Highlights of the Zeno Results from the USMP-2 Mission

I. INTRODUCTIONIn a pure fluid near its liquid-vapor critical point, the otherwise small, statistical fluc-tuations in its density become as large as the wavelength of light when the system is stillonly 10 mK from the critical temperature, To. These large fluctuations scatter light verystrongly, and the previously clear fluid turns milky white, a phenomenon known as criticalopalesence.Indeed, many different systems have critical points, and large fluctuations of some ther-modynamic parameter are a universal feature. Because critical fluctuations become macro-scopic and involve enormous numbers of molecules, many features of critical-point behaviorare controlled by the statistical behavior of the fluctuations, so that many types of systemsexhibit the same behavior near the critical point.Light-scattering from critical fluctuations in a fluid is a simple and accurate techniqueto measure the decay rates of the fluctuations (the inverse of their lifetimes). However,near the critical point, the fluid becomes highly compressible, so the weight of the fluidalone causes severe density gradients in the sample, and distorts these measurements in aterrestrial laboratory.The goal of the Zeno experiment was to build a precision, light-scattering spectrometerto measure the fundamental fluctuation decay rates in a sample of xenon. The spectrome-ter would operate in the low-gravity environment of the Space Shuttle, thus removing thegravity-induced distortion of the measurements. The instrument performed beautifully onits first mission and we were able to make our projected measurements with 1% precisionto within 100 #K of the critical temperature (16.7 °C).A schematic of the light-scattering spectrometer is shown in Fig. 1. Light from a low-power, helium-neon laser was directed into the sample along one of two beam paths deter-mined by the beamsplitter (B 1): the path was chosen by shutters (S 1 and S 2). Lightscattered by the sample (located in the cylindrical thermostat at the center of the figure) wasthen collected by the apertured photomultiplier tubes (PMT 1 and PMT 2), the primarysignal that was processed to obtain the decay-rate information. In addition, photodiodes(PD 1 and PD 2) placed behind partially-reflecting mirrors (M 2 and M 3) measured theintensity of the light entering and exiting the sample. These two signals were then ratioedelectronically to give us information about the turbidity of the fluid, which was an importantmeasurement and our primary method of locating the critical temperature of the sample.