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Physicists shed new light on long-standing problem in nuclear fission

High-precision nuclear structure measurements provide new insights into nuclear fission

Scientists from NPL and the University of Surrey have contributed to an international research collaboration to show the way the angular momentum of the two fragments, resulting from the splitting of an atomic nucleus, is generated. The results are presented by the Nu-Ball in the consortium’s article ‘Angular momentum generation in nuclear fission’, published in Nature.

A series of experiments at the ALTO particle accelerator facility at Irène-Joliot-Curie (IJC) Laboratory in Orsay, France, has revealed that the fragments resulting from nuclear fission obtain their intrinsic angular momentum (or spin) after fission, not before, as is widely assumed. This result was made possible by the ‘Nu-ball’ collaboration, an international group of nuclear physicists aiming to study a wide range of nuclei and their structure.

The group is made up of researchers from 37 institutes and 16 countries, including scientists from NPL’s Nuclear Metrology Group and the University of Surrey’s Physics Department, and is led by the IJC Laboratory.

Although nuclear fission, in which a heavy nucleus splits in two and releases energy, is a well-established reaction being discovered at the end of the 1930s, open questions about the process persist to this day.

The new scientific study addresses the question of why, when a heavy atomic nucleus undergoes fission, the resulting fragments are observed to emerge spinning, even when the original nucleus did not spin at all. There are many competing theories, but the majority state that the spin of the fission fragments is generated before the nucleus splits, leading to a clear correlation of the spins of the two partner fragments.

To reveal the mechanism generating fragment spin, the team induced nuclear fission reactions at the ALTO facility and measured gamma rays, which are emitted in the process. Specifically, they irradiated samples of the uranium isotope 238U and the thorium isotope 232Th with a pulsed neutron beam.

The University of Surrey and NPL scientists, Paddy Regan, Matthias Rudigier, Alberto Boso, Michael Bunce, Peter Ivanov together with PhD students Rhiann Canavan and Shaheen Jazrawi contributed to the conceptualisation of the experiment; the design and commissioning of the NuBALL spectrometer and preparation of the experiment, participated in the measurements, analysed selected data channels and contributed to the scientific discussion and drafting of the final article.

These new insights into the role of angular momentum in nuclear fission are important for the fundamental understanding and theoretical description of the fission process. However, they also have consequences for other research areas, such as the study of the structure of neutron-rich isotopes and the synthesis and stability of super-heavy elements.

NPL and the University of Surrey’s Nuclear groups have established a long-standing collaboration with the groups at Orsay, and were both founding members of the NuBALL collaboration, contributing expertise on gamma-detection, calibration metrology and multi-parameter data analysis for nuclear structure interpretation.

Professor Paddy Regan, NPL Professor of Nuclear Metrology, University of Surrey & NPL Fellow in Nuclear Metrology, stated: “A major step towards this breakthrough in understanding of the fundamentals of nuclear fission was development of the gamma-ray spectrometer “Nu-Ball”, consisting of 184 individual detectors acting in digital coincidence mode. These include UK-based detectors which routinely make up the STFC funded Fast Timing Array (FATIMA) and the NPL-based National Nuclear Array (NANA). Only by exploiting this state of the art digital instrumentation, is it was possible to measure gamma rays from prompt fission with the required precision and accuracy to reveal the underlying reaction mechanism in the angular momentum population in the nuclear fission process. This work represents a textbook example of how addressing high-impact nuclear science problems can drive the development of new technology which can then be exploited within the UK’s sovereign national infrastructure regarding measurement of radioactive materials.’’ 

The lead author of the study, Dr Jonathan Wilson from the IJC Laboratory in Orsay, said: “What really surprised me was the lack of significant dependence of the average spin observed in one fragment on the minimum spin demanded in the partner fragment. Most theories hypothesizing that spin is generated before fission would have predicted a strong correlation. Our results show that the fragment spin emerges after the splitting. It can be illustrated with by the snapping of a stretched elastic band which results in a turning force, or torque.”

Professor John Simpson, from the Science and Technology Facilities Council's Daresbury Laboratory said: “This is a fascinating result that sheds light on the understanding of one of the fundamental decay properties of the atomic nucleus, that of fission. It is great to see the technical advances made in blue skies research, in detectors and instrumentation, produce such excellent results of wide scientific interest and importance."

Image courtesy of Luc Petizon

25 Feb 2021