Tomorrow's job, now I have all the bits for it, is to make a few radioactive sources using something you can buy in the supermarket. Lo-sodium salt. It's about 33% potassium chloride, which is about double the background radiation in most places. The hazards are so small though that we're not actually allowed to label it as radioactive as the stress that could cause outweighs the real risk, which is zero, within the error margins. Some research even suggests it may be beneficial.
@_thegeoff I don't understand the comparison, because I think the units don't match. Do you mean sth like "if you were to fill a large volume with potassium chloride, you'd get 2x normal background dose in the middle of the volume"?
@robryk In this case just a good sprinkling of potassium chloride will double the background. A large 100g chunk will do much the same, as you're mostly just measuring the surface directly in front of (and a few mm from) the GM tube window.
@robryk Our counter only measures a certain cross-section. So if it covers, say, 4 square cm and the source is 16 square cm we'll only get some of it. (Obviously varies depending on both orientation and whether we're looking at alpha, beta or gamma.)
For the potassium it's beta, so only the part of the source roughly in front of the window is likely to be detected. If it's a tall cylinder a lot of the beta from further down will be absorbed source itself.