• An asymmetric fission island was discovered in neutron-deficient sub-lead nuclei, bridging a gap to the well-understood actinide region.
• The study identifies a deformed Z = 36 proton shell in the light fragment as a key factor driving asymmetric fission in this region.
• The extensive dataset of 100 exotic fissioning systems, 75 newly measured, will constrain fission models used for nuclei with extreme neutron-to-proton ratios.

A comprehensive study has mapped an asymmetric fission island by measuring the charge distributions of fission fragments for 100 exotic fissioning systems, 75 of which had never been measured before. The research establishes a connection between the neutron-deficient sub-lead region and the actinide region, revealing that a deformed Z = 36 proton shell of the light fragment significantly influences the fission process in sub-lead nuclei. This new dataset will improve the accuracy of fission models used for estimating fission properties of nuclei with extreme neutron-to-proton ratios.

• What: The study presents measurements of charge distributions of fission fragments for 100 exotic fissioning systems, revealing an asymmetric fission island in the neutron-deficient sub-lead region. This contradicts the typical symmetric fission observed in lighter elements.
• Who: Researchers from CEA, DAM, DIF, Université Paris-Saclay, CNRS, IJC Lab, IGFAE, University of Santiago de Compostela, GSI Helmholtzzentrum für Schwerionenforschung, Technische Universität Darmstadt, and other institutions.
• When: The article was published on April 30, 2025. The experiment was performed at GSI Helmholtzzentrum für Schwerionenforschung. Data was received on 04 October 2024 and accepted on 11 March 2025.
• Where: Experiment S455 was conducted at the beamline infrastructure Cave C at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany.

The discovery of an asymmetric fission island in the sub-lead region challenges existing models that predict symmetric fission for lighter elements. The identification of the deformed Z = 36 proton shell's role provides new insights into the nuclear fission process. This research has significant implications for refining theoretical models used in nuclear astrophysics, particularly in understanding r-process nucleosynthesis, and for energy applications that rely on accurate fission models.

This study provides comprehensive data mapping the asymmetric fission island and highlighting the role of the deformed Z = 36 proton shell in the fission of sub-lead nuclei. These findings contribute to refining fission models used to predict the properties of nuclei with extreme neutron-to-proton ratios, impacting both fundamental nuclear physics and applications in astrophysics and energy.