I just came across a product called NukAlert:
More than just a detector, the NukAlert is a patented personal radiation meter and alarm. Small enough to attach to a key chain, the device operates non-stop, 24/7 and will promptly warn you of the presence of unseen, but acutely dangerous levels of radiation.
Used nationwide by federal, state and local first responders, law enforcement and the Department of Defense, the device is the ideal solution for increasing individual security and awareness of radiological threats. It will be very reassuring to know, with confidence, when you and your family are out of the worst danger.
Well, at least this is what it says on their site. I wish I had one to give it a try… 😉
Anyway, European customers can get one here, US customers go to this site, and Japanese customers can order one here.