Saturation of low - energy antiproton annihilation on nuclei

Recent measurements of very low-energy (p L < 100 MeV/c) ¯ p annihilation on light nuclei reveal apparent suppression of annihilation upon increasing the atomic charge Z and mass number A. Using ¯ p-nucleus optical potentials V opt , fitted to ¯ p-atom energy-shifts and-widths, we resolve this suppression as due to the strong effective repulsion produced by the very absorptive V opt. The low-energy ¯ p-nucleus wavefunction is kept substantially outside the nuclear surface and the resulting reaction cross section saturates as function of the strength of Im V opt. This feature, for E > 0, parallels the recent prediction , for E < 0, that the level widths of ¯ p atoms saturate and, hence, that ¯ p deeply bound atomic states are relatively narrow. Antiproton annihilation cross sections are calculated at p L = 57 MeV/c across the periodic table, and their dependence on Z and A is classified and discussed with respect to the Coulomb focussing effect at very low energies.