Proton-proton correlations observed in two-proton radioactivity of 94Ag

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 predicted(1) to decay through one- and two-proton radioactivity, respectively. The experimental observation of one- proton radioactivity was first reported(2) in 1982, and two-proton radioactivity has now also been detected by experimentally studying the decay properties of Fe-45 (refs 3, 4) and Zn-54 (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 silver(6), Ag-94, which is known to undergo one- proton decay(7). We infer from these correlations that the long-lived state must also decay through simultaneous two-proton emission, making Ag-94 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'.