Abstract
Nuclear fission leads to the splitting of a nucleus into two fragments1,2. Studying the distribution of the masses and charges of the fragments is essential for establishing the fission mechanisms and refining the theoretical models3,4. It has value for our understanding of r-process nucleosynthesis5,6, in which the fission of nuclei with extreme neutron-to-proton ratios is pivotal for determining astrophysical abundances and understanding the origin of the elements7 and for energy applications8,9. Although the asymmetric distribution of fragments is well understood for actinides (elements in the periodic table with atomic numbers from 89 to 103) based on shell effects10, symmetric fission governs the scission process for lighter elements. However, unexpected asymmetric splits have been observed in neutron-deficient exotic nuclei11, prompting extensive further investigations. Here we present measurements of the charge distributions of fission fragments for 100 exotic fissioning systems, 75 of which have never been measured, and establish a connection between the neutron-deficient sub-lead region and the well-understood actinide region. These new data comprehensively map the asymmetric fission island and provide clear evidence for the role played by the deformed Z = 36 proton shell of the light fragment in the fission of sub-lead nuclei. Our dataset will help constrain the fission models used to estimate the fission properties of nuclei with extreme neutron-to-proton ratios for which experimental data are unavailable.
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Code availability
The main analysis and results presented in this paper were performed using the nptool framework. The analysis codes used to generate these results are available in the s455 project of nptool at https://gitlab.in2p3.fr/np/nptool/-/tree/NPTool.2.dev/Projects/s455?ref_type=heads.
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Acknowledgements
The results presented here are based on experiment S455, which was performed at the beamline infrastructure Cave C at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany, in the context of FAIR Phase-0. L.M.F. acknowledge support from TU Darmstadt-GSI through a cooperation contract. We would like to thank the CEA, DAM and DIF for providing us with access to the supercalculator resources, which enabled us to carry out the microscopic calculations. The JSI researchers are supported by the Slovenian Research and Innovation Agency (Grant No. I0-E005). J.P. acknowledges support from the Institute for Basic Science (Grant No. IBS-R031-D1). This work has been partly supported by the Spanish Funding Agency for Research (AEI; Projects PGC2018-099746-B-C21, PGC2018-099746-B-C22, PID2021-125771NB-C21, PID2021-125771NB-C22 and PID2022-140162-I00). A.G.-G and G.G.-J. thank AEI for support (Predoctoral Grant Nos. PRE2018-085934 and PRE2019-087415). J.L.R.-S. is thankful to AEI for support from the Ramon y Cajal programme (Grant No. RYC2021-031989) and thankful for a Postdoctoral Fellowship (Grant No. ED481D-2021/018) from the Regional Government of Galicia. This work was supported by the Royal Society and the UK STFC (Grant Nos. ST/L005727/1, ST/P003885/1 and ST/V001035/1). This work was supported by the Portuguese FCT under EXPL/FIS-NUC/0364/2021. R.G. acknowledges support by the Excellence Cluster ORIGINS from the DFG (Excellence Strategy EXC-2094-390783311).
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Contributions
J.T. was the spokesperson for the experiment. The design and installation of the present experiment was performed by a large number of the R3B-SOFIA collaboration members led by J.T. and in particular P.M., A. Chatillon, L. Audouin, B. Laurent, A.O., F. Wamers, L. Atar, H.A.-P., F.A., P.A., G.A., T.A., J.B., K.B., L.B., T.B., C.C., E.C., J.C., A. Corsi, D.C.-G., A. Cvetinović, E. De F., T.D., M.F., L.M.F, D.G, G.G-J., I.G., E.I.G., R.G., B.G., K.G., A.G.-G., A.-L.H., M. Heil, T.H., M. Holl, A. Horvat, A.J., D.M., T.J., L.J., B. Jonson, B. Jurado, N.K.-N., A.K.-H., O.A.K., P.K., D. Körper, T.K., I.L., J.P., S. Paschalis, M.P., A.R., H.-B.R. J.-L.R., L.R., D. Rossi, H. Scheit, H. Simon, S.S.-D., A.S., Y.L.S., C. Sürder, O.T., I.T., B.V., F. Wienholtz, H.W., J.Z. and M.Z. The data acquisition system was led by A. Chatillon, H.T.J., B. Löher and H.T.T. The online analysis was led by A. Chatillon, P.M. and J.-L.R. J.T., A. Chatillon, P.M. G.B., L. Audouin, A. Heinz, K.B., I.G., L.J., D. Rossi, A.G.-G., G.G.-J., A.S., L. Atar, D.M., E.N., S.M.M., L.R., H.A.-P., V.P., D.G., A.O., M. Holl, Y.L.S., E. De F., N.S.M., S. Paschalis, M.P., R.T., M.T., S. Pirrone, P. Russotto, G.P., L.P., E.I.G., E.V.P., B.G., S.V. and J.V. checked the quality of the captured data. H.W., E.H., E.K., N.K., C.H., S. Pietri, C. Scheidenberger, I.M., F.A., R.K., J.Z., D. Kostyleva, P. Roy and Y.K.T. were in charge of the operation of the FRS. The main offline data analysis and simulation were undertaken by P.M. The microscopic theoretical calculations were led by R.N.B., N.D., N.P., D. Regnier and P.C. All authors read, commented on and approved the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Schematic view of the experimental setup and particle identification plots.
a, Schematic view of the FRagment Separator (FRS) together with the experimental apparatus at R3B (figure generated using Inkscape on Linux). b, Beam identification plot for FRS settings including 200At in the cocktail beam. The projections give the charge Z and mass-to-charge ratio A/Q resolution for the charge Z = 85.
Extended Data Fig. 3 Experimental measured charge distribution and associated yield.
a, Sum of both fragment charge Z∑ = Z1 + Z2 in blue when the fission is induced in the lead targets and in red when it is induced in the carbon target. b, Atomic charge distribution after fission of 190Pb induced in the lead cathodes (blue) and the carbon cathode (red) with the condition Z∑ = 82. c, Atomic charge distribution from the electromagnetic induced fission where the fragmentation/fission contribution has been subtracted with the condition Z∑ = 82. d, Final charge yield for the Coulomb-induced fission of 190Pb. The error bars are depicted.
Extended Data Fig. 4 Measured charge yields.
Experimental measured charge yields from iridium to thorium isotopes.
Extended Data Fig. 5 Calculated shell effects the the fragments.
80Kr shell effects at the Fermi level for neutrons (a) and protons (b). The white dots trace the light prefragment of 174Pt before scission. 94Mo shell effects at the Fermi level for neutrons (c) and protons (d). The white dots trace the heavy prefragment of 174Pt before scission. 82Kr shell effects at the Fermi level for neutrons (e) and protons (f). The white dots trace the light prefragment of 190Pb before scission. 108Pd shell effects at the Fermi level for neutrons (g) and protons (h). The white dots trace the heavy prefragment of 190Pb before scission.
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Morfouace, P., Taieb, J., Chatillon, A. et al. An asymmetric fission island driven by shell effects in light fragments. Nature 641, 339–344 (2025). https://doi.org/10.1038/s41586-025-08882-7
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DOI: https://doi.org/10.1038/s41586-025-08882-7
