Optical absorption and photoluminescence spectroscopy have been used
to investigate the physical changes induced by proton and electron irradiation
in selected thermal control coatings. This study focused on a white paint and
on its two components, a polydimethylsiloxane resin and zinc oxide powder.
Samples were irradiated by either 45 keV protons (fluence up to
1016 protons cm-2) or 400 keV electrons (fluence up to
6×1015 electrons cm-2).
In situ reflectance measurements were made during the test and were
complemented after air introduction by photoluminescence characterization. The
optical properties of the paint are essentially those of the ZnO pigment. The
optical degradation of the material appears to be correlated with the density
of defects created by ionization in a zone close to the surface of the paint
and called the optical thickness of the material.
Two degradation regions of the reflectance properties in the wavelength range
250-2500 nm have been identified: one in the infrared and one in the
visible blue region. The degradation in the IR region disappears on exposure
to air so that no complementary technique could be used for identifying its
origin. The point defects responsible for the optical degradation of the paint
in the blue region are singly ionized oxygen vacancies (F+ centres) either
initially present in the material or induced by irradiation. Irradiation
quenches the green photoluminescence emission. The latter would be excited by
the recombination of doubly ionized oxygen vacancies with photoformed
electrons of the conduction band.
The ScaRaB instruments are a series of Earth Radiation Budget radiometers of which the first flight model (FM1) was launched on a METEOR satellite on 25 January 1994. A second model (FM2) was launched on the Russian Resurs satellite in July 1998. Results obtained from FM1 indicated the need for more accurate ground calibration for FM2 and the spare model. Besides spectral response characterization (from 0.2 µm to 50 µm), channel gains and absolute filtered radiance of the on-board lamps and on-board black bodies have been measured, using different reference sources: laboratory black bodies, an absolutely calibrated integrating sphere, and the Sun. The black bodies ensure the validity of the calibrations in the long-wave spectral domain, while the Sun and the integrating sphere check the short-wave spectral domain. After a brief description of the ScaRaB instrument and its main characteristics, this paper describes the procedures and results, including the associated uncertainties, for the measurements of the broadband channel gains: these need to be accurately calibrated and compared. Points of interest and also weaknesses of the three methods are compared in order to lead to the most realistic uncertainty budget.
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