1. Large Angle Veto (LAV)

1.1 The need for a photon veto system

Photon vetoes are required to suppress the dominant background originating from the decay K+--->p+p0 to the specified level. The average inefficiency for the rejection of the p0 should be smaller than 10-8. The photon vetoes need to have hermetic geometrical coverage up to 50 mr for the photons originating from the kaon decays occurring in the decay region (from 5 to 65m after the final collimator). With such a configuration, only about 0.2 % of the events have one photon from p0 left undetected.
The geometry of the experiment suggests partitioning of the detector into three different angular regions, each instrumented by three different detector technologies:

� Large Angle Vetoes (LAV), covering the angular region between 8.5 mr and 50 mr, distributed along the decay volume and spaced by 6m in the upstream region and by 12m downstream.

� The NA48 Liquid krypton calorimeter (LKR), covering angles between 1 and 8.5 mr

� Small angle vetoes covering the region down to zero degrees (SAC) and the zone around the inner radius of the LKR (IRC) calorimeter. These will have suitable overlap in the angular acceptance to cover the beam pipe and an inner radius smaller than that of the beam pipe.