Exploring the Mysteries of Exotic Nuclei**
Recent breakthroughs in experimental techniques and the creation of radioactive ion beams have profoundly enriched our understanding of exotic nuclei—short-lived, unusual forms of atomic matter. New experimental data reveal that in these exotic nuclei, traditional shell gaps and magic numbers—patterns that help us understand nuclear structure—disappear or transform, a phenomenon known as "shell evolution."
This shell evolution relates to the strong nuclear force, which governs how protons and neutrons interact. As the ratio of neutrons to protons increases, the energy levels of these particles can shift, leading to surprising changes in nuclear behavior. While scientists have studied the development of shell structure in exotic nuclei for the past two decades, many questions remain about how these changes affect nuclear features, including collectivity, nuclear shape changes, and coexistence.
Near specific shell closures, the nucleus can take on different shapes: it can become prolate (like an American football) or oblate (like a pumpkin) as nucleons (protons and neutrons) shift across major shell gaps. If these deformed shapes have energy levels similar to those of a spherical nucleus, they can coexist, creating a complex structure. In exotic nuclei, the barriers preventing these changes diminish due to shell evolution, leading to expectations of enhanced deformation and shape coexistence—particularly in the case of the nucleus around 78Ni, where the proton shell gap is reduced.
To investigate these intriguing phenomena, a new research project will employ gamma-ray spectroscopy to measure excited state lifetimes ranging from a few picoseconds to several microseconds. The team will utilize recoil distance Doppler shift and fast timing methods to gather critical information. Experiments will be performed within international collaborations at major international research facilities RIKEN (Japan) and LNL (Italy). The described experimental study will combine measurements of both very short and long lifetimes of the most exotic nuclei one can reach today.
The project has brought on two talented PhD students to contribute their expertise. One student will be working on the experiment in Italy, while the other will focus on the experiment in Japan. Both students have impressive backgrounds in Nuclear Structure Physics, making them well-equipped to tackle the challenges presented by this exciting research.
This research holds the promise of unveiling the complexities of nuclear structure and enhancing our understanding of the fundamental forces shaping the universe. By exploring the behavior of these exotic nuclei, scientists hope to gain deeper insights into the nature of matter itself.
The present project will look for consequences of the shell evolution on the properties of neutron-rich nuclei, with a focus on the development of deformation and shape coexistence. EVOLUTION will focus on the 78Ni mass region where the experimental information on nuclear shapes and deformation is almost non-existing at present. The project will provide valuable experimental information through gamma-ray spectroscopy by measuring lifetimes of excited states ranging from few picoseconds to several microseconds via recoil distance Doppler shift and fast timing methods, respectively. Multi-nucleon transfer reactions and beta-decay processes will be exploited in combination with high resolution fragment separators coupled to an array of high efficienct gamma-ray detectors. The described experimental study will combine measurements of both very short and long lifetimes of the most exotic nuclei one can reach today.
One of the major challenges for nuclear theory is to predict the properties of all nuclei within a single framework. With newly obtained spectroscopic data in the exotic region of 78Ni, EVOLUTION will provide a good testing ground for the predictive power of nuclear theory. Furthermore, knowledge of the shell structure in this mass region will help to improve mass models, which is important since the pathway of the astrophysical rapid neutron capture process, essential to understanding the origin of the elements in the Universe, passes through the 78Ni region.
Two experiments will be performed in two large-scale international beam factories: The radioactive ion beam laboratory RIKEN (Japan) and LNL-Legnaro National Laboratories (Italy). They will target to cover the physics of exotic nuclei from lighter to heavier exotic nuclei in the 78Ni region.
