s of an apparent LET threshold for the effect [Reed-98]. Figure 19, reproduced from that paper, reveals that 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). Johnston, et al. extended the study to other devices and included heavy ions as well as protons [John-98]. Their findings suggest that at higher LET's corresponding to cosmic rays the follow-on amplifier circuit may also lead to transient effects, and that for protons the nuclear elastic scattering may be important to the process. Another study has pointed to the important combination of direct and indirect mechanisms for the case of Metal-Semiconductor-Metal (MSM) photodetectors for use in digital data links such as the fiber-based data busses already described [Mars-98]. The MSM detector technology offers the advantage of a nearly planar geometry which minimizes the pathlengths (and ionization signature) when traversed by protons. Enhanced proton transient cross sections in the plane of the detector and the signature of a threshold LET for direct ionization effects were both noted for MSM devices in [Mars-98]. The existence of multiple mechanisms for transient effects has important implications for the methods and accuracy of transient rate predictions. It has been suggested [LaBe-97] that the aggregate transient rate must be calculated as the sum of the rates for indirect effects using the Bendel formalism plus the direct contribution using the modified RPP approach and measured Weibull-type LET dependence as described for photodetectors in the previous section. For devices exhibiting large enhancements in the cross section near the plane of the IV-38 device as in figure 18, both mechanisms are obviously important, and the dominant one would depend on the details of the application and the environment. It should be assumed that the rates would depend on how such a device is operated in terms of expected signal levels and detection sensitivity. 4.3 De