Cobalt-60

My hobby is or was electronic design. About five years ago, I wanted to test a p-n junction as a detector of radioactivity. For this, I needed a radioactive source. This was a problem -- but ultimately I got hold of a section of cobalt pencil (cobalt-60). The cobalt was a beautiful blue, encased in a very heavy lead container. As it happens, my experiments didn't work as planned, and I returned the cobalt-60. OBSERVATION: Cobalt-60 is one of the preferred ingredients for a "dirty bomb". My own piece of pencil could have contaminated more than 100 square kilometres, and I obtained it without any clearance. So I would think that, yes, "radiological attacks constitute a credible threat". At least over here.

POSTSCRIPT: If I did it again, I would try a (call it) c-a junction with a neon tube.