Helium Purge Flow Prevention of Atmospheric Contamination of the Cryogenically Cooled Optics of Orbiting Infrared Telescopes: Calculation of He-O Differential Cross Section

摘要

The ability of an He purge gas to protect the cryogenically cooled optics of orbiting infrared telescopes from the primary atmospheric contaminant, atomic oxygen, can be estimated by Monte Carlo calculations simulating the He-O scattering events that would occur in the operating telescope. We report the results of accurate quantum mechanical calculations of the He-O differential cross sections sigma(theta, E) needed in such Monte Carlo calculations. To facilitate the use of these cross sections in Monte Carlo calculations, we have constructed a simple separable function of E and theta which provides an adequate representation of sigma(theta, E) for the entire range of relative collision energies studied, E = 0.02 to 1.66 eV. For high relative collision energies, we have also tested the accuracy of the computationally simpler WKB approximation. (Author)