Exotic insights, unified aims

More than a century after the discovery of the atomic nucleus, no universal model can yet reliably predict its properties as the numbers of protons and neutrons change. Exotic nuclei – highly unstable and complex systems with unique properties – challenge and extend current nuclear theory. An additional piece of the puzzle was recently provided from the combination of experimental results from two leading international facilities and advanced theoretical calculations.

All known atomic nuclei are plotted on the nuclear chart, a graphical representation organised by the number of protons and neutrons. While each data point is a step towards understanding neighbouring nuclei, model predictions break down where experimental data is lacking. This is particularly the case for exotic nuclei. “These highly unstable, short-lived isotopes do not exist on Earth outside of research but are essential for understanding astrophysical processes such as neutron star mergers,” explains Caterina Michelagnoli, ILL scientist responsible for the FIPPS instrument.

The nuclear chart was recently extended with new data on four exotic Bromine (Br) isotopes –  ⁸⁷Br, ⁸⁹Br, ⁹¹Br and ⁹³Br. “The behaviour of these nuclei is of particular interest due to the odd-even asymmetry between their 35 protons and increasingly high numbers of neutrons (52 to 58),” explains Jérémie Dudouet, CNRS researcher at the Institute of Physics of 2 Infinities (IP2I). “These neutron-rich exotic isotopes are highly challenging to produce and investigate, but their study was made possible through the combination of experimental data from two unique international facilities, together with advanced theoretical calculations" continues Dudouet. “The achievement illustrates the potential of collaboration and the effective pooling of economic, technical and human resources.” explains Michelagnoli. 

Experimental measurements began at the GANIL facility, using the high-intensity uranium-238 beam to produce the exotic bromine nuclei via nuclear fission. To detect these rare, far-from-stability events, the researchers required the capabilities of the AGATA (Advanced GAmma Tracking Array) spectrometer – developed through European collaboration – coupled with the VAMOS++ magnetic spectrometer. “Fission fragments are produced in an excited state and emit gamma rays, which serve as their fingerprint,” explains Michelagnoli. “VAMOS++ enables the specific isotope to be identified, while AGATA detects gamma emissions; their combination provides high selectivity and confidence that these never-before-detected gamma rays indeed originate from the exotic isotopes identified.”