In nuclear physics there is a series of numbers that is particularly fascinating: 2, 8, 20, 28, 50, 82 – the so-called magic numbers. However, these numbers have nothing to do with magic. They are called that because atomic nuclei are particularly stable when the number of their protons or neutrons corresponds to such a magical number – but a recent experiment suggests that this magic has its limits.
Stable isotopes are more common in the universe, which is why magic nuclei have an advantage: Helium-4, for example, occurs particularly frequently in the cosmos and is doubly magical, it has two protons and two neutrons in its nucleus. Oxygen-16, by far the most common stable oxygen isotope, is also twice as magical, with eight protons and eight neutrons each. The reason for the stability is that atomic nuclei can be described in terms of a shell model, with closed nuclear shells corresponding to the magic numbers.
Basically, symmetrical nuclei are stable, i.e. those with approximately the same number of protons and neutrons. But what about highly asymmetric but magical nuclei, such as oxygen-28? Couldn’t the isotope also be stable, or at least assume a short-lived, bound state, since it is doubly magical with eight protons and 20 neutrons in the nucleus? A collaboration led by the Japanese physicist Yosuke Kondo from the Tokyo Institute of Technology has now been able to answer this question. The researchers succeeded for the first time in observing oxygen-28, like them in the journal Nature to report.
A proton is knocked out of the nucleus like a billiard ball
However, at the same moment that the isotope was created, it decayed again. It only exists as a short-lived resonance, the study says, because of magic. From the details of the measurement, the researchers actually conclude that oxygen-28 is not a magical nucleus after all. “The magic numbers that were established with stable nuclei do not apply globally,” says Thomas Aumann from TU Darmstadt, who was involved in the work.
The research team also examined oxygen-28 because it is a light element with a large excess of neutrons. It is therefore particularly suitable for testing core models. Oxygen-28 does not occur in nature; the heaviest stable oxygen isotope is oxygen-18. Unstable, i.e. radioactive, isotopes range up to oxygen-24. According to the current work, the strong interaction, the fundamental force that holds atomic nuclei together – even beyond the repulsive effect of the positively charged protons – reaches its limits with oxygen-28, despite the supposedly magical number of nucleons.
Nevertheless, the experiment at the Riken Beam Factory, a particle accelerator near Tokyo, is a great success. The research team shot calcium-48 nuclei onto a beryllium plate with high energy. A fluorine-29 beam emerged as a secondary beam from this impact. This intense particle beam was then aimed at a target made of liquid hydrogen, where the fluorine nuclei collided with hydrogen nuclei.
“There is a high momentum transfer, like in billiards,” says Thomas Aumann, explaining the process. A proton is knocked out of the fluorine-29 nucleus, which is called a “knockout” reaction. This creates oxygen-28. During the laboratory experiment in Wako, in addition to that isotope, oxygen-27 was also produced, which, however, decayed just as immediately as oxygen-28. For both isotopes, there is no longer any support for some neutrons in the nucleus due to the loss of the ninth proton.
Oxygen-28 split into oxygen-24 and four neutrons. These four neutrons were then observed using two special detectors, called Neuland and Nebula. “Detecting four neutrons was a huge challenge; no one had ever succeeded before,” says Aumann. The Neuland neutron detector in combination with the calcium beam was technically the key to success. The detector was developed at the Helmholtz Center for Heavy Ion Research in Darmstadt, where the device was originally intended as a prototype for the Fair particle accelerator facility, which is still under construction. However, the detector was then shipped to Japan, with some effort, as Aumann explains. Virtually his entire working group was sent to Wako to help with the construction.
The magic numbers are probably not generally applicable to predict stability properties. Ultimately, the strong interaction needs to be further researched, says an accompanying article Nature-Study. Only then can we better understand how the elements in the universe are formed, which of them are stable and why, for example, neutron stars stick together.