Zeno: Critical Fluid Light Scattering in Microgravity

The Zeno instrument was designed and developed for flight on the U.S. Space Shuttle to measure the density fluctuation decay times of a liquid-vapor critical fluid using photon correlation spectroscopy. The instrument was flown and operated successfully on two flights: March 4, 1994 on STS-62 as part of the USMP-2 payload and Feb. 22, 1996 on STS-75 as part of the USMP-3 payload. This paper will give an overview of the experiment principles, discuss the major design challenges, and show some performance data. Of particular note was the temperature control of better than 3 microK (rms) for hours, which was essential for the effort to make measurements closer than 1 mK to Tc. Both flights taught us lessons about the difficulty of controlling the local density (at the laser focus) in microgravity. Correlograms were recorded with a custom interfaced ALV 5000 Correlator. In the second flight, corrclograms were recorded from 500 mK down to 2 mK at 24 temperatures, 383 correlograms in all. The fitted decay rates generally gave the desired 1% precision. The phase boundary was located with unprecedented precision of ± 20 µK. More details of the experiment Science Requirements, the personnel, apparatus, and results are displayed on the Zeno homepage at http://www.zeno.umd.edu.