  Absorptive corrections, which are known to suppress proton-neutron
transitions with a large fractional momentum z -> 1 in pp collisions, become
dramatically strong on a nuclear target, and they push the partial cross
sections of leading neutron production to the very periphery of the nucleus.
The mechanism of the pion and axial vector a1-meson interference, which
successfully explains the observed single-spin asymmetry in a polarized pp ->
nX, is extended to the collisions of polarized protons with nuclei. When
corrected for nuclear effects, it explains the observed single-spin azimuthal
asymmetry of neutrons that is produced in inelastic events, which is where the
nucleus violently breaks up. This single-spin asymmetry is found to be negative
and nearly A-independent.
