The multiwire proportional chamber is of the multistep type and has 100 mm long stainless-steel anode wires with a pitch of 2 mm. The detector has a CaF 2 window and was filled with an argon-triethylamine gas mixture. A specific detector response of N 0=37.9 cm −1 was obtained using 1 m of argon gas as radiator. The resolution of the Cherenkov ring radius was measured to be α R −=2.3 mm, which implies a Cherenkov angular resolution of α θ c / θ c=10.3%. The photon position resolution along the signal wire was found to be α y ⩽2.3%.