  The Anti Coincidence Shield (ACS) of the INTEGRAL SPI instrument provides an
excellent sensitivity for the detection of Gamma Ray Bursts (GRBs) above ~
75keV, but no directional and energy information is available. We studied the
ACS response by using GRBs with known localizations and good spectral
information derived by other satellites. We derived a count rate to flux
conversion factor for different energy ranges and studied its dependence on the
GRB direction and spectral hardness. For a typical GRB spectrum, we found that
1 ACS count corresponds on average to ~ 1E-10 erg/cm^2 in the 75keV-1MeV range,
for directions orthogonal to the satellite pointing axis. This is broadly
consistent with the ACS effective area derived from the Monte Carlo
simulations, but there is some indication that the latter slightly
overestimates the ACS sensitivity, especially for directions close to the
instrument axis.
