Researchers?at the Institute of Modern Physics?(IMP)?of the?Chinese Academy of Sciences?(CAS),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?aluminum,phosphorus,sulfur,and argon elements,?and?have?proposed a new approach?to?uncover?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 are 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 this 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 thenewly?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 elements.
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 this study.
Surprisingly,with this new method,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-26,27 and?sulfur-27,28,and?suggest?that?argon-31?may?be a new double proton halo nucleus. Additionally,the study clarifies?that the ground state of?aluminum-22?does?not?exhibit?proton halo structure.?These findings shed light?on future 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 on related?fields.
The corresponding authors?of this study are Prof. ZHANG Yuhu?and Prof. WANG Meng?from?IMP,and Prof.?Yury Litvinov from the GSI Helmholtzzentrum für Schwerionenforschung?in Germany.
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.?

Figure. 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)