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 to decay through one- and two-proton radioactivity, respectively. The experimental observation of one-proton radioactivity was first reported 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 silver, 94Ag, which is known to undergo one-proton decay. 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'.

The nuclear charge radius of 11Li has been determined for the first time by high-precision laser spectroscopy. On-line measurements at TRIUMF-ISAC yielded a 7Li-11Li isotope shift (IS) of 25 101.23(13) MHz for the Doppler-free [FORMULA: SEE TEXT]transition. IS accuracy for all other bound Li isotopes was also improved. Differences from calculated mass-based IS yield values for change in charge radius along the isotope chain. The charge radius decreases monotonically from 6Li to 9Li, and then increases from 2.217(35) to 2.467(37) fm for 11Li. This is compared to various models, and it is found that a combination of halo neutron correlation and intrinsic core excitation best reproduces the experimental results.

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Fine structure in the o. decay of mass-separated Pb and Hg has been studied at the GSI on-line mass separator. Alpha singles spectra as well as a-x-t and a-e-t coincidence events were collected. The o. decay of Pb revealed feeding to a low-lying 0+ state at 328(12) keV inHg.This state can be interpreted as being the bandhead of the deformed rotational band observed previously in in-beam studies. In the a decay of Hg, feeding towards the Srst excited 2+ state at 153 keV and the 0~+ state at 478 keV in Pt was observed. The hindrance factor of the a decay towards the excited 0+ state gives information about the particle-hole character of the states connected in the a decay. A two-level mixing calculation is introduced. From the fixing in Pt and the a-decay hindrance factors, small mixing is deduced for ground states of neutron-de6cient Hg and Pb nuclei.

Very neutron-deficient isotopes of rhodium, palladium, silver, cadmium and indium in the N= 50 region were studied at the GSI on-line mass separator following bombardment of 58, 60, 62Ni and 63, 65Cu targets with 4.0 MeV/u ions of 4~The known 'properties of the t-delayed proton precursor 95mpd were confirmed, but a search for direct proton decay of 95mAg was not successful. Beta-delayed proton activities observed at mass-numbers 94, 96, 97 and 100 were assigned to the precursors 94Rh(66_+14 s), 96Ag(5.1 + 0.4 s), 97 +4 lOOin" . _ Cd(3 z s) and In addition. 94pd(9.0+0.5 s) was identified from fl-y-X studies. Individual decay schemes of 95~, 96pd and 96, 9SAg are reported.