How does lead absorb radiation like x-rays and gamma rays?

29 November 2009

Question

How does lead absorb radiation like x-rays and gamma rays?

Answer

Chris - Well, the reason that lead is a good choice is because it's a very dense substance, because dense substances can get in the way of the radiation and soak it up. And the denser something is the more atoms, and in the case of things like x-rays and gamma rays the more electrons, there are to potentially interact with that ray as it goes through and stop it.So, if you look at the density of lead; lead weighs something like 11 grams per centimetre cubed. Iron, on the other hand, is only seven. So in other words, you can get lots and lots of shielding with lead for much less space than if you use, say iron or concrete, which doesn't have the same density, although both could soak up x-rays in the same way. What happens is that the x-ray, which is effectively a light wave, when it goes through the material it's interacting with the cloud of electrons around each of the atoms. And what could happen is the x-ray, when it does have this opportunity to interact with the electrons, can add some energy to an electron, and this can make the electron depart from the nucleus that it was originally orbiting. This can make an ion, for example, and the electron can then move away or be captured elsewhere.So what that does is basically turn the energy in the x-ray or the gamma ray into other forms of energy inside the material, so it's basically a safe form of energy and a way of neutralizing the effects of the radiation. Lead is a good choice because it's very, very dense, so you can pack in more protection into a smaller area than you would otherwise. But lead is very, very heavy to wear for personal protection! I've worn lead aprons when doing x-rays medically in hospital, and it really is very, very heavy. So I wouldn't recommend it if you can avoid it!Dave - The other effect is because lead has got a very, very positively-charged nucleus. The electrons around the middle of it can absorb a huge amount of energy before they get kicked off the atom. So an electron which is very near to the centre of the nucleus, can absorb a much more energetic gamma ray or x-ray, than say a hydrogen atom, because in a hydrogen atom, the electron can take just a small kick to remove it, and so it can't absorb any more energy.

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