This applet displays data recorded at CERN to study the performance of Cherenkov Ring Imaging prototypes for the LHCb experiment. The display area represents a 24-by-24 array of photon-sensitive pixels. This behaves rather like a CCD used in a digital camera but is much more senstive to light, being capable of detecting single photons, and can be read out much faster. The pixels are shown at approximately their real size (2mm-by-2mm). LHCb will need about 500 times this number of pixels and the signals from each pixel will be captured 40000000 times per second.
When a charged particle passes through a medium with velocity greater than the local speed of light in the medium, Cherenkov radiation is produced. This radiation is in the form of photons which are produced in a cone about the direction of travel of the charged particle. A spherical mirror is used to form a ring image of these photons on a light sensitive detector array.
What is displayed here is a series of ring images formed by illuminating each pixel that has been hit by a photon for each charged particle. A new ring is added about once per second in this demonstration.
The number of photons produced by each charged particle is rather small so this applet allows the "persistence" of the image to be adjusted. Instead of replacing each ring by a new one, the old ring image gradually fades at a rate determined by the persistence setting. By setting the persistence close to (but less than) 100% a number of ring images effectively pile up on top of each other. When set to 100% the images never fade.
The applet also allows the "Saturation count" to be adjusted. This value sets the number of hits on a pixel for which the pixel appears brightest. Pixels having less hits are drawn proportionately dimmer. Pixels having more hits are drawn at maximum brightness regardless of the actual number of hits.
The overall brightness of the display may be adjusted with the brightness control. A setting of 100% usually gives good results.
The display can be temporarily stopped or restarted using the "Stop" and "Go" buttons.