Nuclear isomers—metastable states of atomic nuclei that conserve high excitation energy for unexpectedly prolonged periods—are essential to deepening our grasp of complex nuclear architectures and astrophysical nucleosynthesis processes. Over the course of many years of experimental investigation, the GSI/FAIR consortium has detected a cumulative total of 192 nuclear isomers, a feat unmatched by any other research facility worldwide. Propelled by an innovative integration of particle acceleration and advanced detection instrumentation, these findings testify not only to methodological ingenuity but also to the relentless curiosity driving experimental nuclear science.

Central to the success at GSI is the extraordinary contribution of Dr. Ivan Kojouharov, whose career dedicated to nuclear spectroscopy catalyzed the co-discovery of 143 individual nuclear isomers—more than any other researcher globally. His expertise in developing high-performance germanium detector arrays proved vital in numerous experiments, enabling exquisite resolution when measuring gamma radiation emitted by isomeric transitions. Dr. Kojouharov’s extensive publication record underscores his pivotal role in expanding the nuclear landscape through meticulous data acquisition and nuanced analysis techniques.

The sophisticated accelerator complex at GSI is a cornerstone of this achievement, comprising an elaborate chain of devices starting with the Universal Linear Accelerator (UNILAC), proceeding through the heavy-ion synchrotron SIS18, and culminating in the Experimental Storage Ring (ESR). This multi-tiered infrastructure uniquely facilitates experimental access to virtually all elements present in the periodic table, allowing researchers to induce nuclear reactions and isolate exotic isotopes under controlled conditions. Crucially, the Fragment Separator (FRS) functions as an ingenious “sorting machine,” spatially segregating newly formed nuclear fragments post-collision and enabling targeted scrutiny for isomeric states.

Nuclear isomers themselves exhibit a striking deviation from typical nuclear behavior. While most nuclei shed excess energy instantaneously upon excitation, isomers are hindered from rapid decay by internal quantum mechanical selection rules and nuclear structural idiosyncrasies. This results in metastable configurations capable of holding onto energy for durations ranging from microseconds to years before gamma emission restores the nucleus to its ground state. Studying these isomers unravels intricate nuclear potential landscapes and contributes vital insights into stellar processes such as rapid neutron capture (r-process) nucleosynthesis, which governs the cosmic production of heavy elements.

Beyond fundamental inquiry, nuclear isomers hold promise for innovative technological applications, including usage in medical diagnostic imaging techniques through their unique decay signatures. Moreover, some isomers have sparked interest as candidates for ultra-precise nuclear clocks, which could surpass the accuracy of contemporary atomic clocks by exploiting nuclear transition frequencies less susceptible to environmental perturbations. These applied research directions are intertwined with ongoing efforts to decipher the fine structure of nuclear energy levels and decay pathways.

Prof. Michael Thoennessen’s meticulous compilation consolidates all verified scientific publications on nuclear isomers with half-lives exceeding 100 nanoseconds, standardizing knowledge across a broad temporal scale. This database update complements his longstanding stewardship of isotope discovery statistics—another domain where GSI maintains a premier global standing. Notably, the late Professor Hans Geissel held the record for the discovery of 279 isotopes, a testament to the continuous lineage of excellence fostered at the center.

The international scientific collaboration NUSTAR, hosted for the first major meeting since the February fire at GSI, exemplifies the vibrant research community engaged with the FAIR (Facility for Antiproton and Ion Research) project. NUSTAR scientists delve deeply into nuclear reactions mimicking astrophysical environments, striving to elucidate element formation mechanisms active within stars and explosive cosmic events. The communal synergy at FAIR offers fertile ground for breakthroughs in both experimental nuclear physics and astrophysics.

Looking ahead, the construction of the FAIR international accelerator center presages even more profound discoveries. As one of the world’s largest and most ambitious research infrastructures, FAIR integrates next-generation superconducting fragment separators (Super-FRS) poised to enhance isotope separation capabilities beyond those of the conventional FRS. This technological leap is expected to accelerate the discovery rate of isotopes and nuclear isomers, extending the experimental reach into previously inaccessible regions of the nuclear chart.

Professor Thomas Nilsson, Scientific Managing Director of both GSI and FAIR, articulates the institution’s vision succinctly: to replicate cosmic phenomena within a state-of-the-art laboratory environment and secure a commanding position atop the leaderboards of nuclear discoveries. The convergence of scientific ambition, technical innovation, and interdisciplinary collaboration embodied in FAIR and GSI illuminates promising avenues for unraveling universal mysteries encoded in nuclear matter.

In summation, the record-setting discovery of nuclear isomers at GSI/FAIR underscores a significant chapter in the annals of nuclear physics research. It exemplifies how precision instrumentation, comprehensive accelerator suites, and dedicated scholarship synergistically advance our understanding of atomic nuclei and their astrophysical significance. As FAIR progresses, the anticipation of uncovering new exotic states fuels excitement within the scientific community, reaffirming GSI’s stature as a crucible for nuclear innovation and cosmic exploration.

