Nuclear physicists of Milano University and INFN Milano (GAMMA experiment) have contributed to the discovery of the unexpected way the angular momentum of the two fragments, resulting from the splitting (fission) of an atomic nucleus, is generated. Such outstanding mystery in Nuclear Physics has been solved by measuring gamma rays emitted by U and Th fissioning isotopes.
Publication in Nature
Nuclear fission, in which a heavy nucleus splits in two and releases energy, was discovered at the end of the 1930s by the chemists Otto Hahn and Fritz Strassmann, and the physicists Lise Meitner and Otto Frisch. This physical phenomenon still has fascinating unknown aspects to be revealed.
In the fission process, the fragments are observed to emerge spinning. This observation has been an outstanding mystery in Nuclear Physics for decades: the internal generation of around 6-7 units of angular momentum (or spin) in each fragment is particularly puzzling for systems which start with zero, or almost zero, spin. Up to now, no experimental observations were able to provide a decisive discrimination between the many competing theories for the angular momentum generation mechanism, although the present consensus is that excitation of collective vibrational modes generate the intrinsic spin before the nucleus splits (pre-scission).
A series of experiments at the ALTO particle accelerator facility at IJC Laboratory in Orsay, France, has now revealed that the fragments resulting from nuclear fission obtain their intrinsic angular momentum after fission and not before. This surprising 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 using a high-granularity gamma spectrometer made of more than 100 high-purity and large-volume Germanium detectors from the European GAMMAPOOL network.
The NU-BALL collaboration includes researchers from 37 institutes and 16 countries – among them scientists from the University of Milan and INFN (GAMMA experiment from the Nuclear Physics Committee 3) – and is led by the Irène-Joliot-Curie Laboratory in Orsay.
To reveal the mechanism generating the spin of the fission fragments, the NU-BALL gamma-ray spectrometer was coupled to the LICORNE directional neutron source to measure, with high precision, gamma rays emitted by the fast-neutron-induced fission of samples of uranium 238U and thorium 232Th isotopes. The experimental campaign lasted 7 weeks in 2018.
The new comprehensive data show that there is no significant correlation between the spins in fragment partners, contrary to most theories hypothesizing that spin is generated before fission. This lead to conclude that the fragment spin emerges after the splitting, namely the spin mechanism has a post-scission nature. To explain these observations, it is proposed that collective motion of nucleons in the ruptured neck of the fissioning system generates two independent torques, analogous to the snapping of a stretched elastic band which results in a turning force.
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. Moreover, there are implications for practical applications such as the gamma-ray heating problem in nuclear reactors. Knowledge of the gamma rays emitted in nuclear fission (and parameters such as the number of gamma rays emitted) is also very relevant for calculating heating effects.
The University of Milan and INFN GAMMA experiment (of the Nuclear Physics Committee 3) have a long-standing collaboration with several nuclear physics groups at Orsay, and have contributed expertise on gamma-detectors as well as data analysis and nuclear structure interpretation.
Prof. Silvia Leoni, University of Milan and INFN
J.N. Wilson et al., ‘Angular momentum generation in nuclear fission’, Nature 590, 566 (2021) https://doi.org/10.1038/s41586-021-03304-w
Charles Q. Choi, ‘Mystery of Spinning Atomic Fragments Solved at Last’, Scientific American, February 24, 2021,