Researchers at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences, in collaboration with other institutions, have made significant strides in understanding exotic atomic nuclei. They recently achieved the first precise mass measurements of several such nuclei and, using these mass data, were able to determine the proton dripline for several elements, including aluminum, phosphorus, sulfur, and argon. These findings also led to the proposal of a novel approach to uncover proton halo structures, a fascinating phenomenon in nuclear physics. The results were published in Physical Review Letters on November 27, 2024, and represent a major breakthrough in the study of atomic nuclei and their behavior under extreme conditions.
The atomic nucleus is a quantum system made up of protons and neutrons, typically held together in a compact, stable structure. However, certain nuclei exhibit unusual properties, leading to what are known as “halo nuclei.” These nuclei are characterized by one or more nucleons (protons or neutrons) that are loosely bound and spread out far beyond the typical size of the nucleus. The result is a much larger spatial distribution compared to other neighboring nuclei. The most commonly observed halo structures are neutron halos, which occur when neutrons in a nucleus form a diffuse region around the nucleus. Proton halos, in contrast, are less frequently seen and are harder to detect experimentally, mainly due to the Coulomb barrier—a repulsive force between positively charged protons.
Proton halos are significant because they provide insights into the behavior of protons in extremely low-energy states. These structures are weakly bound and are found in isotopes close to the proton dripline—the boundary beyond which the nucleus can no longer hold onto additional protons. The difficulty in observing proton halos lies in their rarity and the challenges posed by the Coulomb barrier. Despite these difficulties, the researchers at IMP were able to use precise nuclear mass measurements to reveal signs of proton halo structures.
The experiment was conducted at the Cooler Storage Ring (CSRe) at the Heavy Ion Research Facility in Lanzhou (HIRFL). In their work, the team utilized the newly developed Bρ-defined isochronous mass spectroscopy technique, which allowed them to determine the masses of several exotic atomic nuclei for the first time. These included silicon-23, phosphorus-26, sulfur-27, and argon-31. Additionally, they were able to improve the mass precision of sulfur-28 by a factor of 11, enabling a more accurate understanding of the mass behavior of these elements. The precise mass data gathered through this technique were essential for determining the proton dripline for aluminum, phosphorus, sulfur, and argon elements.
One of the key findings of the study was the use of a physical quantity called “mirror energy differences,” which are related to atomic masses. This approach offers a way to probe proton halo structures, as these differences are sensitive to the distribution and binding of protons and neutrons in nuclei. Associate Professor Xing Yuanming, one of the co-first authors of the study, emphasized that mirror energy differences are a valuable tool for identifying proton halo structures. The researchers found that these differences revealed signs of isospin symmetry breaking in several near-proton-dripline nuclei. This symmetry breaking is thought to occur due to the presence of proton halo structures in these nuclei, a conclusion that was supported by corresponding theoretical calculations.
The experimental results also provided new insights into which nuclei might exhibit proton halos. The study identified candidate nuclei such as phosphorus-26, phosphorus-27, sulfur-27, and sulfur-28 as potential proton halo candidates. Among these, argon-31 was proposed as a new double proton halo nucleus. The discovery of a double proton halo in argon-31 could significantly impact our understanding of proton-rich nuclei and their behavior at the extremes of nuclear stability. The researchers also clarified that the ground state of aluminum-22 does not exhibit proton halo characteristics, refining the conditions under which proton halos might exist.
These findings not only enhance our understanding of proton halo structures but also introduce a new method for detecting them in exotic nuclei. The use of mirror energy differences as an indicator for isospin symmetry breaking could serve as a valuable tool in future nuclear research. This new approach could open avenues for further investigation into proton-rich nuclei and provide new ways to explore the underlying forces that govern nuclear structure.
Source: Chinese Academy of Sciences