Instrumentation

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Characterization of Position-Sensitive Microchannel Plate Detectors Microchannel plate (MCP) position-sensitive detectors (PSDs) are now commonly used in a wide range of areas including, imaging spectroscopy, mass spectrometry, electron spectroscopy, astronomy, and atomic collision studies. The particular style of detector we use is composed of a pair of microchannel plates stacked in a chevron configuration above a resistive anode. Particles, or photons, striking the front MCP may start a cascade of secondary electrons. As the MCP's each have a large gain, a burst of 106-107 electrons will impact the resistive anode. By comparing the currents to the four corners of the anode it is possible to determine the position of impact of the initial particle or photon. With the addition of a few components to the bias network it is also possible to use these devices for highly accurate timing. In order to use these devices for quantitative measurements we carried out a fairly comprehensive study of their properties focusing on, the effect of MCP bias, absolute detection efficiency, and the effect of the particle impact angle. We still routinely conduct such tests on the PSDs that we use for our experiments. Further details can be found in Gao et al., Rev. Sci. Inst. 55, 1756 (1984). A higher spatial resolution study is described in Gao et al., Rev. Sci. Inst. 59, 1954 (1988).   Accurate Absolute Pressure Measurements in the 10-6 Torr Range Capacitance diaphragm gauges (CDG's) measure pressure by determining the deflection of a metal diaphragm that separates two reservoirs held at different pressures. The CDG provides an accurate and reliable instrument for making absolute pressure measurements. However, the commercial devices are generally not calibrated below 0.01 Torr. In our laboratory we routinely measure pressures in the 10-6 Torr range and this motivated us to study the behavior of CDG gauges at these lower pressures. In order to use the gauge at such low pressures we enhanced the precision of the CDG measurements by computer averaging the output voltage of the CDG electronics unit. We overcame temperature drift problems by simply checking the zero reading of the CDG at frequent intervals. The linearity of the gauge was demonstrated using a novel technique based on the effect of gravity on the gauge diaphragm. When the CDG is tilted, the force of gravity on the diaphragm results in an apparent pressure which is indistinguishable from a gas pressure. The apparent pressure is very simply related to the tilt angle. Using two simple tilt apparatuses we were able to show that the gauge was linear to within +/-1% in the 10-6 Torr range and that the manufacturer's calibration is valid in that regime. Details of this project can be found in Straub et al., Rev. Sci. Inst. 65, 3279 (1994).   Atomic Oxygen Sensor Project Determination of oxygen atom fluxes and concentrations is difficult and has traditionally been only semi-quantitative. Atomic collision measurements performed in this laboratory have depended on a calibrated mass spectrometer to determine the atomic oxygen number density. However, measurements can only be made intermittently, and several minutes are required for each reading. One simple method of determining the flux of oxygen atoms employs the use of a catalytic probe. This method relies on measuring the temperature rise due to recombination of oxygen atoms on a silver oxide coated probe surface. Sometimes, as in the present design, a second reference probe, identical to the catalytic probe but uncoated, is used to compensate for temperature changes unrelated to atomic oxygen recombination. Variations of this general concept have been widely used with some quantitative success. The specific design we are developing will hopefully overcome some of the shortcomings of previous implementations. Our prototype device, shown to the right, was built by one of our summer students, Brian White.     Physics and Astronomy | Rice Quantum Institute| Rice Space Institute Rice University   Updated March 19, 2001