This will lead to a relatively fast carbonization of the foil surface region, the possible consequences of this still being largely unknown. In the worst case, this could lead to an embrittlement of this layer with subsequent flaking off (including the reflecting Al layer on top), and hence disastrous results for the optical properties of the mirror. In the best possible case, the developing carbon layer (between the Al and the remaining polymer layers) will smoothly fit in between the existing layers, without any mechanical tensions. Further, blistering effects may set in at relatively low doses, compared to metals. Shrinet et al. (12) found that the critical dose for blistering in Mylar and Kapton is around 7 10'5 ions/cm2, which would correspond to typically one year of mirror operation in space in our application. The blisters observed are very large (10-150 /zm). The foils used are, however, not coated with metals, which could result in higher critical doses (12). In this connection, the attention should be drawn to the findings of J. Davenas et al. (9), who reported hydrogen loss from (CH)X foils from zones far deeper than the range of the implanted atoms. A similar phenomenon of long-range transports of irradiation effects (in this case, transmission sputtering through samples thicker than the range by a factor of 2000) has also recently been observed for He irradiation in sulfur (10,13), which was explained by slowly diffusing irradiation created charges, spreading all over the bulk of the sample, even in non-irradiated areas. In an experiment performed recently by us to observe the implantation profiles in double-layer structures with organic foils, some hints for those effects were also found. After irradiation of a metal covered foil (130 keV :!He+ at 0.1 juA/cm2, impinging onto 2000 A Au/2 jam Mylar/100 A Al structure), some Al emission from the rear side of the foil occurs, though the He+ ions should come completely to rest in the front part of the foil. This phenomenon — though not yet precisely examined — could be of some importance in our space mirror application. In particular, energetic electrons, but also the implanted ions, lead to a charge build-up in the foil, which could be the driving force for the above-mentioned long-range foil degrading effects, but also for enhanced Al sputtering — i.e., shortening the mirror's lifetime. Further, we observed that 'He irradiated Al coated mylar foils exhibited some shrinking after only a few juC/cm2 He+ implanted. Simultaneously, blistering started, which could be observed directly during implantation by the naked eye as small sub-mm size sparks in the light of a lamp. Parallel to the onset of blistering, some reduction of the optical reflectivity, estimated to about 10%, was observed. At prolonged irradiation, up to a total dose of 1000 juC/cm2, a gradual release of the mechanical tensions was observed, the formation of new blisters ceased, and no other alterations in mechanical or optical behaviour were found. The foils' behaviour under still higher irradiation doses (corresponding to the solar mirror performance after some years) is not yet known. The creation of ALO;) by collisional transport of oxygen from the foil to the Al layer can be neglected. Also, the worsening of the reflectivity of the Al layer by surface roughening due to sputtering is completely outraged by the blistering effect. The foil erosion by sputtering occurs on a much slower time scale than the destruction by radiation damage and cascade mixing, so that sputtering is not regarded as a lifetime limiting factor. Depending on the mechanical behaviour of such foils, being quickly carbonized under the high intensity irradiation, we estimate a useful foil lifetime of either a few hours to days only, or approximately 10 to 100 years, if the mirror is operated
RkJQdWJsaXNoZXIy MTU5NjU0Mg==