The possibility of observing the neutrinoless double ␤ decay and thus proving the Majorana nature of neutrinos as well as providing a sensitive measure of its mass is a major challenge of today's neutrino physics. As an attractive alternative, we propose to study the inverse process-the radiative neutrinoless doubleelectron capture ͑02EC͒. The associated monoenergetic photon provides a convenient experimental signature. Other advantages include the favorable ratio of the 02EC to the competing 22EC capture rates and, very importantly, the existence of a coincidence trigger to suppress the random background. These advantages partly offset the expected longer lifetimes. Rates for the 0␥2EC process are calculated. High Z atoms are strongly favored. A resonance enhancement of the capture rates is expected to occur at an energy release comparable to the 2P-1S atomic level difference. The resonance conditions are likely to be met for decays to excited states in final nuclei. Candidates for such studies are considered. The experimental feasibility is estimated and found encouraging.

No abstract

The x-ray cascade from antiprotonic atoms was studied for 208 Pb and 209 Bi. Widths and shifts of the levels due to the strong interaction were determined. Using modern antiproton-nucleus optical potentials, the neutron densities in the nuclear periphery were deduced. Assuming two-parameter Fermi distributions (2pF) describing the proton and neutron densities, the neutron rms radii were deduced for both nuclei. The difference of neutron and proton rms radii r np equal to 0.16 ± (0.02) stat ± (0.04) syst fm for 208 Pb and 0.14 ± (0.04) stat ± (0.04) syst fm for 209 Bi were determined, and the assigned systematic errors are discussed. The r np values and the deduced shapes of the neutron distributions are compared with mean field model calculations.

No abstract

Deeply bound KNN, KNNN and KNNNN states are discussed. The effective force exerted by the K meson on the nucleons is calculated with static nucleons. Next the binding energies are obtained by solving the Schrödinger equation or by variational calculations.The dominant attraction comes from the S-wave Λ(1405) and an additional contribution is due to Σ(1385). The latter state is formed at the nuclear peripheries and absorbs a sizable piece of the binding energy. It also generates new branches of quasi-bound states. The lowest binding energies based on a phenomenological KN input fall into the 40-80 MeV range for KNN, 90-150 MeV for KNNN and 120-220 MeV for Kα systems. The uncertainties are due to unknown KN interactions in the distant subthreshold energy region.