Researchers at Chalmers University of Technology have contributed to an international research collaboration that reveals new and unexpected insights into the nuclear fission process. Fission in unstable nuclei with extreme proton-to-neutron ratios has been extensively investigated for the first time. The findings – recently published in Nature – reveal surprising patterns in the charge distributions of fission fragments.
A better understanding of fission – where an atomic nucleus splits and releases energy – is essential to obtain a better knowledge of not only the secrets of how atoms work, but also to a better understanding of how elements are formed in universe and better techniques for energy production on Earth.
In a recent study published in Nature, researchers have investigated the fission of 100 different types of exotic nuclei, where a large portion has never previously been studied this way, thus adding to better knowledge of the fundamental driving forces in nature.
The basis of the article is an experiment which was conducted at the GSI Helmholtz Centre for Heavy Ion Research in Germany. During the experiment, a target of beryllium was bombarded with a heavy beam of uranium at 86.7% of the speed of light within an accelerator facility. The collision that followed produced hundreds of different isotopes that were filtered out and identified according to their mass and charge.
Have contributed with data analysis and interpretations
The study is a large international collaboration and the culmination of about 25 years of work. Associate Professor Andreas Heinz, together with colleagues from the Department of Physics at Chalmers University of Technology, have contributed with performing the experiment, data analysis and interpretation.
Here, Andreas Heinz gives an insight into the research:
“Fission is a process that has been investigated for a very long time, but only for a very limited number of isotopes. Typically, what one does is bombard the isotope of interest with, for example, neutrons, and then one observes fission. For long lived isotopes such as uranium, this is no problem, but with much shorter-lived nuclei, it is much trickier. And those are the ones that we have managed to study in this work,” says Andreas Heinz.
Shell effects affect how the nuclei split
When a nucleus splits into two fission fragments the two fragments typically do not have the same size or the same mass. In many cases one is heavier, and the other one is lighter. The origin of this asymmetry is caused by the shell structure of nuclei, where certain numbers of protons and neutrons are more stable than others. However, how this works in detail is still difficult to understand.
To shed light on this mystery, fission of exotic nuclei with more extreme ratios of protons to neutrons were investigated, in particular nuclei in the platinum, mercury and lead region with relatively few neutrons. While it was known that some of those nuclei fission into a heavy and a light fragment, this work succeeded in identifying the reason: additional stability due to a specific number of protons, 36, in the light fission fragment.
“What we are trying to find out is which shell effects are responsible for the nucleus splitting in one light and one heavy part. This is very difficult to predict, and also difficult to measure experimentally. We have measured a region of nuclei undergoing fission, which has not been investigated very thoroughly up to now. This study finds evidence that it is a shell effect in the number of protons of the light fission fragments which is responsible for a lot of the evolution which we have not previously seen,” says Andreas Heinz, who himself was surprised how clear the influence of this shell effect was visible in the data.
Gives important clues to fission research
The new study adds further pieces to the puzzle that is fission research.
“How the complex nuclear fission process works in detail is still an open question. Today there is not just one model to describe the entire process. Experimentally it is also very difficult to measure all the things one could imagine one wants to measure and to extract exactly the information needed. Since this study has given us access to fission data of many more nuclei, the picture has really changed, and we have now a better understanding of how nuclear shells influence nuclear fission,” says Andreas Heinz.
About the research:
The article An asymmetric fission island driven by shell effects in light fragments was published in Nature, April 30, 2025. Contributors to the article from Chalmers University of Technology are Associate Professor Andreas Heinz, research engineers Matthias Holl, Håkan T Johansson and Hans Törnqvist, Professor Björn Jonson, and Professor Emeritus Mikhail Zhukov. The researchers are currently active at the Department of Physics, except for Matthias Holl who is now active at Lund University.
Contact
- Associate Professor, Subatomic, High Energy and Plasma Physics, Physics