A 400 GeV shower counter

Specifications and test data are given for a lead-scintillator electromagnetic shower counter used to detect beam photons at Fermi National Accelerator Laboratory. The response is independent of beam position to +- 1.4% rms over a 25 cmx25 cm area. Shower penetration losses reach 4.1% at 250 GeV but are easily calculable. The resolution is limited almost entirely by fluctuations in the number of shower tracks sampled: The full-width-at-half-maximum is approximately 43% Esup(1/2), where E is the incident energy in GeV. (Auth.)