The result, published in Science Bulletin, focuses on hafnium-153, a neutron-deficient isotope located near the proton drip line. The proton drip line marks the region where nuclei contain so few neutrons relative to their number of protons that additional proton emission becomes energetically possible. Nuclei approaching this boundary are especially valuable to nuclear physicists because their unusual proton-to-neutron ratios expose weaknesses in existing theories of nuclear structure and help define the limits of nuclear stability.

Hafnium-153 contains 72 protons and 81 neutrons, making it significantly more proton-rich than the stable hafnium isotopes found in nature. Its existence is important because nuclear models must account for whether such a highly unbalanced nucleus remains bound, becomes weakly bound, or rapidly disintegrates. By measuring the isotope’s mass and confirming its survival through the experimental system, researchers obtained new evidence for how nuclear forces operate in an extreme environment where conventional patterns of stability begin to break down.

The experiment began with a beam of bismuth-209 ions generated by HIAF’s Booster Ring, or BRing. These energetic ions were directed onto a graphite target, causing projectile-fragmentation reactions. In this process, the incoming bismuth nuclei collide with carbon atoms and break apart, producing a broad mixture of radioactive fragments. Among the debris were the rare hafnium-153 nuclei, whose production probability was so low that only a handful could be expected even under high-intensity operating conditions.

The resulting mixture, known as a cocktail beam, was then sent through the HIgh rigidity Radioactive Ion Beam Line, or HIRIBL. This beamline separates reaction products according to their magnetic rigidity, a quantity determined by the momentum and electric charge of each ion. Efficient separation was essential because the desired hafnium-153 nuclei were surrounded by far more abundant isotopes and reaction products. HIRIBL allowed the researchers to filter and transport the nuclei of interest toward the facility’s precision measurement system.

The purified radioactive ions were injected into HIAF’s Spectrometer Ring, known as SRing, where the team used isochronous mass spectrometry. This technique is designed to determine the mass-to-charge ratio of short-lived nuclei without requiring them to reach an equilibrium orbit in the storage ring. Ions with different masses circulate with carefully related revolution times, allowing their identities to be inferred from timing signals produced during repeated passes through the ring. Because the method can register individual ions, it is particularly powerful for studying isotopes that are created only a few times.

Despite hafnium-153’s extremely low production cross section—the probability that the isotope will be formed in a collision—the researchers detected ten individual ions. Each event carried significant scientific weight because the isotope’s identification depended on the combined analysis of its timing behavior, magnetic rigidity, and position within the mass spectrum. The measurements indicate that hafnium-153 is either bound or weakly bound, meaning it can exist as a nuclear system rather than immediately ejecting a proton. The finding agrees with predictions from several contemporary nuclear mass models, while also supplying an experimental benchmark for future refinements.

The result was independently supported by experiments at the Radioactive Isotope Beam Factory operated by RIKEN in Japan. The near-simultaneous observations by two separate facilities strengthen the case that hafnium-153 has been reliably identified and demonstrate the international importance of mapping the proton-rich frontier. Independent confirmation is especially valuable in rare-isotope research, where conclusions may initially rest on only a few detected events and where backgrounds from neighboring nuclei can be difficult to eliminate.

Researchers say the observation highlights the combined performance of HIAF’s major components rather than the capability of a single instrument. The high intensity of the heavy-ion beam increases the number of rare nuclear reactions, HIRIBL provides the separation power needed to isolate fragile radioactive products, and SRing supplies the single-ion sensitivity required for precision mass measurements. Together, these systems create a platform for investigating nuclei that cannot be produced in useful quantities with conventional stable-beam experiments.

HIAF’s first physics result also points toward a broader research program involving the discovery of new isotopes, measurements of nuclear masses and lifetimes, and studies of matter under extreme proton-neutron imbalance. As beam intensity and experimental efficiency improve, the facility is expected to reach still rarer regions of the nuclear landscape. Each newly observed isotope can alter the boundaries used in nuclear models, offering clues about the origin of the elements, the behavior of matter inside explosive stellar events, and the fundamental forces that hold atomic nuclei together.

Subject of Research: Rare isotope discovery and nuclear structure near the proton drip line

Article Title: Discovery of isotope hafnium-153 near the proton drip line via isochronous mass spectrometry: extending the nuclear landscape with HIAF

News Publication Date: 2-Jul-2026

Web References: https://doi.org/10.1016/j.scib.2026.06.056

References: Science Bulletin, DOI: 10.1016/j.scib.2026.06.056

Image Credits: Photo by Institute of Modern Physics of the Chinese Academy of Sciences

Keywords

Hafnium-153, rare isotopes, nuclear physics, proton drip line, HIAF, isochronous mass spectrometry, radioactive ion beams, nuclear structure, heavy-ion accelerator, nuclear landscape

The initial phase of this extensive research occurred between 2002 and 2012 at Oak Ridge National Laboratory (ORNL) in Tennessee, where the Holifield Radioactive Ion Beam Facility served as a crucible for pioneering nuclear experiments. This facility earned the distinction of being named a historic physics site by the American Physical Society in 2016, a tribute to its instrumental role in advancing nuclear science. ORNL scientists and their collaborators focused particularly on isotopes of tin and its neighboring elements, probing transitions in nuclear energy states and establishing foundational knowledge about the so-called “doubly magic” nature of tin-132.

