increasing shielding. The exact thickness at which proton dose becomes important varies with orbit. Proton dose may be a concern for low-Earth orbits or highly elliptical orbits that encounter the inner Van Allen belts, for orbits encountering high fluences from Solar Energetic Particle (SEP) events, or for missions reaching proton belts surrounding other planets. For circular orbits between about 1,500 and 5,000 km (the heart of the belts) the multi-year mission doses from protons can easily exceed 100 krad(Si). Whether in an orbit dominated by protons or one with mixed electron/proton exposures, the components most sensitive to TID effects are often buried deep in the spacecraft or protected with spot shielding. Consequently they may receive a substantial fraction of their dose from protons. 3.2 Is a rad always a rad? The widely accepted unit for total ionizing dose from ionizing radiation is the rad (from Radiation Absorbed Dose). The rad is defined as 100 ergs per gram of energy absorbed in the exposed material, and 100 rads is equal to 1 Grey (Gy). For the case of heavy ions (including protons), the exposure in rads is determined from the ion LET and the particle fluence according to equation 1. Note that the LET and corresponding dose for a given particle fluence are material dependent quantities. As an example, a 100 MeV proton has an LET in silicon of 5.93 X 10 MeVcm2/mg. For a fluence of 1 proton per square centimeter, the corresponding dose would be 9.5 X rad(Si). This relation is depicted versus proton energy as figure 8. IV-18 Dose per Unit Fluence for Protons in Si 1.0E-05 1.0E-06 1.0E-07 1.0E-08 Proton Energy (MeV) Figure 8. On average, each proton deposits dose according to its LET and the relation expressed in equation 1. The dependence in the above figure describes dose deposition from protons in silicon. The basis for the use, or even the existence, of the rad is that the effects of the ionizing radiation in question, whether it be electrons, photons, protons, or other ions, will be equivalent f