In the late 1940s, Maria Goeppert Mayer noticed that nuclei containing certain numbers of protons or neutrons—specifically, 2, 8, 20, 28, 50, and 82—tend to be more stable than similarly composed isotopes. Her observation led her to propose the nuclear shell model, in which protons and neutrons slot into discrete energy levels, much like electrons do in atoms. (She and Hans Jensen, who proposed the model independently, were awarded a share of the 1963 Nobel Prize in Physics.) Isotopes that have so-called magic numbers of protons, neutrons, or both are akin to the noble gases: They gain stability because their outermost occupied shells are full, and jumping to the next shell requires considerable energy input.
Recent research, however, suggests that for nuclei that are brimming with neutrons, the magic nature of certain nucleon counts can come and go. The latest evidence comes from a team, led by researchers at the Tokyo Institute of Technology, that has produced and measured oxygen-28 nuclei for the first time. Despite its having a doubly magic eight protons and 20 neutrons, the nucleus appears to decay rapidly and, during its passing existence, host an incomplete outer neutron shell.
The researchers developed an intricate experimental setup at the RIKEN Radioactive Isotope Beam Factory in Wako, Japan, specifically to produce and detect 28O. They fired an intense beam of neutron-rich calcium nuclei at a beryllium target to produce a plethora of species, including fluorine-29, which is identical to 28O save for an extra proton. The team isolated and channeled the 29F toward a reservoir of liquid hydrogen, which on occasion would knock off a proton from the incoming isotopes to form 28O. The trickiest part was confirming the presence of the neutron-rich isotope. To do so, the researchers relied on sensitive detectors to spot all five of the anticipated decay products: four neutrons plus an 24O nucleus.
The experiment delivered little evidence that 28O derives any stability enhancement from its supposed doubly magic status. The researchers suspect that 28O exists as a fleeting resonance that quickly emits two pairs of neutrons, to form first 26O and then the short-lived but comparably stable 24O. They also use measurements of both 28O and 29F to argue that some of the oxygen nucleus’s outermost neutrons overcome the energy gap and bleed into another shell, preventing the tidy shell closure predicted by theory.
Future experiments are needed to verify the measurements and pin down 28O’s nuclear structure. But the RIKEN results are consistent with other findings that suggest magic numbers are not absolute. Isotopes of elements such as neon and magnesium that are slightly heavier than oxygen do not seem to sport closed shells when they are packed with 20 neutrons. And 24O, the final decay product of 28O, manages to arrange its 16 neutrons—not a previously known magic amount—into a closed valence shell. With powerful facilities, including RIKEN and the new Facility for Rare Isotope Beams, nuclear physicists should have ample opportunity to probe more nuclei that are chock-full of neutrons. (Y. Kondo et al., Nature 620, 965, 2023.)
