Researchers at the Accelerator Laboratory of the University of Jyväskylä, Finland, have made a groundbreaking discovery in the study of radioactive lanthanum isotopes. By precisely measuring their atomic masses, the team identified an intriguing anomaly in the nuclear binding energies, which has important implications for our understanding of how heavier elements are created in the cosmos. This finding provides new insights into the nuclear processes that occur in extreme astrophysical environments, particularly in events like neutron-star mergers, where rapid processes of element formation occur. The research, which challenges existing models of nuclear structure, was published in the journal Physical Review Letters.
The Challenge of Studying Neutron-Rich Isotopes
Nuclear binding energies—the energy required to hold the protons and neutrons together in an atomic nucleus—are a critical factor in our understanding of how elements form, especially in astrophysical contexts. For elements heavier than iron, their creation in stars and during cosmic events like neutron-star collisions depends on neutron-capture processes. These processes are governed by the nuclear binding energies of neutron-rich isotopes, which are notoriously difficult to study due to their short-lived nature.
In this study, researchers focused on lanthanum isotopes, specifically the neutron-rich versions of the element. These isotopes were produced at the Ion Guide Isotope Separation On-Line (IGISOL) facility, a sophisticated facility that allows for the creation of such rare isotopes. However, the short lifespans of these isotopes present a significant challenge, making it difficult to accurately measure their properties.
High-Precision Mass Measurements
Using the JYFLTRAP Penning trap mass spectrometer at the University of Jyväskylä, the research team achieved an unprecedented level of precision in measuring the masses of six lanthanum isotopes. This method, known as phase-imaging ion cyclotron resonance (PI-ICR), enabled the team to determine the masses of these isotopes with high precision. Notably, this work allowed for the first-ever measurement of the masses of lanthanum-152 and lanthanum-153, which had never been measured before.
Professor Anu Kankainen, the leader of the study and a part of the ERC CoG project MAIDEN, highlighted the importance of these measurements in enhancing our understanding of nuclear structure. These precise mass values provide essential input for calculations that are central to understanding how heavy elements are synthesized in astrophysical environments.
Neutron Separation Energies and Their Astrophysical Importance
One of the key objectives of the study was to determine the neutron separation energies of these lanthanum isotopes. The neutron separation energy is the amount of energy required to remove one neutron from a given nucleus. This value is critical in determining the rates of neutron capture in stellar environments, especially in the context of the rapid neutron capture process (also known as the r-process).
The r-process is responsible for the formation of many of the heavier elements beyond iron, and it takes place in environments with a high flux of neutrons, such as during neutron-star mergers. The famous observation of GW170817, a neutron-star merger event, provided direct evidence of the r-process at work, and the discovery of the r-process’s role in forming heavy elements has opened up new avenues of research. Understanding the neutron separation energies of neutron-rich isotopes is crucial for improving our models of the r-process and refining the calculations for how these elements are created in the cosmos.
According to Professor Kankainen, the precise measurements of the lanthanum isotopes provided new insights into the astrophysical neutron-capture rates in the r-process. These insights help improve our understanding of how elements are formed in extreme environments, such as during neutron-star collisions.
The Unexpected “Bump” in the Data
While analyzing the mass data and calculating the two-neutron separation energies, the researchers were surprised to discover a “bump” in the data, which occurred when the number of neutrons in the lanthanum isotopes increased from 92 to 93. This bump represents a sudden, localized increase in the neutron separation energy, which had not been predicted by any current nuclear mass models.
This phenomenon suggests a possible sudden change in the nuclear structure of these isotopes, a discovery that could significantly impact our understanding of nuclear physics. As Ph.D. researcher Arthur Jaries, a key member of the research team, explained, this bump was a complete surprise. Existing models, which are used to predict nuclear masses and properties, fail to account for this feature. This discrepancy raises new questions about the underlying structure of these isotopes and calls for further investigation using complementary methods, such as laser spectroscopy or nuclear spectroscopy.
Implications for Theoretical Models
The discovery of the bump in the neutron separation energy has profound implications for theoretical nuclear models. The precise mass measurements have not only improved the accuracy of astrophysical calculations but also highlighted significant gaps in our current understanding of nuclear structure. According to the researchers, the new data has reduced the uncertainties in mass-related calculations by a factor of up to 80 in some of the most extreme cases.
These improved measurements have a direct impact on our understanding of the r-process, which is responsible for the formation of heavy elements like gold, platinum, and uranium. The discovery of the bump could provide important clues for the ongoing search for a unified theory of nuclear structure, particularly for isotopes far from stability, like those involved in the r-process.
In particular, the bump’s significance lies in its potential to help resolve the mystery of the rare-earth abundance peak in the r-process. This peak, which represents a concentration of rare-earth elements in the solar system, has long been a puzzle for scientists. The discovery of the bump in the lanthanum isotopes could provide a missing piece of the puzzle, improving our models of how this peak forms and, by extension, refining our understanding of the r-process as a whole.
The Road Ahead: New Research and Model Development
This discovery is just the beginning. According to Professor Kankainen, the observed bump calls for the development of new, more accurate nuclear mass models. The current models, while powerful, are unable to explain the observed features, and they will need to be refined in light of these new findings.
Future research will likely involve using additional experimental techniques, such as laser spectroscopy or nuclear spectroscopy, to investigate the underlying nuclear structure of these isotopes and further explore the causes of the bump. Moreover, theoretical physicists will need to revisit their models and incorporate these new measurements into their predictions to refine our understanding of the nuclear processes involved in the formation of heavy elements in the universe.
Conclusion
The precise measurements of neutron-rich lanthanum isotopes at the University of Jyväskylä have opened up a new chapter in the study of nuclear physics and astrophysics. By revealing an unexpected anomaly in the neutron separation energies, researchers have uncovered a feature that current nuclear models cannot explain. This discovery not only has the potential to transform our understanding of how heavy elements are formed in the cosmos, but it also highlights the need for new theoretical models to better describe the complex nuclear structure of neutron-rich isotopes.
As the team continues its research, the results of this study are expected to have significant ramifications for our understanding of the r-process and the astrophysical neutron-capture reactions that drive the creation of the heaviest elements in the universe. With further investigations and advancements in both experimental techniques and theoretical models, this work will undoubtedly lead to important new insights into the origins of the elements and the fundamental nature of nuclear matter.
Reference: A. Jaries et al, Prominent Bump in the Two-Neutron Separation Energies of Neutron-Rich Lanthanum Isotopes Revealed by High-Precision Mass Spectrometry, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.04250. On arXiv: arxiv.org/abs/2408.06221