Researchers at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences, together with their collaborators, have achieved the first precise mass measurements of several exotic atomic nuclei. Using this mass data, they have determined the proton dripline for the elements aluminum, phosphorus, sulfur, and argon, and have proposed a new approach to uncovering proton halo structures.

The results were published in Physical Review Letters on November 27.

The atomic nucleus is a quantum many-body system composed of protons and neutrons, typically exhibiting a size similar to that of neighboring nuclei. A halo is an exotic nuclear structure found in weakly bound nuclei, characterized by one or more valence nucleons that display an extended spatial distribution, resulting in a radius significantly larger than that of neighboring nuclei.

In previous experimental studies, neutron halos have been more frequently observed, while proton halos have been less common.

“It is challenging to experimentally observe proton halo nuclei, because the Coulomb barrier restricts the formation of proton halo structures,” said YU Yue, a PhD student from IMP and the co-first author of the study. “However, with the aid of precise nuclear masses, we could reveal signs of the proton halo.”

The researchers conducted the experiment at the Cooler Storage Ring (CSRe) at the Heavy Ion Research Facility in Lanzhou (HIRFL). Using the newly developed Bρ-defined isochronous mass spectroscopy technique, they determined the masses of several exotic atomic nuclei, including silicon-23, phosphorus-26, sulfur-27, and argon-31, for the first time. They also improved the mass precision of sulfur-28 by a factor of 11.

The high-precision mass data enabled the researchers to fix the location of the proton dripline for aluminum, phosphorus, sulfur, and argon.

Using the new masses, the researchers extracted a physical quantity known as mirror energy differences. “We propose that mirror energy differences, which are solely related to atomic masses, can be used to probe proton halo structures,” said Associate Prof. XING Yuanming from IMP, another co-first author of the study.

With this new method, the researchers found isospin symmetry breaking in some (near) proton-dripline nuclei. Further study suggested that this should be due to the existence of proton halo structures in these nuclei, a conclusion that was supported by relevant theoretical calculations.

The experimental results support the existence of proton halos in candidate nuclei such as phosphorus-26 and -27, and sulfur-27 and -28, suggesting that argon-31 may be a new double proton halo nucleus. The study also clarifies that the ground state of aluminum-22 does not exhibit a proton halo structure. These findings shed light on potential experimental and theoretical research on proton halo nuclei.

The study suggests that mirror energy differences can serve as a sensitive indicator for detecting isospin symmetry breaking and revealing proton halo structures. This new approach is expected to facilitate further research in related fields.

This work was supported by the Strategic Priority Research Program of CAS, the National Key R&D Program of China, the National Natural Science Foundation of China, the Youth Innovation Promotion Association of CAS, and the Gansu Natural Science Foundation.

A schematic diagram of proton and neutron distributions for mirror nuclei argon-31 and aluminum-31. Left side is the weakly bound proton-rich nucleus argon-31, which is predicted to exhibit a significant halo structure in this work. Right side is the deeply bound neutron-rich nucleus aluminum-31. (Image by IMP)