Recent quartz crystal microbalance measurements made in the Marshall Space Flight Center, Photo-deposition Facility, for several materials, recorded a significant loss of deposited contaminants when the deposition surface of the microbalance was illuminated by a deuterium lamp. Theses measurements differ from observations made by other investigators in which the rate of deposition increased significantly when the deposition surface was illuminated with vacuum ultraviolet radiation, generated by a variety of sources. These observations suggest that the accelerated deposition of molecular contaminants on optically sensitive surfaces is dependant upon the contaminant being deposited and the VUV source and that both must be addressed during the materials screening process.
Computer models that predict the rate at which molecular contamination will deposit on
optical surfaces typically use outgassing source terms, measured with quartz crystal
microbalances, as a basis for the prediction. The American Society of Testing and
Materials, Standard Test Method for Contamination Outgassing Characteristics of
Spacecraft Materials (Method E-1559), is probably the best know technique used by the
aerospace community to measure the outgassing rates or source terms of materials.
A simple method for the insitu calibration of quartz crystal microbalances, based on the
heat of enthalphy of Adipic Acid, has been developed and demonstrated by the Marshall
Space Flight Center, Environmental Effects Branch. The calibration has been
demonstrated over a sample temperature range of 25 to 66 degrees Celsius and deposition
rates of 2 x 10-11 grams/cm2-s and greater, for several measurement system
configurations. This calibration technique is fully compatible with the American
Society for Testing and Materials, Method E-1559, as well as other methodology. The
calibration requires no modification of outgassing facilities employing an effusion cell
and does not degrade the performance or function of typical vacuum systems.
The orbiters undergo periodic maintenance during which extensive mechanical and electrical system inspection, upgrades and/or refurbishment are performed. For sensitive payloads, new materials added to the orbiter midbody (payload bay) area is of concern for the first flight after an orbiter maintenance down period (OMDP). Discovery, (OV- 103), recently underwent an OMDP, and the Hubble Space Telescope Servicing Mission 2 will be its first flight after OMDP. Due to the contamination sensitivity of the Hubble Space Telescope, a program was implemented to monitor the nonvolatile residue and particle levels of the midbody during the OMDP.
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