How cleaner datasets can unlock new understanding of the universe

While much of the experimentation at ILL uses neutrons to probe deep into the atomic structure of materials, in research teams such as the nuclear and particle physics group at the facility, neutrons are a critical tool to investigate the structure of even smaller entities – sub-atomic particles.

Nuclear physics, in particular, involves investigating the structure of atomic nuclei – the region at the centre of all atoms, made up of protons and neutrons. It is a global and varied area of research, having led to applications throughout its history that include nuclear power sources, radiocarbon dating in geology and archaeology, and magnetic resonance imaging – one of the most important imaging devices in medicine. How do protons and neutrons bound to allow for the existence of the elements? What is the origin of the elements? Those are the typical questions that nuclear researchers want to answer.

The nuclear and particle physics group operates four of the ILL’s 40 plus state-of-the-art instruments for the study of nuclear physics and neutron particle physics. One of these is FIPPS, a gamma-ray spectrometer designed for the study of atomic nuclei which are produced in thermal neutron-induced reactions. Its intense, pencil-like neutron beam can be directed at a small sample of either stable or radioactive materials. If a neutron is captured and bound to a nucleus in the sample, its so-called binding energy is released in the form of gamma radiation which is detected by FIPPS. One special sample that can be used is uranium, where the neutron can induce a nuclear fission reaction and the nuclei of the sample’s atoms are split in two. In this process a large amount of energy is released, part of it again in form of gamma radiation.

The gamma rays emitted after these neutron-induced reactions are characteristic for each nucleus, and allow researchers to draw conclusions on its inner structure. To develop effective models for the complex interactions of the protons and neutrons within an atomic nucleus, we need plenty of experimental data from these reactions. FIPPS is a powerful tool for instigating these and detecting the resulting gamma emissions, but as it’s a highly sensitive instrument, there is a lot of data produced in each experiment. This can make it difficult to extract the most useful information from the masses of data picked up by the detectors.

Data analysis is also made difficult as nuclear fission is always followed by beta-decay – a radioactive decay which results in further gamma radiation being released and detected by the FIPPS instrument. When fission reactions are the subject of interest in the experiment, this further gamma radiation can create unhelpful background data that may hide the useful physics message. The signals corresponding to the most exotic nuclei, for example the ones of interest for the understanding of the stellar nucleosynthesis, may “disappear” because of the large background. Many international collaborations have searched new techniques for an improvement of selectivity in this kind of fission studies.

The instruments at ILL are some of the most advanced in the world for studying nuclear physics, with the machinery, experimental set-up, and scientific methods constantly being refined and developed to open new avenues in fundamental scientific research. Recent post-doctoral research at ILL has aimed to develop techniques that help to deal with the challenge of background radiation being picked up during fission experiments, which compromises the quality of the datasets. This involves the first development at a neutron beam and gamma array of what is known as an active fission target, which can identify on event by event basis whether the radiation detected was a result of fission or beta decay.

The active target makes analysis easier and allows the study of rarer processes that emerge from nuclear fission. Not all products of fission are produced with the same probability, and so a tool for supressing the unwanted gamma radiation from beta decay makes spotting these rare events easier. The new active target is highly effective at selecting the gamma rays produced from fission. Those data attracted the interest of the international nuclear physics community and many discoveries are on the way.

By improving how we analyse the extensive data produced in these experiments, scientists can better access and understand nuclear structures for improved and more efficient nuclear reactors dedicated to energy production. The insights enabled through this analysis can also contribute to our understanding of the universe – in areas of astrophysics requiring knowledge of a large range of nuclei, especially unstable ones such as those produced in fission, one can reveal how the elements in the universe are produced.

The first instrument built as part of the Institut Laue-Langevin’s current modernisation programme, Endurance, FIPPS is one of the many tools to have been created or upgraded to improve the scientific capabilities at ILL. Among mechanical advances, there is a substantial investment in the holistic enhancement of how each instrument operates, including the accompanying software and sample environments. ILL’s instruments are constantly being developed and upgraded to meet advances in the field and the evolving needs of its users, and enabling scientists to push the boundaries of what is achievable with its tools.  

Contact: Caterina Michelagnoli