BaBar seen in a new light

22 March 2000

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Clever lighting makes a plain but vital section of a particle detector look more like something from Star Trek. The image shows a support tube made at the Lawrence Berkeley National Laboratory for an experiment called BaBar, which was installed a year ago at the Stanford Linear Accelerator Center in California. The tube now supports the part of BaBar known as the DIRC (for Detection of Internally Reflected Cherenkov light). This is basically a set of very precisely ground quartz bars that are housed inside of rectangular "bar boxes". There are 12 bars per box, and 12 boxes altogether. The 12 boxes fit in the support tube, which slides over a 1.6-m diameter detector that reveals the tracks of charged particles produced in collisions at the heart of the experiment.

Charged particles passing through the quartz bars radiate Cherenkov light, which travels by internal reflection to one end of the bar (the other end is mirrored). The light then enters a large volume and finally strikes an array photomultiplier tubes, which record an arc of light. The curvature of this arc yields information about the velocity of the original charged particle, and so provides a vital clue in identifying the type of particle.

Credit: Ernest Orlando Lawrence Berkeley National Laboratory