The production and experimental identification of new isotopes represent a foundational pursuit in nuclear physics research. The discovery of each previously unobserved nuclide expands the empirical boundaries of the nuclear chart and provides indispensable benchmark data for refining nuclear theoretical models, constraining nucleon-nucleon interaction parameters, and elucidating the fundamental limits of nuclear binding stability [
1,
2]. Theoretical predictions estimate there are 7000–9000 bound atomic nuclei in total [
3,
4]; to date, only just over 3300 isotopes have been experimentally identified. The vast uncharted territories of the nuclear landscape remain one of the most pressing and challenging frontiers in contemporary nuclear science. Specifically, the proton drip line for heavy nuclei with atomic numbers
Z > 70 has long eluded precise experimental exploration, and the identification of new isotopes in this proton-rich frontier carries profound implications for advancing nuclear structure theory and astrophysical nucleosynthesis research.
Global efforts to expand the nuclear landscape have been extensively advanced via state-of-the-art heavy-ion accelerator facilities. The Radioactive Ion Beam Factory (RIBF) at RIKEN, Japan, has sustained long-term systematic research programs and achieved prominent progress in new isotope discovery through projectile fragmentation and in-flight fission techniques over recent decades. The Facility for Rare Isotope Beams (FRIB) at Michigan State University, USA, has rapidly validated its superior discovery capability by identifying five new neutron-rich rare-earth isotopes within its inaugural operational year. In the near future, the superconducting fragment separator at the Facility for Antiproton and Ion Research (FAIR) in Germany will further complement global heavy-nucleus research endeavors. In China, the Institute of Modern Physics (IMP), Chinese Academy of Sciences, has established a distinguished research legacy in synthesizing neutron-deficient isotopes, with recent breakthroughs including the discovery of
156W [
5],
160Os [
5], and
235Bk [
6] via fusion-evaporation reactions at the Heavy Ion Research Facility in Lanzhou (HIRFL).
Building on its accumulated technical expertise and research experience, IMP has successfully constructed the High Intensity Heavy-ion Accelerator Facility (HIAF), a new-generation comprehensive accelerator complex dedicated to advancing frontier nuclear science. The expansion of the nuclear landscape constitutes one of HIAF’s core scientific objectives. Beyond exploring uncharted nuclear territories, HIAF is designed to address a broad spectrum of cutting-edge scientific questions, including the investigation of exotic nuclear structures far from stability, the cosmic origin of heavy elements, and the intrinsic properties of strongly interacting quantum matter. Furthermore, the facility serves as a critical interdisciplinary platform for heavy-ion-based applications in life sciences, space science and materials science.
Now, writing in the
Science Bulletin, a research team at the IMP, Chinese Academy of Sciences, has reported the first successful production and unambiguous identification of the near-proton-drip-line
153Hf isotope (
Z = 72,
N = 81), achieved via isochronous mass spectrometry (IMS) during the inaugural commissioning campaign of the HIAF Spectrometer Ring (SRing) [
4]. This work marks the first landmark physics achievement of the HIAF-SRing system, demonstrating the facility’s robust capability for exploring extreme nuclear states. Leveraging the unique zero-background detection property and single-ion sensitivity of the IMS technique, this experiment achieves precise discrimination of rare
153Hf nuclides from complex isobaric contaminants, enabling definitive isotope identification without reliance on auxiliary spectral correction or statistical validation. The newly discovered
153Hf isotope features a single neutron hole in the
N = 82 closed shell, serving as a critical probe for tracing the evolution of shell closure magic numbers in proton-rich heavy nuclear systems. Notably,
153Hf is the crucial gateway to extending systematic studies of the evolution of this shell closure, and its observation is a decisive step toward locating the proton drip line in this heavy-element region [
7]. Figure 1 highlights this gap on the nuclear chart, which the HIAF-SRing work aims to fill. This finding fills a long-standing gap in the heavy-element nuclear chart, provides rigorous experimental constraints for prevailing nuclear mass models, and paves the way for subsequent precise mass measurements and systematic studies of proton-drip-line nuclear properties. It is noteworthy that an independent parallel effort to search for new proton- and neutron-rich isotopes beyond
Z = 50 has been recently reported at RIKEN Nishina Center for the first experimental study, via utilizing a 345-MeV/nucleon
208Pb primary beam at the RI Beam Factory [
8]. The neutron-rich nuclei have been produced by in-flight fission of
238U as well as projectile fragmentation of various high-
Z beams. Notably, that work successfully identified 22 previously unobserved exotic isotopes — among which
152Hf,
153Hf, and
193Hf are included, remarkably pushing forward the known boundaries of the nuclear landscape chart [
8].
Key research highlights are summarized as follows:
1) First production and unambiguous identification of the new 153Hf isotope
This study realizes the first experimental production and definitive identification of the neutron-deficient 153Hf isotope near the heavy-element proton drip line, resolving a decades-long experimental blank in the hafnium isotope chain. The detected 153Hf signals exhibit no spectral overlap with known nuclides, with candidate peak deviations exceeding 10 standard deviations from adjacent known isotopes, ensuring fully unambiguous nuclide assignment. The finite production cross section and measurable lifetime of 153Hf further confirm its particle-bound nature, consistent with multiple state-of-the-art nuclear theoretical predictions.
2) First physics result from the HIAF-SRing facility in the initial commissioning campaign
The discovery of 153Hf represents the first scientific breakthrough of the newly commissioned HIAF-SRing spectrometer system, validating the facility’s core performance and frontier research capability. Conducted during the facility’s technical commissioning phase with only 10% of its designed maximum beam intensity, this achievement demonstrates the exceptional reliability and high precision of HIAF’s beam transmission, ion selection and spectral detection systems. Benefiting from the facility’s scalable operational parameters, HIAF is poised to achieve a two-order-of-magnitude improvement in experimental luminosity in future upgrades, enabling more extensive exploration of exotic rare isotopes.
3) Innovative isotope discovery via zero-background and single-ion sensitive IMS technique
Unlike conventional in-flight separation methods that suffer from low mass resolution and ambiguous signal discrimination, the IMS technique deployed at HIAF-SRing achieves zero-background detection and single-ion-level sensitivity. By establishing a direct correlation between ion revolution time and mass-to-charge ratio, the method realizes one-dimensional high-precision separation of isobaric nuclides, eliminating false identification risks caused by background noise or signal pileup. This technique provides a revolutionary technical approach for discovering ultra-rare isotopes with picobarn-level production cross sections, offering universal advantages for future exotic nucleus research.
As the first scientific output of HIAF-SRing, this result achieves the unambiguous discovery of the 153Hf isotope near the proton drip line via high-precision IMS based on the HIAF-SRing facility, validating the facility’s exceptional frontier research capability. Moreover, as the inaugural scientific outcome of the HIAF commissioning project, this finding not only enriches and completes the heavy-element nuclear chart, offering crucial experimental data for the validation and refinement of nuclear theoretical models, but also verifies the superior comprehensive performance of China’s new-generation heavy-ion accelerator in exploring extreme nuclear physics regimes. The HIAF facility possesses enormous untapped operational performance and research potential, which will enable further expansion of the explored regime of nuclear landscape and promote the development of fundamental nuclear physics research in the extreme proton-rich region. The innovative application of zero-background single-ion-sensitive IMS breaks through the technical bottlenecks of traditional rare isotope detection methods. Subsequent facility optimizations and parameter upgrades will further empower HIAF to broaden the boundaries of nuclear exploration, facilitate in-depth investigations into exotic nuclear structures and stellar nucleosynthesis mechanisms, and deliver unique contributions to advancing the global frontier of nuclear physics research.
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