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Lightweight Mirror Accelerates Physics Research

Photonics Spectra
Jan 2000
Daniel C. McCarthy, Senior News Editor

Quarks traveling in large dissociated packets have been scarce since the big bang. Quantum physicists wishing to isolate and study quarks must separate larger particles by colliding them in giant accelerator facilities like the Phenix project at Brookhaven National Laboratory in Upton, N.Y.

The collider smashes very heavy beams of gold nuclei together to generate extremely hot dense plasma. This excites a phase transition among the protons and neutrons that produces a sort of soup full of quarks and, theoretically, a sketch of what the universe would have looked like moments after the big bang. Phenix's detector configuration at this stage requires the use of extremely lightweight mirrors, which Brookhaven developed with the help of Opticon Corp.

Lacking a method to detect quarks themselves, physicists instead detect electrons produced by the decay of quark pairs, signified by the radiation these electrons emit when they pass through a cloud of ethane gas. The mirrors focus this fluorescence, called Cerenkov radiation, into the detector. "The Cerenkov detector sees only really fast particles," said Tony Frawley, a Phenix collaborator and builder of the mirror system. "The problem is that all the particles that go through our detector also go through other detectors, so we want to keep to a minimum the number of parts we have."

The Phenix project's 96-mirror array focuses the fluorescent signature of quarks into a detector, a task that requires the entire array to weigh about 150 lb, the same as the gas within the detector chamber. Courtesy of Tony Frawley.

Because the quarks actually pass through the mirrors, the optical materials must have extremely low mass. At the center of each spherical mirror is a substrate made of Rohacell foam over which Opticon used a thin-film epoxy replication process to apply the mirror surface and reflective coating. The finished mirrors are about 0.5 in. thick with the reflective aluminum coating on one side. The detector incorporates more than 100 of these optics, each measuring 90 × 50 cm.

The mirrors' optical quality, only 1.5-mrad reflection accuracy, was not as great a priority as the density of the material, said Frawley, who quipped that their specifications barely make the mirrors fit to shave in. Despite this, the 96-mirror array weighs only about 150 lb, roughly the same amount of mass as the gas in the chamber.

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