Contaminations of inner surface of magnesium fluoride windows in the ‘Expose-R’ experiment on the International Space Station

Abstract

A series of experiments was carried out previously on board of the International Space Station in 'EXPOSE-R', a multi-user expose facility, provided by European Space Agency attached to the external surface of the Russian Segment. In one experiment, spores of microorganisms and species of higher plant seeds, in heat-sealed polymer bags were irradiated by solar radiation passed through MgF 2 windows in a high space vacuum. After sample exposure, it was found that in many cases the inner surfaces of windows were contaminated. Analysis of the contamination revealed the presence of chemical groups CH 2 , CH 3 , NH, OH, CvO, Si-CH 3 (Demets et al. in 2015). Their presence in deposits was explained by photofixation of gaseous precursors -some of the vapours of glues and additives in polymeric materials in the core facility of 'Expose-R'. Carbon-, oxygen-and silicon-containing groups may be deposited from outer intrinsic atmosphere. This atmosphere is connected with sample compartments and core facility. However, the presence of NH groups on inner surfaces of windows was not expected. This paper shows that the process responsible for carbon-, nitrogen-and oxygen-containing group formation can be a photopolymerization of caprolactam, which is released from the outer Nylon 6 layer of polymer bags under Solar vacuum ultraviolet radiation.