Andrew Wike asked:
How does lead absorb radiation like x-rays and gamma rays?
Chris - Well, the reason that lead is a good choice is because itís 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 doesnít 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 itís 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 itís basically a safe form of energy and a way of neutralizing the effects of the radiation. Lead is a good choice because itís 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! Iíve worn lead aprons when doing x-rays medically in hospital, and it really is very, very heavy. So I wouldnít 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.
Andrew Wike asked the Naked Scientists: How does Lead absorb radiation? and why is it used over other metals in x-rays? What do you think? Andrew James Wike, Sun, 29th Nov 2009
I'm not too sure if it absorbs it, more like blocks it..I think !!
Lead is very dense and is a good electrical conductor. I suspect that lead does absorb the radiation so that the radiation becomes part of the lead. I haven't studied the dynamics of the absorption but it probably contributes to the energy of electrons in the lead. Some types of radiation could affect the nucleons causing the lead to become radioactive. Vern, Sun, 29th Nov 2009
One of the odd things about lead is that it's not very dense.
If lead does absorb radiation, does it then become full ?...what happens to the radiation it absorbs ? neilep, Mon, 30th Nov 2009
In the case of gamma and X rays it just warms the lead up a bit. For alpha and beta it might knock some of the atoms out of their nice orderly place in the crystal latice (yes, lead is crystaline). In the case of neutrons it might get "full" eventually, but the neutron flux to do that would need to be big- rather more than you get from a nuclear bomb. Bored chemist, Mon, 30th Nov 2009
Thank you all it makes sense now. Andrew James Wike, Tue, 1st Dec 2009
In a rigid atom structure with tightly bound electrons, only specific energy quanta can be absorbed. Electrons canít just absorb any energy, itís all to do with the structure of the particular atom. This explains why some materials are transparent. The light isnít powerful enough for any of the tightly bound electrons in the glass, so it just passes right through. In the case of metals, however, their atoms are surrounded by very loosely bound electrons, so all metals will absorb any light that hits them ( and then re-emit most of it ). Coloured materials reflect particular frequencies of light, and not others. In the case of lead, itís simply down to the fact that itís atoms are surrounded by more electrons so there is more chance that a beam of x-rays or gamma rays will be absorbed. Lead is a more efficient absorber. It has nothing to do with the mass of the nuclei. niki, Mon, 3rd Jun 2013
This is a common misconception. Gamma and X rays are absorbed by large nuclei, those with largest atomic number. Lead contains the largest stable nucleus, cheap, and it's also not too toxic. Because it's so dense it is also a good alpha and beta shield, as is any thick metal. It doesn't do much for neutrons. Sean Sjahrial, Wed, 26th Feb 2014
If the energy of the incoming photon is enough to free the electron from its valance shell then the electron leaves the atom through the photoelectric effect, in which the electron leaves with kinetic energy equal to the energy of the photon that acted upon it minus the energy of its valance shell. This usually takes place at lower photon energies.
Reply #11 is only true for very high energy electromagnetic radiation, for which lead is not the preferred absorber anyway. Up to a few MeV the dominant interactions are elastic, photoelectric and compton scatter, which are all dependent on the electron density of the absorbing material. Lead is useful because it has a high density of electrons, is cheap and easy to machine, and unlike uranium (which is a really good absorber) and other higher-atomic number metals, is not radioactive or particularly toxic. Nevertheless concerns about the chemical toxicity of lead have prompted the use of tungsten in clinical x-ray shields.
Lead is a dense metal with high atomic number.