c Geosynchronous any satellite 35790 x 35790 0° 1.1 rad/year 1.1 Mrad/year overshielding = > 1-2 krad/year G PS-II operational 22600 x 22600 9.1 Grad/year 1.9 Mrad/year (or GLONASS = end of life 19132 X 19132 64.8°) overshielding = > 1-10 krad/year Table 1. Yearly radiation doses for various types of satellites in circular orbits (overshielding example is given for Radpak TM shield case [SEI]). 1 000 Dose VS A Ititude 9 Years & 55° ( 10 mm AI) 1 000 10 000 100 000 CIRCULAR ORBIT ALTITUDE in km Figure 3. Indication of the dose received behind 10 mm Aluminum for 50 to 60° orbit as a function of altitude. Trend for present and new spacecrafts or constellations (Space Station, Celestri, Skybridge, GPS-II, etc) [Calv-99]. This increased regime needs enhanced strategies of procurement, modeling of radiation effects and radiation hardness assurance. THE INTERACTIONS: PERMANENT EFFECTS OF THE CUMULATED DOSE III-6 2.3.1. INTERACTIONS AND TRACK RADII. UNIFORM OR DISPERSED DOSE? In matter, the interaction of photons, ions, protons or high-energy electrons present in the environment generates secondary electrons that are themselves very energetic with respect to the energies of the valence electrons or even of core electrons of atoms. photo ELECTRONS PRIMARIES Oxide SECONDARIES Silicium Tracks : D = 1 mm (photon 1 MeV) = 0.3 um (proton 10 MeV) 0 = 1.5 um (photon 10 keV) 0 = 500 A (Oxygen 100 MeV) Figure 4. Cascades of electrons resulting from the primary interaction. Thus, secondary electrons can ionize atoms, generating electron-hole pairs or collective excitations of valence electrons (plasmon modes). Oscillations of plasmon modes can eventually generate electron hole pairs. As long as energies of the generated electrons and holes are greater than the minimal energy required for an electron-hole pair, they can in turn generate supplementary pairs. Consequently, one lone photon of high energy, or electron or proton or ion, can create thousand or million of electron-hole pairs. 2.3.2. EQUIVALENCE BETWEEN FLUXES AND DOSE RATE