Fitch Cherenkov counter
In order to trigger only on kaons in the beam, we have prepared a Fitch-type differential Cherenkov counter, whose structure is as shown in the followings. Kaons and pions injected into a 40 mm-thick acrylic radiator emit Cherenkov light with characteristic polar angle according to each velocity. The refraction index of the radiator is optimized so that Cherenkov light from pions is totally reflected at the rear surface while that from kaons in the chosen momentum range is emitted from the rear surface. The Cherenkov light from pions is reflected by a mirror around the radiator and detected by an inner photomultiplier (PMT) ring (π-ring). The light from kaons is reflected by a backward parabolic mirror and detected by an outer PMT ring (K-ring). Each ring is composed of 14 PMTs with a Winston cone at the entrance.
This counter is very similar to the counter developed for the KEK E246 experiment, but the momentum range is different. The designed momentum range is 740−800 MeV/c, whereas the original momentum range was 620−700 MeV/c. We modified the focal length of the K-ring parabolic mirror to compensate for the Cherenkov emission angle shift due to this momentum difference. The PMT hit multiplicity in each ring is used for an online trigger. Using the threshold level of a multiplicity N > 7, it is expected that we can achieve more than 99% trigger efficiency for K+s, while the π+ misidentification as K+ is negligible.