Properties of cathodoluminescence for cryogenic applications of SiO₂-based space observatory optics and coatings

Abstract

Disordered thin film SiO 2 /SiO x coatings undergoing electron-beam bombardment exhibit cathodoluminescence, which can produce deleterious stray background light in cryogenic space-based astronomical observatories exposed to highenergy electron fluxes from space plasmas. As future observatory missions push the envelope into more extreme environments and more complex and sensitive detection, a fundamental understanding of the dependencies of this cathodoluminescence becomes critical to meet performance objectives of these advanced space-based observatories. Measurements of absolute radiance and emission spectra as functions of incident electron energy, flux, and power typical of space environments are presented for thin (~60-200 nm) SiO 2 /SiO x optical coatings on reflective metal substrates over a range of sample temperatures (~40-400 K) and emission wavelengths (~260-5000 nm). Luminescent intensity and peak wavelengths of four distinct bands were observed in UV/VIS/NIR emission spectra, ranging from 300 nm to 1000 nm. A simple model is proposed that describes the dependence of cathodoluminescence on irradiation time, incident flux and energy, sample thickness, and temperature.

Feasibility of Detecting Spacecraft Charging and Arcing by Remote Sensing

“…This cathodoluminescence spectral radiance can range from 10 −9 to 10 −5 W • sr −1 • m −2 • μm −1 for different polymeric, glassy, and multilayer and nanodielectric composite materials (see Fig. 2 and [1,4,5]). This phenomenon has been seen in the laboratory countless times (see [6,7]).…”

“…The highest electron current fluxes seen in GEO are about 0.4 nA • cm −2 at an effective thermal energy of about 20 keV. Taking the glow radiance to be proportional to the beam energy and current flux [4], one may correct the maximum expected spectral power density from L Charging to be 6.…”

“…For example, if each bombarding electron were to yield all its energy in a 1 μm bandwidth, then L C at Earth would still be only 4.9 × 10 −21 W • m −2 • μm −1 , and the signal-to-noise ratio against skyglow is ∼0.06. If one can make observations from space, and where the zodiacal background is also negligible, the skyglow can be eliminated, so that L Total L Charging (4) and the signal-to-noise ratio is as good as possible. Unfortunately, the zodiacal light background for GEO satellite observation is strongest near eclipse seasons, when the satellite is near the plane of the ecliptic.…”