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Angular momentum generation in nuclear fission

Wilson, J. N.; Thisse, D.; Lebois, M.; Jovančević, N.; Gjestvang, D.; Canavan, R.; Rudigier, M.; Étasse, D.; Gerst, R-B.; Gaudefroy, L.; Adamska, E.; Adsley, P.; Algora, A.; Babo, M.; Belvedere, K.; Benito, J.; Benzoni, G.; Blazhev, A.; Boso, A.; Bottoni, S.; Bunce, M.; Chakma, R.; Cieplicka-Oryńczak, N.; Courtin, S.; Cortés, M. L.; Davies, P.; Delafosse, C.; Fallot, M.; Fornal, B.; Fraile, L.; Gottardo, A.; Guadilla, V.; Häfner, G.; Hauschild, K.; Heine, M.; Henrich, C.; Homm, I.; Ibrahim, F.; Iskra, Ł. W.; Ivanov, P.; Jazrawi, S.; Korgul, A.; Koseoglou, P.; Kröll, T.; Kurtukian-Nieto, T.; Le Meur, L.; Leoni, S.; Ljungvall, J.; Lopez-Martens, A.; Lozeva, R.; Matea, I.; Miernik, K.; Nemer, J.; Oberstedt, S.; Paulsen, W.; Piersa, M.; Popovitch, Y.; Porzio, C.; Qi, L.; Ralet, D.; Regan, P. H.; Rezynkina, K.; Sánchez-Tembleque, V.; Siem, S.; Schmitt, C.; Söderström, P.-A.; Sürder, C.; Tocabens, G.; Vedia, V.; Verney, D.; Warr, N.; Wasilewska, B.; Wiederhold, J.; Yavahchova, M.; Zeiser, F.; Ziliani, S. (2021) Angular momentum generation in nuclear fission. Nature, 590 (7847). pp. 566-570. ISSN 0028-0836

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When a heavy atomic nucleus fissions, the resulting fragments are observed to emerge spinning this phenomenon has been an outstanding mystery in nuclear physics for over 40 years . The internal generation of around 6-7 units of angular momentum in each fragment is particularly puzzling for systems which start with zero, or almost zero, spin. There are currently no experimental observations which enable decisive discrimination between the many competing theories for the angular momentum generation mechanism. Nevertheless, the present consensus is that excitation of collective vibrational modes generate the intrinsic spin before the nucleus splits (pre-scission). Here we show that there is no significant correlation between the spins in fragment partners, which leads us to conclude that angular momentum in fission is actually generated after the nucleus splits (post-scission). We present comprehensive data showing that average spin is strongly mass dependent, varying in saw-tooth distributions. We observe no significant dependence of fragment spin on the mass or charge of the partner nucleus, confirming the uncorrelated, post-scission nature of the spin mechanism. To explain these observations, we propose that collective motion of nucleons in the ruptured neck of the fissioning system generates two independent torques, analogous to the snapping of an elastic band. A parametrisation based on occupation of angular momentum states according to statistical theory well-describes the full range of experimental data. This new information on the role of spin in nuclear fission is not only important for the fundamental understanding and theoretical description of fission, but also has consequences for the γ-ray heating problem in nuclear reactors, for the study of the structure of neutron-rich isotopes, and for the synthesis and stability of super-heavy elements .

Item Type: Article
Keywords: Nuclear Fission; gamma-ray detection; nu-ball spectrometer; HpGe detectors; Nuclear fission; radioactivity
Subjects: Ionising Radiation > Radioactivity
Divisions: Medical Physics
Identification number/DOI: 10.1038/s41586-021-03304-w
Last Modified: 12 Jul 2021 14:58
URI: http://eprintspublications.npl.co.uk/id/eprint/9056

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