Where did heavy elements (e.g., gold, platinum, and rare earths) originate? Of roughly 7,000 atomic nuclei predicted by theoretical models, only approximately 3,300 have been observed. Each newly discovered isotope adds a piece to the “nuclear chart”—a vast map of every possible combination of protons and neutrons. Filling in this chart is essential for understanding the fundamental nuclear properties—which combinations are stable, how strongly nucleons bind, and how nuclear shapes evolve—and for unraveling how heavy elements were formed. Approximately half of all elements heavier than iron are created by the rapid neutron-capture process (r-process) in explosive cosmic events, such as neutron-star mergers. During the r-process, the nuclei rapidly capture neutrons and undergo beta decay, tracing a path far from the neutron-rich side of stability. The neutron-rich rare-earth region around Ce (Z = 58) represents a critical stepping stone toward this r-process path.

A research team at RIKEN discovered seven new isotopes in this region. The experiment was conducted at the RI Beam Factory (RIBF), one of the world’s most powerful facilities for producing unstable nuclei. Intense 345-MeV/u uranium-238 beams impinged on a beryllium target, inducing in-flight fission and producing neutron-rich fragments. The BigRIPS fragment separator, using dipole magnets to bend and separate fragments, collects specific RI beams of interest during flight. A key advantage of this method is ultrafast identification, within hundreds of nanoseconds, which enables the detection of short-lived isotopes with no chemical limitations. Each fragment was identified by its atomic number and mass-to-charge ratio using time-of-flight, magnetic rigidity, and energy loss measurements. Production cross-sections showed smooth mass-number dependence, and p-value tests (p < 0.05) confirmed the statistical significance of each discovery.

In this study, the team first observed seven isotopes: cesium-152, barium-155, lanthanum-158, cerium-159, cerium-160, gadolinium-173, and terbium-175. These extremely neutron-rich nuclei extend the known chart to previously unexplored territories. The rare-earth region is particularly interesting because these highly deformed nuclei should resist triaxial deformation and maintain axially symmetric shapes, a prediction that can be tested closer to the r-process path.

This discovery demonstrates the world-leading capability of RIBF; the facility has now contributed over 200 new isotopes via in-flight separation, accounting for 52% of all isotope discoveries worldwide in the 2020s. With a planned upgrade to increase the beam intensity, RIBF is poised to push further into an unexplored neutron-rich territory, promising detailed insights into both nuclear structure and heavy-element synthesis.

(Written by Toshiyuki Sumikama on behalf of all the authors)

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