10 keV, respectively), as well as 3-30 keV electrons. Besides this, particles of much higher energies are occasionally emitted as the result of solar flares or storms. In our calculation, we focus for simplicity on the solar wind only, and neglect the disturbing influence of the Earth's magnetic field and shadow. With our MONTE CARLO computer code TRIM (2) (including the currently most reliable formulae for the nuclear interaction potential (3) and the electronic stopping power (4)), we simulated the effect of the solar wind on a hypothetical solar mirror foil. Figure 2 shows the depth distributions for various angles of incidence. From their mean values, we derive the actual implantation, damage and ionization distributions, depicted in Fig. 3a-e. (The ionization distributions were added, as it became recently evident that light ions, implanted in organic foils, often redistribute according to their ionization distributions (5).) It is seen that the particle flux accumulates to considerable concentrations in the case of hydrogen even after only a few years of operation. The damage in the foil surface, especially in the first 350 A, is also astonishingly high, and quickly reaches some displacements per atom (dpa). In this depth regime, hydrogen creates the majority of the defects, due to its high abundance in solar wind. At greater depth, damage is essentially due to helium irradiation. By cascade mixing, Al is implanted in the foil only in a small amount, whereas carbon and hydrogen especially are driven into and even through the Al layer to some extent. The TRIM calculations further indicate a sputtering yield of 0.01 atoms/incident H+ ion, 0.1 atoms/incident a particle, and approximately 0.5 atoms/incident heavier ion. Due to the dominance of hydrogen in the solar wind, the total erosion yield is essentially determined by this component, and yields a value of about 0.15 A/year. Neglecting the additional effect of micrometeorite impact (which will perforate the foil instead of gradually eroding the reflecting layer), we find a maximum lifetime rmax of about 635 years, corresponding to the complete removal of the 100 A thick Al coating. During this time, the distributions of the implanted solar wind are altered by the surface shift due to sputtering, and, by cascade mixing, the foil composition changes, see Fig. 3f-h. DISCUSSION From these results, we may deduce the following: Considerable quantities of hydrogen are implanted, associated with a remarkable damage to the foil surface region. Whereas most hydrogen (and helium) is supposed to diffuse out of the foil immediately (6) (so that the large gas concentrations calculated here are probably only hypothetical numbers), the damage may be the essential lifetime limiting factor, as organic polymers do not have the ability to withstand intense irradiation unchanged, as do metals to some extent. It is known that there exist 2 groups of organic polymers, in one of which crosslinks between molecules are formed under irradiation (e.g., photoresists), and the molecules of the other group (e.g., hydrocarbons) tending to degrade rapidly (7). If the foil used belongs to the second group, a great amount of the cracked polymer fraction will leave the foil as small molecules, as H2, CO, CO2, COH4, O2, etc., already in the initial phase (after a dose of typically some 1012 ions/cm2 (8), accumulated after only a few hours of exposure in space). Foils belonging to the other group are stable against irradiation damage for a longer time before considerable decay sets in.
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