cal signal by an optoelectronic photodetector, the signal level may be only a few hundred or thousand electrons prior to amplification. Several studies have demonstrated how the photodetector, by virtue of its low signal level, can be sensitive to false signals from direct ionization by incident protons. For details and additional information, please see the review article [Mars-96, and references therein]. The sensitivity of the photodetector is perhaps not SO surprising in view of the fact that this optoelectronic detector functions to capture digital information at rates into the Gbps regime from optical signals with average powers of only a few uW. This results in valid signals of only a few hundred electrons in some cases. Also, the photodiode must necessarily be large enough to capture the optical signal. For typical multimode fiber, this corresponds to surface areas of thousands of square microns (the device examined in our study has a 75 micron optical aperture with an 80 micron diameter junction). Photodiode physical cross sections can easily exceed 10 -5 cm 2 and due to their extreme sensitivity, the error cross sections can be correspondingly large. Figure 13 depicts the disk-shaped planar photodiode structure under reverse bias conditions and indicates various particle trajectories that deposit charge by direct ionization. The sketch beneath shows resulting current pulses sensed in the receiver circuit which decay with an RC time constant determined by the circuit bandwidth. Also depicted is the received signal provided in a no-return-to-zero (NRZ) protocol containing the digital information. The ratio between the high and low current levels (the "extinction ratio") is typically about 10. Receiver circuits are almost always designed to accommodate a range of incident average optical powers and automatically adjust the decision level, or threshold, to be midway between the high and low levels. As suggested in the figure, data can be disrupted if ion- induced current exceeding the threshold current is s