New Scientist has an article on thework underway to build better radiation detectors for use in cargo scanning – and here’s why:

Firstly, they wrongly flag around 2 per cent of all containers as suspect, mainly because they cannot distinguish between a plutonium bomb and the radioactive potassium-40 found in bananas. More importantly, they fail to detect the most dangerous nuclear material of all: highly enriched uranium (HEU).

"D’oh!" springs to mind..