Borromean structures in medium-heavy nuclei
Borromean nuclear cluster structures are expected at the corresponding drip lines. We locate the regions in the nuclear chart with the most promising constituents, it being protons and α particles and investigate in details the properties of the possible Borromean two-α systems in medium-heavy nuclei. We find in all cases that the α particles are located at the surface of the core nucleus as dictated by Coulomb and centrifugal barriers. The two lowest three-body bound states resemble a slightly contracted 8 Be nucleus outside the core. The next two excited states have more complex structures but with strong components of linear configurations with the core in the middle. α-removal cross sections would be enhanced with specific signatures for these two different types of structures. The even-even Borromean two-α nucleus, 142 Ba, is specifically investigated and predicted to have 134 Te-α-α structure in its ground state and low-lying spectrum.