  An internal polarized hydrogen storage cell gas target is proposed for the
AD--ring to determine for the first time the two total spin--dependent cross
sections $\sigma_1$ and $\sigma_2$ at antiproton beam energies in the range
from 50 to 200 MeV. The data will allow the definition of the optimum working
parameters of a dedicated Antiproton Polarizer Ring (APR), which has recently
been proposed by the PAX collaboration for the new Facility for Antiproton and
Ion Research (FAIR) at GSI in Darmstadt, Germany. The availability of an
intense beam of polarized antiprotons will provide access to a wealth of
single-- and double--spin observables, thereby opening a new window to QCD
transverse spin physics. The physics program proposed by the PAX collaboration
includes a first measurement of the transversity distribution of the valence
quarks in the proton, a test of the predicted opposite sign of the
Sivers--function, related to the quark distribution inside a transversely
polarized nucleon, in Drell--Yan (DY) as compared to semi--inclusive Deep
Inelastic Scattering, and a first measurement of the moduli and the relative
phase of the time--like electric and magnetic form factors $G_{E,M}$ of the
proton.
