Preshower response to high energy photons where clusters from two-gamma decays of neutral pions can merge in the FMS.

The fraction of merged clusters is dominated by contributions at larger rapidities (where the tower size is actually smaller).

An optical simulation is used to study the response in PS3 after the Pb converter. Single photon efficiencies are about 60% which is consistent with the thickness of the converter. The rejection of two-photon clusters in comparison with single-photon clusters is mainly limited by the small number of interactions in the converter (i.e. a fairly large relative statistical fluctuation).