Physicists are no magicians, but ask them about how protons and neutrons are arranged in atomic nuclei and you’ll be sure to hear the term magic. Just like electrons fill up a series of onion-like shells of different energy around an atomic nucleus, protons and neutrons are each thought to occupy a series of shells within the nucleus. In this nuclear shell model, nuclei in which protons or neutrons form complete shells, without any space left for more particles, are called magic because they are more stable than their nuclear neighbours. Nuclei with complete proton and neutron shells are termed doubly magic and are exceptionally stable.
In a study just published in Physical Review Letters, a team of researchers working for the MINIBALL and HIE-ISOLDE collaborations at CERN provide the first direct proof that the tin nucleus tin-132 (132Sn), which is considered to be doubly magic, does indeed merit this special status. The result is the first to emerge from the recently commissioned HIE-ISOLDE accelerator, and shows that this accelerator is a key facility to unravel the inner workings of atomic nuclei.
The nucleus 132Sn has 132 nucleons – 50 protons and 82 neutrons – and is one of only 10 species, out of 3200 known nuclei, that qualify as doubly magic and act as benchmarks for testing the nuclear shell model. What’s more, in the nuclear chart, which organises all known nuclei according to their number of protons and neutrons, 132Sn is close to nuclei thought to be produced in an astrophysical process, the r-process, responsible for creating heavy elements in the cosmos. This process is not fully understood, so studying 132Sn could cast light on the origin of heavy elements in the cosmos.
Previous studies that explored the doubly magic nature of 132Sn were indirect, deducing the doubly magic nature of 132Sn by studying the properties of its nuclear neighbours. In this new study, the HIE-ISOLDE/MINIBALL team examined 132Sn directly. The team took 132Sn isotopes produced by the ISOLDE facility, accelerated them in HIE-ISOLDE to an energy of 5.49 MeV per nucleon, and then focused them at a target of lead-206 (206Pb) inside the MINIBALL gamma-ray detector array. This excited the nucleons in the 132Sn nuclei to higher-energy states. These collective excitations, which have low chances of occurring, decayed with emission of gamma-ray photons, which MINIBALL detected.
By analysing the number of gamma-ray photons detected, the authors measured the strengths of these excitations for the first time. From these strengths, they found more pronounced excitations in 132Sn compared to those of its nuclear neighbours. This was predicted by theory and is a crucial feature of doubly magic nuclei. It thus confirms the doubly magic nature of 132Sn.
“These results were only possible due to a unique combination: ISOLDE, the prime facility for producing radioactive isotopes; the new HIE-ISOLDE accelerator, which provides the ideal energy per nucleon for this type of experiment; and MINIBALL, which can detect gamma rays from the decay of the excitations with high efficiency and superior energy resolution,” explains Peter Reiter, a member of the MINIBALL collaboration.
If these results were not enough proof, the researchers also compared the observed strengths with several new state-of-the-art theoretical shell-model calculations for 132Sn, finding a remarkable agreement between the observations and all calculations. This further reinforces the conclusion that 132Sn is doubly magic. Who said physicists are not magicians?