A classic atomic nucleus, as found inside most everyday elements, is essentially a roughly spherical lump of protons and neutrons, firmly bound by the strong nuclear force. However, when the nuclei become more exotic, for example when they have an unusually large number of protons or neutrons or are simply extremely heavy, they sometimes develop very remarkable structures. For example, the nuclei of the heaviest, artificially created elements can be flattened, egg-shaped or even pear-shaped. And some atomic nuclei have a kind of halo of protons or neutrons - individual nuclear building blocks that extend like clouds far beyond the actual radius of the atomic nucleus.
A team led by Yu Yue from the Institute of Modern Physics at the Chinese Academy of Sciences has now discovered several exotic atomic nuclei with such a proton halo using a new technique. As the team reports in the journal "Physical Review Letters", one of the discovered nuclei may even have a double halo: Argon-31 could be surrounded by two of these proton clouds at once. For the study, the working group measured the masses of atomic nuclei with unusually few neutrons. Silicon-23, for example, has five neutrons fewer than the "normal" silicon-28; argon-31 even lacks nine neutrons compared to the stable argon-40.
These particularly proton-rich atomic nuclei are very unstable; they decay in milliseconds. They are close to the so-called "drip line" - the limit at which excess nuclear components simply "drip out" of the nucleus. Such exotic formations at the limit of stability provide information about the behaviour of nuclear components under extreme conditions and provide insights into the nature of matter itself. The halos are a consequence of these exotic binding ratios. Strongly bound particles have a high probability of being located in the centre of the nucleus. The particles of the halo, on the other hand, are more weakly bound and therefore further out in the nucleus - they surround it like a kind of skin or halo.
Neutron halos are now known from several neutron-rich nuclei. Proton halos, on the other hand, are very difficult to detect. Yu's team therefore used the comparison with so-called "mirror nuclei": Atomic nuclei in which protons and neutrons are virtually swapped. The mirror nucleus of argon-31, for example, is the isotope aluminium-31 - it has 13 protons and 18 neutrons. If these nuclei are exactly symmetrical, they should have a calculable energy difference. However, using the precisely measured atomic masses, the experts showed that the real energies deviate from the symmetry. This indicates that the binding energy in argon-31 is different than expected. Using such analyses, the team came to the conclusion that the nuclei phosphorus-26 and -27 as well as sulphur-27 and -28 each have one halo proton - and argon-31 even has two.
