  The problem of detecting a nuclear weapon smuggled in an ocean-going cargo
container has not been solved, and the detonation of such a device in a large
city could produce casualties and property damage exceeding those of September
11, 2001 by orders of magnitude. Any means of detecting such threats must be
fast and cheap enough to screen the millions of containers shipped each year,
and must be capable of distinguishing a threatening quantity of fissionable
material from the complex loading of masses of innocent material found in many
containers. Here we show that radiography with energetic X-rays produced by a
10 MeV electron accelerator, taking advantage of the high density and specific
atomic properties of fissionable material, may be a practical solution.
