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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Bored Chemist wrote ...
Aargh! Why is it that people think 1 lead is particularly dense? 2 The density is the reason why it's a good Xray shield?
An atom of lead weighs roughly twice as much as an atom of silver yet the two metals have about the same density. That means that the lead atoms are not actually densely packed at all. Another way to look at it is that gold has atoms that are a little lighter than lead, but it's nearly twice as dense.
The big advantage to using lead is that it has a large nuclear charge and so it scatters Xrays well. The scattering cross section varies as the square of the atomic number. That's what makes lead a good choice in spite of its low density. (It's also relatively cheap and easy to work with).
Other materials are sometimes used. Concrete and steel because they are cheap. Tungsten ( because it actually is dense, so a thinner layer can sometimes be used than if lead were chosen) And, rather oddly, depleted uranium. Uranium is very dense and has an even higher scattering potential than lead. It's a bit radioactive but the alpha radiation from lead can be stopped with a sheet of aluminium foil (or a couple of sheets of paint).
When a beam of monoenergetic photons of incident intensity Io, penetrates a substance of mass thickness x and density Ï, it emerges with intensity I given by this here exponential attenuation law: I/Io = exp{-(μ/Ï)x}
where mass thickness is mass per unit area, given by multiplying the thickness t by the density Ï, i.e., x = Ït.
"Overall attenuation depends on both the target’s mass density and its atomic number."*
* Boyes J, The Effect of Atomic Number and Mass Density on the Attenuation of X-rays
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
Aluminum is one of the worst metals to try shielding x-rays with, it's virtually transparent. You'd be almost as well off using plastic. Both lead and steel work reasonably well, but the steel has to be quite a bit thicker.
Registered Member #3766
Joined: Sun Mar 20 2011, 05:39AM
Location:
Posts: 624
what about mercury? Nice and dense, in the sixth period like lead, and it's liquid! Just have to be careful with it, hazardous material and whatnot. [EDIT] I get the feeling that Proud Mary is about to shoot this down...
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
AlexRU73 wrote ...
Copper shield not good?
It would not have the same problems as Hg - re-radiating its own strong characteristic spectrum - but of course you would have to use a much thicker layer than you would with Pb.
You'll see that copper's strong characteristic emissions fall below 10 keV - soft rays that are easily stopped by a few millimetres of Al. You could use so-called 'graded Z' (i.e. Atomic Number) shielding - a Cu layer to block most of the radiation, and then an Al layer to block the low energy fluorescence of the Cu.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Proud Mary wrote ...
Bored Chemist wrote ...
Aargh! Why is it that people think 1 lead is particularly dense? 2 The density is the reason why it's a good Xray shield?
An atom of lead weighs roughly twice as much as an atom of silver yet the two metals have about the same density. That means that the lead atoms are not actually densely packed at all. Another way to look at it is that gold has atoms that are a little lighter than lead, but it's nearly twice as dense.
The big advantage to using lead is that it has a large nuclear charge and so it scatters Xrays well. The scattering cross section varies as the square of the atomic number. That's what makes lead a good choice in spite of its low density. (It's also relatively cheap and easy to work with).
Other materials are sometimes used. Concrete and steel because they are cheap. Tungsten ( because it actually is dense, so a thinner layer can sometimes be used than if lead were chosen) And, rather oddly, depleted uranium. Uranium is very dense and has an even higher scattering potential than lead. It's a bit radioactive but the alpha radiation from lead can be stopped with a sheet of aluminium foil (or a couple of sheets of paint).
When a beam of monoenergetic photons of incident intensity Io, penetrates a substance of mass thickness x and density Ï, it emerges with intensity I given by this here exponential attenuation law: I/Io = exp{-(μ/Ï)x}
where mass thickness is mass per unit area, given by multiplying the thickness t by the density Ï, i.e., x = Ït.
"Overall attenuation depends on both the target’s mass density and its atomic number."*
* Boyes J, The Effect of Atomic Number and Mass Density on the Attenuation of X-rays
Exactly, And, as I said, lead isn't very dense. That's why tungsten sometimes gets used; it really is dense.
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
Well whatever the case, lead works well as shielding, it's relatively cheap, easy to get, and because it's soft and has a low melting point, it's easy to work with and form into the desired shapes. Yeah it's toxic, but in metallic form it's fairly safe to work with so long as you wash your hands after handling it and don't try to sand or grind it.
Registered Member #3766
Joined: Sun Mar 20 2011, 05:39AM
Location:
Posts: 624
Proud Mary just posted this in another thread-
Proud Mary wrote ...
In fact, X-rays which pass through the body are much less dangerous than soft (low energy) X-rays, which are wholly absorbed, transferring all their energy into the tissues.
In diagnostic radiography, aluminium filters are employed to block these soft X-rays, which carry the greatest health risk, but contribute nothing to the image.
so it looks like aluminum is good for something after all.
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Alternative Shielding
