Internal pair creation induced by nuclear coulomb excitation in heavy-ion collisions

Internal electron-positron pair creation caused by nuclear Coulomb excitation in heavy-ion collisions is compared with the spontaneous and induced positron production in overcritical quasimolecules with Z₁ + Z₂$?170. Using the rotation-vibration nuclear model in the calculation of the quadrupole Coulomb excitation and the conversion coefficient for electron-positron pair formation, the total cross section for the system ²³⁸₉₂U-²³⁸₉₂U was found to be ${\sigma }_C^{e^{+},e^{?}}=0.125mb$ at E_kin^c.m. = 797 MeV, which is four times smaller than the cross section ${\sigma }_{ind}^{e^{+},e^{?}}$ for the corresponding vacuum decay process if a K-shell vacancy production of L₀ = 10^? is assumed. Evaluation of the ratio $R=d{\sigma }_{ind}^{e^{+},e^{?}}({?}_{c.m.})/d{\sigma }_C^{e^{+},e^{?}}({?}_{c.m.})$ between the differential cross sections with respect to the ion angle ?_c.m leads to R = 60 at ?_c.m. = 160°. In contrast to the induced positron spectrum, the decay of excited nuclear levels in U is not followed by positrons with kinetic energy above 0.9 MeV. Therefore a unique determination of the decay of the vacuum in overcritical fields is experimentally possible provided that L₀ is not drastically reduced. In addition, similar calculations have been performed for pair creation resulting from nuclear Coulomb excitation for the systems U-Cf, Th-Th, Pb-Pb, Ni-Ni, Xe-Ba, Xe-Nd and Xe-Ce. The occupation of vacant bound states of superheavy electronic molecules by conversion electrons of γ-rays from nuclear transitions with E_γ < 1 MeV is also discussed.