  The PHENIX MPC-EX detector is a Si-W preshower extension to the existing Muon
Piston Calorimeters (MPC). Covering the pseudorapidity range 3.1 <eta < 3.8,
the MPC-EX and MPC access high-x partons in the projectile nucleon The combined
detectors will make possible a measurement of the prompt photon single spin
asymmetry A_N in 200GeV transversely polarized p+p collisions, which will help
elucidate the correlation of the transverse motion of valence partons in the
proton with the proton spin.
