI production method has established a new framework for investigating exotic decay modes, including single- or two-proton radioactivity.

Unlocking the mysteries of nucleosynthesis in the universe and constructing a unified nuclear model are long-standing challenges in modern physics. The advancement of RI beam production techniques, as demonstrated by this ²⁰⁸Pb beam experiment, represents a definitive step forward. The event-by-event-tagged high-intensity RI beams will serve as powerful instruments for future reaction studies and continue to expand the frontiers of nuclear physics.

(Written by S. Michimasa on behalf of all authors.)

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