The stability and spontaneous decay of naturally occurring atomic nuclei have been much studied ever since Becquerel discovered natural radioactivity in 1896. In 1960, proton-rich nuclei with an odd or an even atomic number Z were predicted1 to decay through one- and two-proton radioactivity, respectively. The experimental observation of one-proton radioactivity was first reported2 in 1982, and two-proton radioactivity has now also been detected by experimentally studying the decay properties of 45Fe (refs 3, 4) and 54Zn (ref. 5). Here we report proton–proton correlations observed during the radioactive decay of a spinning long-lived state of the lightest known isotope of silver6, 94Ag, which is known to undergo one-proton decay7. We infer from these correlations that the long-lived state must also decay through simultaneous two-proton emission, making 94Ag the first nucleus to exhibit one- as well as two-proton radioactivity. We attribute the two-proton emission behaviour and the unexpectedly large probability for this decay mechanism to a very large deformation of the parent nucleus into a prolate (cigar-like) shape, which facilitates emission of protons either from the same or from opposite ends of the ‘cigar’.
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Z.J. acknowledges support from a Polish grant. The support of the Université Catholique de Louvain, Belgium, is acknowledged by I.M. L.G. acknowledges support from the Russian RFBR and the Russian Ministry of Industry and Science. The support of the US National Science Foundation is acknowledged by S.L.T. Author Contributions R.K. was in charge of the development of the ion sources of the on-line mass separator and of the operation of this facility. I.M. suggested and built the charged-particle detector, applied the ‘tagging’ method in the data analysis and established the reported phenomenon, J.D. set up the γ-ray detectors and the related electronics and data acquisition. All authors except L.G., M.H. and P.V.D participated in the measurements. L.G. contributed the theoretical interpretation of the data. The manuscript was prepared by I.M. and E.R. with the active participation of all authors, especially J.D., H.G., M.H., S.L.T. and P.V.D.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Mukha, I., Roeckl, E., Batist, L. et al. Proton–proton correlations observed in two-proton radioactivity of 94Ag. Nature 439, 298–302 (2006). https://doi.org/10.1038/nature04453
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