o ensure that the incident proton has sufficient range to penetrate both the device packaging and the sensitive volume of the device itself. The analysis is greatly facilitated (and more accurate) if the non-ionizing energy loss rate through the active volume of the device is constant, and if the incident proton beam is reasonably monoenergetic. In some cases, the active volume of a device may be very thin, and also close to the surface of the device. If such a device is irradiated with protons normally incident to the surface, the forward directed energetic recoils may deposit their energy below the active device region. As shown in figure 12, it can take several microns to reach "recoil equilibrium" inside a device [Dale94]. Since recoil equilibrium applies to devices shielded spacecraft subject to irradiation from all sides, we also should approximate condition in laboratory radiation tests. If it is questionable whether a device will be in recoil equilibrium for the case of protons incident from the front, the unit may simply be irradiated from the back to achieve equilibrium. This effect may have been at least partly responsible for deviations of Si CCD damage factors from NIEL observed at higher proton energies in figure 10 [Dale94]. Lack of recoil equilibrium was cited in [Summ88] as a possible explanation for the deviations of Si JFET damage factors from NIEL apparent in figure 9. However, Barry et al. [Barr95] eliminated recoil equilibrium as an explanation for the deviation of their LED damage factors from NIEL by also irradiating devices from the backsides with identical results. Although this issue has not been well studied, it is nonetheless straightforward to perform proton irradiations SO that recoil equilibrium is satisfied. SENSITIVE VOLUME 11 X X 0.15 um 20 MeV PROTONS 63 MeV PROTONS 150 MeV PROTONS OVERLAYER THICKNESS (um) Figure 12 The approach to displacement damage equilibrium is shown for several proton energies. It is also shown in [Dale94], that the overlayer thickness required to achieve