The penetration limit of thin films

One sensor of the Helios micrometeoroid experiment is covered by a thin film consisting of 3000 Å parylene and 750 Å aluminium. Micrometeoroids must penetrate this film before they are detected. In order to study the effects of the film on the detection of micrometeoroids simulation experiments were performed with iron, aluminium, glass and polyphenylene projectiles in the mass range of 5 × 10 −13g < m < 2 × 10 −10g and in the speed range of 1.5 km/ sec < ν < 13 km/ sec. The bulk densities of the projectiles ranged from 1.25 g/cm 3 (polyphenylene) to 7.9 g/cm 3 (iron). By measuring the speed of the projectiles before and after the film penetration the speed loss Δν caused by the film was determined. The angle of incidence was varied in three steps (0°, 30° and 60°). This deceleration strongly depends on the projectiles' densities: Vertically impacting iron projectiles of m = 10 −11 g and ν 1 = 3 km/ sec were subject to a relative speed loss of Δν/ ν 1 = 4%, aluminium projectiles of the same mass and speed showed Δν/ ν 1 = 8%, glass projectiles Δν/ ν 1 = 9% and polyphenylene projectiles Δν/ ν 1 = 14%. The total charge of the plasma produced upon impact on a gold target of a projectile which had penetrated the film before that was compared with the plasma produced by a projectile without a penetration. For iron projectiles these two signals did not differ significantly even at an angle of incidence of 60°. Whereas polyphenylene projectiles showed an attenuation of the charge signal by a factor of 10 after the penetration already at an angle of incidence of 0°. Polyphenylene projectiles impacting the film at an angle of incidence of 60° could no longer be detected behind the film. This experiment defined the penetration limit of the Helios film. Comparison with other penetration data yielded a penetration formula which is applicable to projectiles with diameters in the submicron to centimeter range. This penetration formula gives the penetration limit of a film as a function of the projectile's mass, speed and density.