  The P326 experiment at the CERN SPS has been proposed with the purpose of
measuring the branching ratio for the decay K^+ \to \pi^+ \nu \bar{\nu} to
within 10%. The photon veto system must provide a rejection factor of 10^8 for
\pi^0 decays. We have explored two designs for the large-angle veto detectors,
one based on scintillating tiles and the other using scintillating fibers. We
have constructed a prototype module based on the fiber solution and evaluated
its performance using low-energy electron beams from the Frascati Beam-Test
Facility. For comparison, we have also tested a tile prototype constructed for
the CKM experiment, as well as lead-glass modules from the OPAL electromagnetic
barrel calorimeter. We present results on the linearity, energy resolution, and
time resolution obtained with the fiber prototype, and compare the detection
efficiency for electrons obtained with all three instruments.

Instrumentation and Detectors