The nice thing about radiation is that it is easy to detect even minicule amounts of it fairly easily.
I am surprised how little they cost.
I just did a search on Google and found a site, where you can get a pocket model for a couple hundred dollars.
Maybe people should be be encouraged to buy them, or the government could sell it to them at a reduced price, and if a lot of people had them, it would be harder for the terrorists to transport them in trucks or vans, or store them in a garage in a city, without someone noticing it.
Many years ago I worked in a lab where we were doing studies on radioactive material. We had to ascertain background levels each time we tested. This was during a-bomb tests in Nevada. After each one of those detonations the background readings rose significantly. The mantels for gas lights emit measurable radiation.