hey are by design very large and operate at very high data rates [27]. A recent analysis suggests that a combination of direct ionization and recoils are responsible for the anomalous angular dependence of proton upsets in optocouplers [28]. Charge-coupled devices (CCDs) can also be sensitive to direct ionization by protons because of their large collection depths [29]. For more information on these devices and their susceptibility to protons the reader is directed to the third segment of this short course [30]. 3.1.2 Nuclear Reaction Effects As mentioned above, direct ionization by light particles usually does not produce a high enough charge density to cause upsets. Unfortunately, this does not mean that we can ignore II-11 these lighter particles. Protons and neutrons can both produce significant upset rates due to indirect mechanisms. As a high-energy proton or neutron enters the semiconductor lattice it may undergo an inelastic collision with a target nucleus. This may result in the emission of alpha (a) or gamma (y) particles and the recoil of a daughter nucleus (e.g., Si emits a-particle and a recoiling Mg nucleus), or a spallation reaction, in which the target nucleus is broken into two fragments (e.g., Si breaks into C and O ions), each of which can recoil. Any of these reaction products can now deposit energy along their paths by direct ionization. Because these particles are much heavier than the original proton or neutron, they can deposit higher charge densities as they travel and therefore may be capable of causing an SEU. These inelastic collision products typically have fairly low energies and do not travel far from the particle impact site. They also tend to be forward-scattered in the direction of the original particle; this has consequences for the SEU sensitivity as a function of the angle of incidence [31]. Historically neglected, recent experiments and simulations have shown that elastic collisions may become important for very sensitive devices [32,33]. Low-energy secondary particles (prima