Subject of Research: Discovery and characterization of nuclear isomers

Article Title: Discovery of nuclear isomers

Web References: DOI: 10.1016/j.adt.2025.101767

Image Credits: © L. Weitz, GSI/FAIR

Keywords

Physics, Nuclear physics, Nuclear reactions

The BARB project—or Biomedical Applications of Radioactive Ion Beams—builds on a vision nearly half a century old yet only now truly realized, thanks to recent instrumental and accelerator advances. The core innovation lies in the simultaneous utilization of therapeutic and diagnostic radiation emitted by radioactive ions during treatment. This capability addresses range uncertainty, a long-standing challenge in particle therapy that arises because clinicians cannot precisely ascertain the ion beam’s stopping location once it penetrates the body. Small errors in range lead to either underdosing the tumor or damaging surrounding healthy tissue. The BARB team’s approach overcomes this by harnessing the positron emission from radioactive ions to generate in-beam positron emission tomography (PET) images, thus allowing real-time tracking of the beam’s path inside the body.

At the heart of this achievement is a sophisticated detector system developed by researchers at LMU Munich, designed specifically for small animal research. This high-resolution in-beam PET scanner can detect the annihilation photons produced as emitted positrons interact with electrons in the tissue, thereby precisely localizing the radioactive beam as it deposits its therapeutic dose. Originally developed under the ERC Consolidator Grant project “SIRMIO,” the PET detector has been further optimized within BARB to handle the unique challenges posed by radioactive carbon ion beams, enabling image-guided treatment with unprecedented accuracy.

Professor Marco Durante highlights the transformative nature of this advance: “Particle therapy is growing rapidly as one of the most precise forms of radiation treatment, but its clinical impact is limited by uncertainties in imaging during treatment. Our work demonstrates that using the same ion beam for therapy and in situ imaging breaks this barrier, paving the way for highly accurate, safe, and versatile particle therapies.” The ability to verify dose delivery in real time has exciting implications, particularly for treating tumors near critical structures—such as spinal cords or vital organs—where even millimeter-scale inaccuracies can have severe consequences.

The researchers presented proof of concept by treating osteosarcoma, a malignant bone tumor, in mice. The tumor was located in a sensitive neck region in close proximity to the spinal cord, traditionally a prohibitive area for intensive radiation due to potential neurological damage. By employing a radioactive carbon ion beam (^11C isotope), the group administered a high therapeutic dose of 20 gray directly to the tumor with submillimeter precision. The mice experienced complete tumor control without paralysis or other significant neurological side effects, underscoring the method’s safety and efficacy at preclinical scale.

This breakthrough is facilitated by the tandem integration of advanced accelerator facilities and nuclear imaging instrumentation. The “FAIR Phase 0” experiments at GSI/FAIR provided the intense beams of radioactive ions necessary to perform these realistic therapies, overcoming previous limitations where available beam intensities and control were insufficient. Moreover, collaboration between research pillars APPA and NuSTAR at FAIR, and international partners such as LMU and QST-Chiba, exemplifies how interdisciplinary approaches accelerate cutting-edge biomedical developments.

One of the key technical challenges addressed was real-time image reconstruction paired with continuous PET data acquisition during irradiation. Co-first author Giulio Lovatti, working as a doctoral student at LMU, emphasized the complexity of extracting meaningful beam localization from PET signals under ongoing beam delivery conditions. Their solution enabled the first demonstration of fully image-guided tumor therapy using radioactive ion beams, a milestone expected to refine treatment planning and delivery in future clinical settings.

Beyond cancer therapy, the BARB project opens new avenues in radiation oncology and other medical fields. The approach to image-guided irradiation could enhance the treatment of metastases difficult to target precisely and enable safer therapies for small, sensitive areas, including non-malignant cardiac applications such as ventricular ablations for arrhythmias. Such versatility illustrates the broad translational significance of this methodology.

Looking forward, the team plans to investigate additional short-lived isotopes for radioactive ion therapy that may generate even stronger imaging signals with faster feedback, optimizing treatment monitoring efficiency and accuracy. The future integration of these therapies with the fragment separator Super-FRS under construction at FAIR promises to increase radioactive beam intensities, thereby enabling more effective and clinically applicable protocols.

The success of BARB also lays important groundwork for Professor Durante’s subsequent ERC Advanced Grant project, “Heavy Ion FLASH (HI-FLASH),” which explores ultra-high dose-rate irradiation techniques. This continuity reflects a vibrant and rapidly evolving research landscape, wherein fundamental nuclear physics insights increasingly drive medical technologies that directly benefit patients.

In conclusion, the work presented by the BARB collaboration represents a paradigm shift in particle therapy—a sophisticated synergy of physics, biology, and medical technology that moves beyond traditional radiation treatments. By transforming the ion beam itself into both a precise weapon against tumors and an intrinsic imaging tool, this pioneering research ushers in a new era of safe, efficient, and personalized radiotherapy, paving the way for future clinical translation and improved patient outcomes worldwide.

Subject of Research: Animals

Article Title: Image-guided treatment of mouse tumours with radioactive ion beams

News Publication Date: 19-Aug-2025

Web References: DOI: 10.1038/s41567-025-02993-8

References: Nature Physics publication by the BARB collaboration

Image Credits: © GSI/FAIR