Nuclear "magic numbers" collapse beyond doubly magic nickel 78

Scientists at the RIKEN Nishina Accelerator Research Center and their collaborators used the center's heavy-ion accelerator, the RI Beam Factory, to demonstrate that nickel-78, a "double magical" nickel isotope rich in neutrons, with 28 protons and 50 neutrons, still retain a spherical shape that makes it relatively stable despite the significant imbalance in the number of protons and neutrons. They also discovered a surprise – observations from the experiment suggest that nickel-78 could be the lightest nucleus with 50 neutrons to have a magical nature. Lighter isotones, that is nuclei with the same number of neutrons but a different number of protons, would inevitably be deformed, despite the magic number of neutrons.

Understanding the validity of magic numbers in extremely neutron-rich nuclei is essential to understanding why our universe has the mix of nuclei we see today. Elements heavier than iron are not synthesized during the normal combustion of stars, but are mainly created by two processes called process s and process r, which involve nuclei capturing additional neutrons. The process r, in which the neutrons are rapidly absorbed, is particularly important because it is responsible for the creation of certain neutron-rich nuclei. During the process, the nuclei accumulate neutrons until they reach a state in which they can no longer accept them (this state is called a point of waiting), and then undergo a process called beta decay, in which they lose a neutron but gain a proton. , allowing them to start accepting new neutrons. The process r, which accounts for about half of the production of heavier nuclei than iron, can only take place in extremely neutron-rich environments, such as supernova explosions and neutron star fusions, like the one observed in 2017.

The precise location of these "waiting points" is however not well understood. The process is complicated by the fact that the magic number of protons or neutrons – which equates to the idea of ​​chemically closed electron layers – makes the nuclei more resistant to the capture of additional neutrons. A well-known magic number is 50 neutrons, but it is not known if this number is preserved for nuclei extremely rich in neutrons.

To get an answer, the group decided to experiment with nickel-78, a doubly magical isotope that only became accessible to experimentation recently, thanks to powerful accelerators such as the RI Beam Factory in Japan, the one used in this study. To perform the experiment, published in Nature, the researchers combined the observations of the MINOS detector operated by the CEA in France and the DALI2 detector operated by RIKEN, both located in the RIBF complex. They generated a beam of uranium 238 and used it to bombard a beryllium target, forcing uranium to fission into isotopes such as 79 copper and 80 zinc, both with 50 neutrons.

These two beams were then sent to a hydrogen target, sometimes producing nickel 78, at the center of the research.

Using gamma-ray detectors, the group has shown that nickel-78 is relatively stable, as predicted by calculations, by maintaining a spherical rather than deformed shape. Ryo Taniuchi of the University of Tokyo and the RIKEN Nishina Center for Accelerator-Based Science said: "We were pleased to be able to demonstrate experimentally that nickel-78 retains the spherical shape provided by the calculations. were surprised to discover that the nucleus also has a competing form, which is not spherical, and that any isotone lighter than the one we used would be subject to this deformation and would not retain its magical nature. "

Pieter Doornenbal of the Nishina Center said: "This is an important discovery because it gives us new insights into how magical numbers appear and disappear in the nuclear landscape and affect the nucleosynthesis process that led to the isotope abundance the current universe.We intend to make further experiments with even lighter isotones with 50 neutrons to experimentally demonstrate this discovery. "