Heavy shielding can render active interrogation of cargo useless. This 
shielding attenuates both the probe and product particles.  The product of 
these two attenuation factors can make detection impossible.  Intermediate 
energy protons produce fissions as effectively as neutrons or high energy 
gamma rays but are much more penetrating than either of these convention 
probes. We have: 1) Performed experimental measurements of delayed gamma 
rays and neutrons produced by proton interactions with a wide range of 
targets; 2) used these data to validate Monte Carlo studies; and 3) Studied 
accelerator designs and costs for a system that might be used for proton 
interrogation.