Science Questions

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

Sun, 29th Nov 2009

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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.


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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 !!

More klevur peeps will answer this I am sure.

neilep, Sun, 29th Nov 2009

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.
It's about as dense as silver but each atom is (about) twice as heavy so they must be packed quite loosely in lead.
Anyway there are two ways that radiation gets blocked. Neutrons get stopped by absorbtion onto the nuclei of atoms but that's an unusual form of radiation.
The other thing that happens is that X and gamma rays get scattered by the electrons in materials. Lead has a high atomic number and, to maintain neutrallity, the nucleus is surrounded by a lot of electrons.
That's essentially what makes it a goos radiation sheild. Each time an Xray is scattered it loses some energy and once it's lost that energy it's no longer there.

The conductivity of lead dosn't enter into it for most radiation screening (it would for blocking radio waves or visible light but they are not in the original question). Lead glass is used for blocking Xrays and it's a very good insulator.

Lead isn't the only thing that gets used as shielding but it has the advantage of being fairly cheap.
For some purposes concrete is more ecconomical and sometimes tungsten is used- it's not as good on an atom-for-atom basis but it's nearly twice as dense so there are a lot more atoms of it in a given volume. Bored chemist, Sun, 29th Nov 2009

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

From what i remember in my general science studies is that lead can deflect most gamma energy. Mr. Scientist, Mon, 30th Nov 2009

Thank you all it makes sense now. Andrew James Wike, Tue, 1st Dec 2009

If a material "blocks" some form of radiation, it either absorbs or reflects it. (Or a mix of both, I suppose).

X-rays and gamma-rays are just very high-frequency (extremely short wavelength) electromagnetic radiation, like light. When these are absorbed, the energy is turned to heat. Materials don't become 'full' of light or electromagnetic radiation, so that's a non-issue. Because lead is a big atom it has a lot of electrons. This high density of electrons will tend to make it more opaque (absorbing) to high-energy EM radiation than other materials.

For 'nuclear' radiation (alpha particles (i.e. protons), neutrons) these will get absorbed into the lead nuclei (which are much bigger, and more stable, than other common elements), where they will accumulate. Does lead have several stable isotopes? If so, then it can absorb a limited amount of radiation "permanently". Otherwise, some of the lead atoms will become transmuted into something else (possibly radioactive)... although these will still be distributed among all the other sheilding lead atoms, so it can probably take a very high dose of nuclear radiation before it becomes substantially radioactive itself? techmind, Fri, 4th 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.

Compton scattering occurs at higher (mid-range) photon energies, what happens here is the photon comes in, strikes the electron, freeing it and giving some kinetic energy to its momentum and then deflects off at a lower energy at the opposite angle and same direction as the departing electron.

If neither of these happen and the photon is greater than or equal to 1.022MeV of energy, then pair production can happen when it flies close to the nucleus of the atom, when this happens the photon converts all of its energy into two particles, an electron and a positron which leave at 180 degrees apart traveling with momentum (kinetic energy) equal to the (photon energy-1.022MeV) / 2.

Still another reaction that can happen is the electron can completely absorb the photon, but in this case the photon energy is not enough to free the electron, so the electron emits its excess energy in one or more "characteristic x-rays."

Secondary photons (gamma or x-ray from interactions like compton scattering and bremsstrahlung) in  a radiation shield are responsible for what is referred to as "buildup." Buildup through a dense shield like lead emits a strong secondary radition field out the other side. This secondary radiation field needs to be stopped, since the secondary radiation is usually at a much lower energy, water is chosen as a cheap, lightweight shield. When you get x-rays at the hospital, your body tissue absorbs the secondary radiation from the lead apron you're wearing.

I know this is an old post but I hope it helps.
BechtelEngineer, Mon, 5th May 2014

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.

At very high photon energies, where Compton scatter dominates, concrete and steel - or even water - are cheaper alternatives.  alancalverd, Mon, 5th May 2014

Lead is a dense metal with high atomic number.
Lead, leaded glass, steel, and concrete are used in nuclear research as shielding against neutron radiation and it can shield all types of harmful radiation x-rays, alpha-rays and gamma rays. mediray, Fri, 27th Jun 2014

Iron is the most dense which you can see looking at the fusion fission graph iron peaks at the top of the fusion aspect. Where lead is often seen as the stable point in fission biproducts because of its weird properties. This weird stabilization of lead is why it absobs so well. Otherwise iron the most dense or say bismith loads of protons, would be better at absorbing. But they aren't so it has to do with leads structure. mike, Tue, 25th Nov 2014

I have an upcoming research about common household materials that can be an alternative to lead, any suggestions? John Ruiz, Mon, 13th Jul 2015

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