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 they have; 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 neutralising 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...

Comments

Ever since I saw this map showing gamma ray emissions in the galaxy, I've wondered if in the same way photons of a frequency of visible light can be used for solar energy, if in some way some other material could be used to do the same for photons of higher frequency, where the higher the frequency? the more energy you get.
So this evening I was exploring what happens to that energy when say- lead or my favourite and much safer - bismuth absorbs it.

I learned a bit from what was written above, but it seems to me that gamma can ultimately convert to energy in the domain of electrons, so seems to me there should be a way to get that energy back out.

I've also wondered just what is the maximum frequency an electromagnetic wave can have. I imagine at some point, the wavelength would be so small, it would even be smaller than quantum particles... I once read 10 to the minus 23rd is a wavelength the width of an electron, so what would 10 to say- the minus 40th? go through?

Either way, we are basking in high energy photons day in and day out - seems there has to be a way to catch them analogous to solar energy.

I recently was reading about the abilities of Perovskite. It is amazing the things that can be done with it. I can't wait to see the results in computing in the future or even the possibilities for solar power. I was wondering about radiation though. Could this be dangerous to health, especially in regards to using LiFi computing?( Data transfer through light waves) Are TeraHertz dangerous? Especially seeing as how cell phones are only 2.4 ghz?
At what range do things potentially get dangerous ?

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