Physicists measure the “double-magic” atomic nucleus – zirconium-80 is even lighter and therefore more closely bound than expected –

Deformed and magical: The zirconium isotope 80Zr has 40 protons and 40 neutrons and therefore has a “magic” number that is particularly favorable in terms of quantum mechanics for both core components. This has consequences, as measurements have now confirmed: The zirconium 80 core is lighter than the sum of its core components. In addition, this zirconium core is clearly deformed – instead of a sphere, it resembles an elongated ellipse, as the team reports in the specialist magazine “Nature Physics”.

The atomic nucleus probably also has a structure similar to the electron shell of the atom. According to his theory, there are quantum mechanically conditioned shells in it, which are successively filled by the protons and neutrons. This could explain why atomic nuclei with certain numbers of neutrons and protons are particularly stable – these “magic numbers” each correspond to a full core shell. The atoms in which both protons and neutrons have such a full core shell are doubly magical.

One of these double-magic isotopes is zirconium-80, whose nucleus consists of 40 neutrons and 40 protons. “This core is of particular interest because its proton and neutron shell structures have to be very similar,” explain Alec Hamaker from Michigan State University and his colleagues. According to theory, the core building blocks of such double-magic atoms are particularly tightly bound to one another.

Mass reveals binding energy in the core

Whether this is really the case should be readable from the mass of this atomic nucleus. “Because the mass of an isotope is a sensitive indicator of the binding forces in the core and the shell structure behind it,” the researchers explain. Under the extreme conditions in the atomic nucleus, part of the mass of the core building block becomes E = mc according to Einstein’s formula2 converted into binding energy. Because they are particularly strongly bound, double-magic nuclei should therefore be lighter than neighboring isotopes.

For zikronium-80, it was predicted more than 30 years ago that its core would have to be strongly deformed by the forces within it – elliptical instead of spherical. Hamaker and his colleagues have now measured the weight of the zirconium 80 core ten times more accurately than before. To do this, they first generated a beam of the short-lived zirconium isotopes 80 to 83 by shooting molybdenum atoms at a beryllium plate.

The resulting positively charged atomic nuclei were directed into a magnetic Penning trap, where the researchers determined their mass. It can be determined from the speed at which the atomic nuclei rotate in the magnetic field.

Even easier than expected

“Such measurements have been made before, but never with this precision,” says Hamaker. “And that revealed some interesting physics.” The measurements showed that the zirconium-80 core is even lighter than the theory predicts. “According to our values, 80Zr is substantially lighter and therefore more strongly bound than was to be expected from the systematic trends,” write the physicists.

They explain this with the fact that the same number of neutrons and protons in the nucleus leads to additional interactions which further increase the binding energy of the atomic nucleus. At the same time, this means that zirconium-80 must have a clearly deformed core – it is elongated, elliptical instead of round. “Theorists predicted this over 30 years ago, and now the evidence is in,” says Hamaker. (Nature Physics, 2021; doi: 10.1038/s41567-021-01395-w)

Quelle: Michigan State University Facility for Rare Isotope Beams

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