The term “doubly magic” refers to the nuclear configuration of tin-132, which possesses fully occupied outer shells of both protons and neutrons, granting it exceptional stability compared to neighboring isotopes. This extraordinary stability manifests as a higher energy barrier required to remove a proton or neutron from the nucleus, making tin-132 a nuclear archetype. The precision measurements conducted at ORNL provided critical data that helped characterize this unique behavior in detail, influencing theoretical models of nuclear structure for years to come.

More recently, complementary experimental efforts at CERN’s ISOLDE facility in Switzerland employed advanced laser spectroscopy techniques to measure the charge radii of exotic tin isotopes near nuclear shell closures at neutron numbers N=50 and N=82. These measurements, which involve discerning the subtle shifts in nuclear charge distribution, provide nuanced understanding of how nuclear size and shape evolve across isotopic chains. The collaboration between the teams at ORNL and CERN has formed a comprehensive picture of the nuclear landscape in this region of the nuclear chart.

Alfredo Galindo-Uribarri, a prominent physicist at ORNL and a key figure in these studies, emphasized the importance of integrating historical data with modern spectroscopic measurements. He noted that the combined analyses afford essential insights into the evolution of nuclear properties across isotopes, enabling physicists to refine theoretical frameworks that model nuclear interactions. The interplay between experimental precision and theoretical innovation underscores the dynamic nature of modern nuclear physics research.

The recent results, published in the esteemed journal Physical Review Letters, underscore how minute variations in neutron number influence nuclear properties such as charge radii, shell closures, and overall nuclear stability. These findings contribute critically to the bedrock of nuclear physics by facilitating improved predictive models. Such models are indispensable for applications ranging from the optimization of nuclear reactors to the assessment of nuclear weapon resilience.

Understanding how nuclei change structure when moving away from stability is pivotal for interpreting nucleosynthesis processes—how elements heavier than iron are forged in stellar environments. The tin isotopes investigated lie in a region critical to the rapid neutron capture process (r-process), a sequence of reactions believed to produce many of the heavy elements in the universe during cataclysmic astrophysical events such as neutron star mergers and supernovae.

The experimental methods combined at two premier facilities—radioactive ion beams at ORNL and laser spectroscopy at CERN—have enabled scientists to bridge gaps in knowledge that isolated studies could not resolve. At ORNL’s Holifield facility, teams generated exotic tin isotopes and meticulously recorded their nuclear transitions. Meanwhile, ISOLDE’s sophisticated laser systems measured charge distributions with unparalleled precision. Together, these data sets allowed for cross-validation and enhanced understanding.

These measurements also bolster the endeavor to benchmark and challenge theoretical approaches such as nuclear shell models and ab initio calculations that attempt to simulate complex many-body nuclear systems from first principles. Adjustments informed by empirical data on tin isotopes recalibrate parameters to better fit reality, improving both accuracy and reliability. Such enhancements are critical as nuclear theory increasingly supports practical technologies and informs fundamental research on matter’s building blocks.

The longevity and collaboration evident in this research narrative—from initial experiments at ORNL’s Holifield facility to cutting-edge laser spectroscopy at ISOLDE—exemplify the power of sustained, international scientific partnerships. Insights gleaned from tin isotopes resonate not only within nuclear physics but ripple into interdisciplinary domains including astrophysics, material science, and energy research.

This work highlights how probing nuclei at the edges of stability reveals unexpected nuances in nuclear forces and shell structures, challenging long-accepted paradigms and inspiring fresh questions. The mysteries of nuclear matter continue to unravel with each isotope measured and modeled, promising deeper comprehension of both terrestrial and cosmic phenomena.

In sum, the collaboration among institutions and the combination of legacy data with state-of-the-art techniques have culminated in a seminal contribution to nuclear science. The detailed understanding of tin isotopes’ charge radii and nuclear configurations enriches foundational knowledge, extending the frontiers of science while offering tangible benefits for technology and national interests alike.

Subject of Research:
Not applicable

Article Title:
Charge Radii Measurements of Exotic Tin Isotopes in the Proximity of 𝑁 =50 and 𝑁 =82

News Publication Date:
25-Nov-2025

Web References:
DOI link to Physical Review Letters article
CERN ISOLDE facility
American Physical Society historic physics site announcement
Nature article on doubly-magic tin-132

Image Credits:
Alonda Hines/ORNL, U.S. Dept. of Energy

Keywords

Physical sciences, Particle physics