Probing nuclear structure through experiment

Every nucleus has a different structure or shape according to the shell model. Some nuclei possess a special number of protons and neutrons, called magic numbers and they prefer to be spherical in shape. When more nucleons (neutrons or protons) are added to these spherical and stable nuclei, the motion of excess nucleons inside the nucleus can lead to a collective motion of nucleons and ultimately to a deformed, unequal mass distribution. During this process, the nucleus’ energy increases as the number of nuclear energy levels increases tremendously. In order to explain the behaviour of these energy levels at high-excitation energies, we investigate the average statistical properties such as Nuclear Level Density– which is the number of available levels per unit energy, and the Gamma-ray Strength Function (gSF) – which is the probability for a nucleus to emit or absorb a gamma-ray. The gSF of deformed isotopes/nuclei show some increased probability for gamma-decay called resonances, which are not observed for the spherical nuclei. So, how do these statistical properties evolve across the isotopic chain of an element?

Scientists at iThemba LABS, together with their international collaborators, recently measured the statistical properties of well-deformed Samarium nuclei, and in particular investigated the scissor resonance (SR) – which is a resonance caused by the proton and neutron deformed clouds oscillating against each other like the blades of a scissor. With the two nuclei being only two neutrons apart and having a similar deformation, the strength of the SR is expected to be comparable. The results surprisingly showed that the magnitude of the SR in the two nuclei were not comparable. They concluded that a possible explanation may be that the calculated deformation is underestimated and so a dedicated experiment to measure the deformations is desirable.

The international team of the project at the Oslo Cyclotron Laboratory at the University
of Oslo, Norway. (Photo credit: Victor Modamio)

A full explanation of the research and its findings is available at the following link (Physical Review C) https://journals.aps.org/prc/abstract/10.1103/PhysRevC.103.014309

This work was published by K. L. Malatji, K. S. Beckmann , M. Wiedeking et al., in Physical Review C 103 (2021) 014309