Lots of harmless things are slightly radioactive—kitty litter, ceramic tiles, even bananas. So most detectors are set to ignore low radiation levels. Basic shielding would be enough to mask all but the strongest sources. “The radiation portals that were deployed in the aftermath of 9/11 are essentially fine, except for three problems: They won’t find a nuclear bomb, they won’t find highly enriched uranium, and they won’t find a shielded dirty bomb,” says Stephen Flynn, a terrorism expert and president of the Center for National Policy. “Other than that, they’re great pieces of equipment.