.* Differences in the acceptances of the shower detector, particularly close to the veto crystals (see Figure 4-3). This effect has three sources, the first comes from a different shower profile, since positrons start to shower earlier, the second is due to missing photons in the p 0 decay, since they are not emitted collinear and the third is caused by scattering of the p +->e+ n e positron within the target.
* Self-vetoing due to backsplashing shower particles. Here also a photon induced shower could generate a signal in the plastic veto hodoscope when charged particles from the shower are leaving the calorimeter through the front face. This would result in vetoing a valid event.
* Positron annihilation before entering the calorimeter can occur in the target, in the hodoscope and in air. Bremsstrahlung of the positron could lead to a misinterpretation of the event, as well as photon conversion in the target or in the hodoscope.
* Instrumental inefficiencies of the MWPCs and the hodoscope.
* Missing events can be caused due to photons traversing the calorimeter without undergoing an interaction. Positrons, as well as the photons, can leak through gaps between crystal modules. Thus, precise dimensions for the modules are crucial and only a thin crystal wrapping was considered. The achieved alignment precision during the final assembly was in the order of 0.2 mm.
The main contribution to the systematical error is the current accuracy of the p +->e+ n e BR measurement, which is of 0.3%. Together with the above mentioned uncertainties, the systematical error is estimated to be in the range of 0.5%. In order to keep the statistical error comparably low 1*107 s of beam time at a rate of 2*106 stopped pions will be necessary.