The ALICE HMPID is based on proximity focusing Ring Imaging Cherenkov (RICH) counters
and consists of seven modules mounted in an independent support cradle, which
will be fixed to the space frame, at the two o’clock position.
photons, emitted when a fast charged particle traverses the 15
mm thick layer of liquid C6F14 (perfluorohexane),
are detected by a photon counter, which exploits the novel technology of a thin
layer of CsI deposited onto the pad cathode of a multi-wire proportional chamber
(MWPC). The HMPID detector, with its surface of about 12 m2,
represents the largest scale application of this technique.
Cherenkov photons refract
out of the liquid radiator and
the CsI-coated pad cathode, located at a suitable distance (the `proximity gap`)
that allows the contribution of the geometrical aberration to the Cherenkov
angle resolution to be reduced. The electrons
released by ionizing particles in the proximity gap, filled with CH4,
prevented from entering the MWPC sensitive volume by a positive polarization of
the `collection` electrode close to the radiator.
Cherenkov rings in the
HMPID proto-3 (64x40 cm2, 1/6
of an HMPID module). The colour coding represents the
signal induced on the pads.