High Threshold Cherenkov Counter

The High Threshold Chekov Counter uses the Cherenkov effect to detect fast-moving particles. Charged particles moving faster than the local speed of light c/n with index of refraction n and c the speed of light in a vacuum, emit light in a cone with an opening angle determined by the particle's velocity and the index of refraction of the medium.

Layout
48 elllipsoidal mirror sections gather the Cherenkov light emitted by fast-moving charged particles. Each mirror section is designed with a curved surface obtained by revolving an ellipse about its major axis, with one focus at the point where collisions happen and the second away from the beamline and out of the path of scattered particles. Photomultiplier tubes (PMTs) are then placed at the location of the second focus of each ellipsoidal mirror section to detect the Cherenkov light. The mirror segments are designed so that they completely surround the beamline for particles scattered through angles from 5-35 degrees, with no gaps or shadowing between adjacent mirrors.

The HTCC must be located between the target and the forward drift chambers that measure the trajectories and momentum of charged particles. To avoid disrupting the particles passing through the HTCC before passing through the detectors, Because scattered particles pass through the HTCC before they pass through the tracking detectors, the HTCC's mirrors are made from extremely low-density materials--precisely machined substrates of 100% closed-cell foam with a density of 31 mg/cc. Both sides of the mirrors are laminated with acrylic to form a rigid substrate with a smooth optical-quality surface.

How it Works
Cherenkov photons that strike the PMT photocathode excite electrons via the photoelectric effect, which are then accelerated by an applied electric field through a series of 14 stages called "dynodes". The applied high voltage is divided approximately evenly between each of the dynodes. The charge of the signal is amplified at each stage by the liberation of additional electrons.

Links/Sources
CLAS12 High Threshold Chekov Counter: http://www.jlab.org/Hall-B/clas12-web/subsystems/htcc-intro.html