Proton energy, passing the whole potential difference in the gap, shown as a colour map and its levels in the (L,B) parameter space on logarithmic scale for the black hole mass M = 3 × 10⁹M_⊙. The dark diagonal valley corresponds to the minimal energy ~10¹³eV of the protons able to pass the gap’s full potential. This minimal energy does not depend on the black hole mass. The energies of about ≥ 10¹⁹eV are only reachable for the “big” opened gaps with a height of about H> 0.1R_H.