Three-body correlations for the ground-state decay of the lightest two-proton emitter 6 Be are studied both theoretically and experimentally. Theoretical studies are performed in a three-body hyperspherical-harmonics cluster model. In the experimental studies, the ground state of 6 Be was formed following the α decay of a 10 C beam inelastically excited through interactions with Be and C targets. Excellent agreement between theory and experiment is obtained demonstrating the existence of complicated correlation patterns which can elucidate the structure of 6 Be and, possibly, of the A=6 isobar.

Neutron elastic-scattering angular distributions were measured at beam energies of 11.9 and 16.9 MeV on 40,48 Ca targets. These data plus other elastic-scattering measurements, total and reaction cross sections measurements, (e, e ′ p) data, and single-particle energies for magic and doubly magic nuclei have been analyzed in the dispersive optical model (DOM) generating nucleon self-energies (optical-model potentials) which can be related, via the many-body Dyson equation, to spectroscopioc factors and occupation probabilities. It is found that for stable nuclei with N ≥ Z, the imaginary surface potential for protons exhibits a strong dependence on the neutron-proton asymmetry. This result leads to a more modest dependence of the spectroscopic factors on asymmetry. The measured data and the DOM analysis of all considered nuclei clearly demonstrates that the neutron imaginary surface potential displays very little dependence on the neutron-proton asymmetry for nuclei near stability (N ≥ Z).

The interaction of an E/A=70-MeV (7)Be beam with a Be target was used to populate levels in (6)Be following neutron knockout reactions. The three-body decay of the ground and first excited states into the α+p+p exit channel were detected in the High Resolution Array. Precise three-body correlations extracted from the experimental data allowed us to obtain insight into the mechanism of the three-body democratic decay. The correlation data are in good agreement with a three-cluster-model calculation and thus validate this theoretical approach over a broad energy range.

The interaction of an E/A=57.6-MeV 17 Ne beam with a Be target was used to populate levels in 16 Ne following neutron knockout reactions. The decay of 16 Ne states into the three-body 14 O+p+p continuum was observed in the High Resolution Array (HiRA). For the first time for a 2p emitter, correlations between the momenta of the three decay products were measured with sufficient resolution and statistics to allow for an unambiguous demonstration of their dependence on the long-range nature of the Coulomb interaction. Contrary to previous measurements, our measured limit Γ < 80 keV for the intrinsic decay width of the ground state is not in contradiction with the small values (of the order of keV) predicted theoretically.PACS numbers: 25.10.+s, 23.50.+z, 21.60.Gx, 27.20.+n Introduction -Two-proton (2p) radioactivity [1] is the most recently discovered type of radioactive decay. It is a facet of a broader three-body decay phenomenon actively investigated within the last decade [2]. In binary decay, the correlations between the momenta of the two decay products are entirely constrained by energy and momentum conservation. In contrast for three-body decay, the corresponding correlations are also sensitive to the internal nuclear structure of the decaying system and the decay dynamics providing, in principle, another way to constrain this information from experiment. In 2p decay, as the separation between the decay products becomes greater than the range of the nuclear interaction, the subsequent modification of the initial correlations is determined solely by the Coulomb interaction between the decay products. As the range of the Coulomb force is infinite, its long-range contribution to the correlations can be substantial, especially, in heavy 2p emitters.