Why are most of the cores deformed

Deformed atomic nuclei

Much like the electron shell of an atom, the atomic nucleus can also be described using a shell model. But there are also significant differences, as physicists only discovered last year: some atomic nuclei change their shape spontaneously. In the podcast, Norbert Pietralla from the Technical University of Darmstadt explained how he and his team were able to prove this phenomenon.

We ourselves and everything around us are made up of atoms. And these in turn consist of an outer shell of negatively charged electrons and a positively charged nucleus.

Norbert Pietralla: “An atomic nucleus consists of electrically positively charged protons and electrically neutral neutrons. The protons are electromagnetically repelled from each other because they have the same charge. But protons and neutrons are held together by the strong nuclear force, which is stronger than the Coulomb repulsion - and in this way the protons and neutrons in the atomic nucleus hold together. "

Similar to the electrons in the shell, which are distributed over certain energy levels or so-called atomic orbitals, the nucleons - i.e. the protons and neutrons - can also be assigned to different orbitals within the nucleus depending on their energy.

Norbert Pietralla

“These orbitals or shells describe the probabilities of the nucleons that are located in these orbitals. That means, depending on the occupation of a corresponding shell, this nucleon - proton or neutron - will be at different locations in the atomic nucleus with a different probability, just as electrons in different orbitals in the atom can be at different locations with different probability. "

If all shells are closed, i.e. occupied with the maximum possible number of nucleons, the atomic nucleus has a uniformly spherical shape.

“If the shell is not fully occupied, however, interactions between protons and neutrons can lead to an atomic nucleus assuming something other than a spherical shape. This happens spontaneously when such a shape is energetically more favorable than the spherical shape. This is due on the one hand to the details of the nuclear forces and on the other hand to the way in which the quantum shells in the atomic nucleus can be occupied by protons and neutrons. "

Depending on the arrangement of the nucleons, the shape of the atomic nucleus can be very different.

“So there are a lot of atomic nuclei that deform in such a way that one axis is longer than the other two. Something like that can be found in everyday life, for example in the form of a cigar. A cigar has an axis of symmetry, two short and one long axis. In an atomic nucleus that deforms into a cigar - physicists call it prolate - the long axis is about thirty percent longer than the two short axes. There are also very extreme forms of this deformation, then one speaks of super deformation, where the long axis is about twice as long as the two short axes. "

Other atomic nuclei are shaped like a disc, more rarely are nuclei with a pear or lemon-like shape. To measure the shape of atomic nuclei, Pietralla and his colleagues at TU Darmstadt bring electrons to high energies in an accelerator and then let them collide with the atomic nuclei. The nuclei are stimulated - they receive additional energy - and the electrons are deflected from their original path. Based on this scattering of the electrons, the researchers can determine whether nuclei have changed from the ground state to an excited state. In their experiments, the researchers examined, among other things, zirconium-96 - an isotope of the metal zircon with 40 protons and 56 neutrons in the core.

“We were interested in the strength of electromagnetic transitions between states of the atomic nucleus. Our investigations showed an unexpectedly high deformation of the excited states. We discovered this through unexpectedly high electrical quadrupole transition radiation, which we were able to measure for the first time. "

This quadrupole transition radiation occurs when a cigar-shaped nucleus is excited and a little later releases the energy absorbed in the form of electromagnetic radiation. The stronger the deformation along the longitudinal axis, the stronger this transition radiation. The team discussed its results with scientists from Tokyo who simulate core states on the computer.

"It was only in discussions with our colleagues that we realized that this is an example of shell development in a nucleus that was previously only discussed by this group as a function of nucleon numbers."

Linear accelerator S-DALINAC

So far, the researchers have assumed that the shape of a nucleus changes when additional nucleons are added - for example in different isotopes of an element. These each contain the same number of protons, but different numbers of neutrons.

“That this can also occur in one and the same nucleus - simply through the excitation of certain nucleons, which leads to a dynamic change in the shell structure - that was new and we were able to demonstrate this for the first time in this nucleus by measuring electromagnetic transition rates. "

It is interesting for the physicists whether they can find other nuclei that also take on different shapes. The team from Japan wants to calculate the interactions and energies in the core with powerful computers and thus make predictions that the researchers in Darmstadt can then check.

“The Japanese colleagues - but also colleagues here at TU Darmstadt - are already very far advanced in calculating nuclei with a large number of valence nucleons, i.e. nucleons outside of closed shells. We will definitely continue this research and we are also looking for other nuclei in which this meaning of the shell evolution, the shell development, is particularly pronounced. "

However, many nuclei that are of particular interest to the scientists are difficult to study in the accelerator because they are very unstable, for example. With the new international accelerator facility FAIR, which is currently being built in Darmstadt, physicists will be able to produce such unstable atomic nuclei in a targeted manner. In this way, experiments will be possible with which scientists will be able to find out even more about the structures and interactions in atomic nuclei in the future.