Angular and energy dependence of proton upset in optocouplers

Transient upsets from protons in high-speed optocouplers were investigated over a range of incident angles and energies. At energies below 50 MeV, ver y large increases in cross section occurred at angles above 60 degrees, con sistent with the increase in cross section that is expected when direct pro ton ionization begins to contribute to the cross section. The angular depen dence of the cross section increases the number of transient upsets expecte d in orbit compared to upset rate calculations that do not take the angular dependence into account. Laboratory alpha particle measurements were used to measure critical charge in these devices. The critical charge and area o f the photodiode provide a way to identify devices that are sensitive to di rect ionization at large angles.