ge. For the classic reaction recoil mechanism, no angular dependence would be expected. In [LaBe-97] the explanation is offered in terms of a combination of mechanisms. Figure 18 depicts the situation in which the device response is dominated by strikes to the optocoupler's internal photodetector with nuclear reactions causing transients when they occur, regardless of the angle of incidence. However, for protons traversing the plane of the photodetector sufficient signal can follow from direct ionization across the longer pathlengths. IV-36 Angle of Incidence (degrees) Figure 17. Proton-induced transients in optocouplers exhibit an enhancement in the cross section when the beam is directed in the plane of the package. For the classic reaction recoil mechanism, no angular dependence would be expected. In [LaBe-97] this behavior is explained in terms of a combination of direct ionization through the plane of the photodetector and indirect reaction mechanisms. Direct Ionization Across Long Pathlengths Proton 380 m Proton And Nuclear Reaction Recoils. Figure 18. This drawing from [LaBe-97] depicts the situation in which the device response is dominated by strikes to the optocoupler's internal photodetector with nuclear reactions causing transients when they occur, regardless of the angle of incidence. For protons traversing the plane of the photodetector, sufficient signal can follow from direct ionization across the longer pathlengths. IV-37 1.2E-07 0 degrees 1.0E-07 85 degrees 87.5 degrees 8.0E-08 + 90 degrees 92.5 degrees 6.0E-08 95 degrees 4.0E-08 2.0E-08 0.0E+00 Proton Energy (MeV) Figure 19. Data supporting the role of the direct ionization mechanism for optocoupler transients shows an apparent LET threshold for the effect [Reed-98]. The increased cross section seen when protons traverse the plane of the diode does not occur with protons above a certain LET (corresponding to ~100 MeV protons). The following year Reed, et al. reported additional data supporting the role of the direct ionization mechanism in term