  The Lancaster experiments with a cylindrical wire moving in superfluid 3He-B
are discussed, where the measured critical velocity of pair creation is much
below the Landau critical velocity. The phenomenon is shown to be analogous to
the instability of the electron-positron vacuum in an adiabatically alternating
strong electric potential of both signs, where the positive- and negative-root
levels cross and thus the instability treshold is twice less than in the
conventional case of a single static potential well.
