eV/nucleon to hundreds of GeV/nucleon), they do not become trapped and are not significantly attenuated by spacecraft shielding. GCRs that hit the atmosphere form a cascade of secondary particles, as mentioned in the next section. GCRs with polar trajectories can penetrate to low altitudes because of the reduced geomagnetic rigidity in the polar regions. Spacecraft with high inclination angles are therefore at greatest risk of encountering SEE due to GCRs. Secondary Particles Secondary particles are produced when GCRs strike the Earth's atmosphere and produce a shower of particles in the atmospheric environment. Because this component of the radiation environment is important only for microelectronics operating in the Earth's atmosphere, a description of the secondary particle environment will be delayed until Section 6.3, Terrestrial and High-Altitude Single-Event Effects. ATOMIC MASS Figure 2.5 Particle composition of galactic cosmic rays [16]. Note that Hydrogen and Helium nuclei (i.e., protons and alpha-particles) account for the vast majority of GCR flux, while heavy ions comprise only about 1%. BASIC MECHANISMS FOR NON-DESTRUCTIVE SINGLE-EVENT EFFECTS All non-destructive single-event effects are caused by the same fundamental mechanism: collection of charge at a sensitive region of a microcircuit following the passage of an energetic particle through the device. In this section we look at the release of mobile carriers along the path of an incident particle, the collection of these carriers, and how the resulting transient currents interact with the circuit to generate a single-event upset. Charge Deposition By definition, as ionizing radiation passes through a target material electrons and holes are released along the path of ionizing particles. There are two primary methods by which carriers are released: direct ionization by the incident particle and ionization by secondary particles created by nuclear reactions between the incident particle and the target material. Direct ionization can cause SEU if the