  Absorptive corrections, known to suppress proton-neutron transitions with
large fractional momentum $z\to1$ in pp collisions, become dramatically strong
on a nuclear target, and push the partial cross sections of leading neutron
production to the very periphery of the nucleus. The mechanism of $\pi$-$a_1$
interference, which successfully explains the observed single-spin asymmetry in
polarized $pp\to nX$, is extended to collisions of polarized protons with
nuclei. Corrected for nuclear effects, it explains the observed single-spin
azimuthal asymmetry of neutrons, produced in inelastic events, where the
nucleus violently breaks up. The single-spin asymmetry is found to be negative
and nearly $A$-independent.
