On Geiger Counters in New York

*This letter went to the Public relations office of the CIA on the date in question*in the news (CBS) New York City Police are using geiger counters. I know 'someone' must have already thought of this, but here it is enumerated for your perusal. An X-ray machine works by directing a beam of particles against a radioactive 'lump' that is excited by the beam, and gives off a flash of radiation that hits the film (impeded by teeth bones tumors etc.) What the cops need to be doing is sweeping a truck or cargo container with a beam like this, so that even a small amount of radio active 'stuff' gives off a flash of radiation, easily measured. are they missing something here?