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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

  • International consortium discovers new insights into the mechanism of nuclear fission, with results published in Nature
  • The paper shows the way the angular momentum of the two fragments, is generated
  • New insights are important for the fundamental understanding and theoretical description of the nuclear fission process

London February 2021 – Scientists from the National Physical Laboratory (NPL) and the University of Surrey have contributed to an international research collaboration to explain how the internal angular momentum of the resulting fission fragments can be generated, following the splitting of a heavy atomic nucleus. The results are presented by the Nu-Ball consortium’s article ‘Angular momentum generation in nuclear fission’, published in Nature. (https://www.nature.com/articles/s41586-021-03304-w).

The nu-ball collaboration consists of nuclear scientists from 37 research institutes from 16 countries, including seven scientists from NPL’s Nuclear Metrology Group and from the University of Surrey’s Physics Department. 

A series of experiments at the ALTO particle accelerator facility at Irène-Joliot-Curie (IJC) Laboratory in Orsay, France revealed that the resulting radioactive nuclear species resulting from nuclear fission appear to obtain their intrinsic angular momentum (or ‘spin’) after fission has occurred, rather than before as has been previously assumed. This new interpretation of the nuclear fission process was made possible following detailed analysis of spectroscopic nuclear structure data taken using the ‘Nu-Ball’ gamma-ray spectrometer. Nu-Ball is also the name of the international consortium of nuclear scientists who have developed a unique gamma-ray measurement system using detectors which combine precise energy determinations with sub-nanosecond timing information. By correlating gamma-ray cascades from different combinations of fission fragments down to coincidence times of the order of one billionth of second (one nanosecond), the nu-Ball measurements allowed the detailed investigation of the complex quantum decay signatures between correlated pairs of nuclear fission fragments with unprecedented precision.

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 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 had virtually no spin prior to fission. There are many competing theories, but the majority state that the spin of the fission fragments is generated before the nucleus splits, which should lead to a clear correlation of the spins between 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 UK’s core technical contribution to the Nu-Ball collaboration includes the provision of specialist Lanthanum-Bromide fast-timing scintillation detectors. These are routinely used in the NPL-based National Nuclear Array (NANA) and the STFC funded Fast Timing Array (FATIMA) for core traceability measurements of radioactive materials used in industry and also for nuclear structure studies of very rare radioactive species

Nu-Ball members based at NPL and the University of Surrey, Paddy Regan, Matthias Rudigier, Alberto Boso, Michael Bunce, Peter Ivanov, together with STFC-funded PhD students Rhiann Canavan and Shaheen Jazrawi, contributed to the conceptualisation of the overall experimental campaign; the design and commissioning of the full Nu-BALL spectrometer; preparation and participation of the individual experiments. UK team members also analysed selected data channels and contributed to the scientific discussion and drafting, revision of the final article.

The new insights into the role of angular momentum in nuclear fission presented in this paper are important for a better theoretical understanding of the nuclear fission process, but also have consequences for other areas of fundamental research, including the structure of very neutron-rich radioactive isotopes and the synthesis of super-heavy elements. There are also direct applications from this work in terms of understanding the production and future management of radioactive waste in nuclear fission-based energy production. This comes from a better understanding of the so-called gamma-ray heating problem in nuclear reactors which required precise knowledge of the intensity and energy of the gamma ray energy signatures emitted following nuclear fission.

Professor Paddy Regan, NPL Professor of Nuclear Metrology, University of Surrey & NPL Fellow in Nuclear Science, said: “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."

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