Oxygen-28, which has eight protons and 20 neutrons, should, in theory be stable. Physicists at the GANIL and LPC Caen, led by Olivier Sorlin and Miguel Marqués working within the SAMURAI collaboration at the RIKEN Research Center in Japan, have now shown that this is not the case.
Protons and neutrons in nuclei occupy discrete orbitals in much the same way as electrons do in atoms. Magic nuclei are those with the precise number of protons or neutrons required to fill a spherical set of related orbitals (a shell). Most nuclei with magic neutron or proton numbers are characterized by a stronger binding and greater stability. They are therefore more abundant in nature. This is the case, for example, for nuclei with eight, 20 or 50 protons or those with an equivalent number of neutrons.
In doubly-magic nuclei, that is, those with a magic number of protons and a magic number of neutrons, both proton and neutron shells are filled, making the binding even stronger. These nuclei are among the most stable in nature. Some examples include oxygen-16 (eight protons and eight neutrons) or calcium-40 (20 protons and 20 neutrons). This remarkable property makes them ideal for testing models of the forces that structure the atomic nucleus.
A difficult nucleus to study
Oxygen-28 exists only in an unbound form and in the very rare cases in which it is produced in collisions, it disintegrates into oxygen-24 in just 10 to 22 seconds. To compare the properties of oxygen-28 with theoretical models, physicists need to be able to instantly detect the four neutrons that the nucleus emits, which is no easy task.
To overcome this problem, researchers from the RIKEN laboratory, working at the NEULAND and NEBULA neutron detectors and the SAMURAI spectrometer, recently developed a new technique that involves studying a related nucleus: fluorine-28, which has nine protons and 19 neutrons. Easier to produce than oxygen-28, fluorine-28 disintegrates by emitting a single neutron. The researchers reckoned that if oxygen-28 is indeed doubly-magic, its properties should be similar to those of the flurione-28 nucleus.
When a nucleus is magic, nucleons (protons and neutrons) completely fill the lowest energy orbitals available, as mentioned. The new study reveals, however, that the fluorine-28 neutrons occupy a large fraction of the valence orbital.
The result is the first to reveal that the doubly-magic nucleus, so important for making nuclei stable, probably disappears in oxygen-28 because of its unbound nature and the nuclear forces present in this very high-neutron-number nucleus. It also implies that new effects strongly influencing the dynamics of nucleons inside nuclei could be at play, say the researchers.
Reference: Extending the Southern Shore of the Island of Inversion to 28F. A. Revel et al. (SAMURAI21 collaboration) Phys. Rev. Lett. 124 152502