This nucleus can be treated as a proton plus semi-magic 128Sn core within the particle-core coupling scheme. It was discovered that this addition of one proton resulted in an overall enhancement of the collective electric quadrupole strength by a factor of 1.4 in stark contrast to previous empirical expectations. The enhanced collectivity, which is an early signal of the emerging nuclear deformation and collectivity that becomes dominant away from shell closures, is due to constructive quadrupole coherence in the wavefunctions stemming from the long-range part of the proton-neutron residual interactions. The understanding of proton and neutron motion within atomic nuclei is an ever-evolving topic and the advent of accelerated beams of exotic nuclei has led to many surprises. This recent study used a novel technique of measuring electromagnetic properties of an exotic nucleus combined with state-of-the-art calculations to deconvolute the nature of proton and neutron interactions, paving the way for a more accurate understanding of these building blocks of nature.
The Science
Atomic nuclei are finite many-body quantum systems that exhibit increased stability or “shell structure” when the proton and/or neutron numbers equal 2, 8, 20, 28, 50, 82, or 126. If both the proton and neutron numbers are equal to one of these “magic” values, the nucleus is said to be “double magic”. Nuclear deformation and collective excitations are postulated to develop when the long-range part of the proton-neutron residual interaction overcomes the short-range pairing interaction, where the latter is similar to Cooper-pair formation in superconductors and favors spherical shapes. Atomic nuclei adjacent to shell closures provide a simple and unique laboratory for exploring the nature of nucleon interactions.
The Impact
A novel technique that measures the electromagnetic properties of radioactive nuclei has demonstrated that 129Sb, a proton plus semi-magic 128Sn core, has a collective strength that is a factor of 1.4 larger than that of the 128Sn core. Two state-of-the-art shell model calculations indicate that this enhancement is due to constructive proton-neutron interactions, where adding one nucleon to a core near a double-shell closure has a pronounced effect. The pronounced enhancement signals the beginning stage of the emerging nuclear deformation and collectivity that becomes dominate as we move away from shell closures.
