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'.

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The beta decay of 33Mg (N=21) presented in this Letter reveals intruder configurations in both the parent and the daughter nucleus. The lowest excited states in the N=20 daughter nucleus, 33Al, are found to have nearly 2p-2h intruder configuration, thus extending the "island of inversion" beyond Mg. The allowed direct beta-decay branch to the 5/2{+} ground state of the daughter nucleus 33Al implies positive parity for the ground state of the parent 33Mg, contrary to an earlier suggestion of negative parity from a g-factor measurement. An admixture of 1p-1h and 3p-3h configurations is proposed for the ground state of 33Mg to explain all of the experimental observables.

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The higher-spin structure of 38 Cl (N = 21) was investigated following the 26 Mg( 14 C, pn) reaction at 30 and 37 MeV beam energies. The outgoing protons were detected in an E − ∆E Si telescope placed at 0 • close to the target with a Ta beam stopper between the target and telescope. Multiple γ rays were detected in time coincidence with the protons using an enhanced version of the FSU γ detection array. The level scheme was extended up to 8420 keV with a likely spin of 10 . A new multishell interaction was developed guided by the experimental information. This FSU interaction was built by fitting to the energies of 270 experimental states from 13 C to 51 Ti. Calculations using the FSU interaction reproduce observed properties of 38 Cl rather well, including the spectroscopic factors. The interaction has been successfully used to interpret the 1p1h and 2p2h configurations in some nearby nuclei as well. I. INTRODUCTIONThe evolution of shell structures, specially with increasing proton-neutron imbalance, can provide valuable insights into the finite many-body problem. The exploration of an exotic region in the chart of nuclides with extreme N/Z ratios, the so called "island of inversion" region, has cast doubt on the persistence of the classical magic numbers and revealed the fragility of the shell gaps that lead to the magic numbers. The nuclei with Z = 10 ∼ 12 and N ≈ 20 have been found to have ground states dominated by the intruder configurations from the upper f p shell orbitals [1-6] and the anomalous property was interpreted as the reduction of the N = 20 shell gap. The immediate question emerged after the revelation of the shell gap reduction was how does this change happen along an isotopic or isotonic chain.Explaining this trend of shell evolution or structural evolution has been a great challenge to the nuclear structure models. The monopole parts of the shell model Hamiltonian have long been recognized to play the major role in the evolution of shell structure. While some models [7,8] are very successful in explaining the very neutron rich sd shell isotopes, they were unable to explain the intruder states of some nuclei within the same isotopic chain [9], or simply some other sd nuclei which are not even very neutron rich [10]; meaning that their monopoles are not well determined to explain the shell gap evolution. This demonstrates the need of a more comprehensive shell model treatment for the intruder states of the sd-shell nuclei which are sensitive to the shell gaps and, hence, very informative to describe the shell gap evolution.The current experimental investigation focuses on the structure of moderately neutron rich 38 Cl with Z = 17 and N = 21 having the valence protons in the sd shell and one valence neutron within the f p shell. Both normal and intruder states of 38 Cl are valuable to understand the N = 20 shell gap evolution. This nucleus has long been recognized as providing a window into the interactions between π0d 3/2 and νf 7/2 nucleons, since the first 4 states (2 − , 5 − , 3 − , and 4 ...