When atoms become radioactive, what kind of radiation comes out?

We put this question to Dr Ian Farnan from the Department of Earth Sciences at Cambridge University:

The caesium-137 which is probably the one that people are most worried about because it has this half-life of about 30 years. That would be a beta emitter. Most of the elements that we're worried about in fact are beta emitters, so that's the problem. The thing that you need to worry about is the half-life. So the iodine which has been released has about an 8-day half-life, so that will mean after about 40 days, say, it will be at 3% of its initial value and within 10 half-lifes, it's down to a thousandth of its initial value or amount. So that's something that dissipates very quickly whereas the caesium will stay around for a lot longer. But as I say, there'll probably be less caesium than there is iodine.