  Systematic alpha-nucleus folding potentials are used to analyze alpha-decay
half-lives of superheavy nuclei. Preformation factors of about several per cent
are found for all nuclei under study. The systematic behavior of the
preformation factors and the volume integrals of the potentials allows to
predict alpha-decay energies and half-lives for unknown nuclei. Shell closures
can be determined from measured alpha-decay energies using the discontinuity of
the volume integral at shell closures. For the first time a double shell
closure is predicted for Z_magic = 132, N_magic = 194, and A_magic = 326 from
the systematics of folding potentials. The calculated alpha-decay half-lives
remain far below one nanosecond for superheavy nuclei with double shell closure
and masses above A > 300 independent of the precise knowledge of the magic
proton and neutron numbers.
