O.k, so we have an X-ray machine pumping out X-rays, then we have lead shielding stopping the X-rays going where they are not welcome. My questions are.......

1)What happens to the X-rays?
2)Does the lead reach a point where it becomes useless as a shield due to X-ray saturation?
3)Is the lead changed in any way by the X-rays?

So many questions, perhaps I`m suffering from lead density!

I can have a tendency to be too technical in these explanations, so please let me know if my writing is too opaque. I'm working towards a doctorate in physics and it's a pleasure to try to explain this stuff (and hopefully build interest in my field!)

The first thing to understand is what X-rays are: they're photons, particles of light, which are basically little bundles of energy. (Understanding what they are isn't trivial, since sometimes they act like waves and sometimes they act like particles, but we're dealing with how they interact with matter (lead), so in this case they behave like particles.) X-rays are dangerous because each photon carries a lot of energy, as compared to visible light or infrared.

Lead is chosen for shielding because it's cheap and because it's very dense, so a photon traveling through a lead block has a very good chance of being stopped by the lead. By stopped, I mean absorbed; the photon essentially collides with an electron. The photon stops existing, and the electron has a lot more energy than it did before (the photon energy has to go somewhere!)

What does it mean for an electron to have more energy? In the everyday world there are a lot of different ways to store energy, but an electron is so small and so basic there's only one way: it's got to move faster. Having absorbed an X-ray, it now has so much energy that it's no longer bound to its atom, and it bounces around inside the lead block (we call this an excited state of its atom). As it collides with other electrons, it gradually loses its energy; usually it only travels a couple of millimeters within the lead block before it is recaptured by a lead atom. Over the course of the collisions the energy is converted to heat (which is really just very low-energy photons); if the lead is shielding a very high-energy radiation source, it will feel slightly warm to the touch.

Not every photon will be stopped by the lead (it's a matter of statistics; no matter how thick your shield, there's always an outside chance the photon will escape), but almost all of them are.

So, to sum up and respond to your specific questions:

1. The basic mechanism is that the X-ray is destroyed in an interaction with one electron, and the energy it carries is gradually converted to heat.

2. The lead cannot be saturated by X-rays, as these are destroyed and the energy is re-radiated safely as heat. When used as a shield for X-ray radiation, the lead will therefore not become useless over time. (There are other types of radiation for which lead does not work well as a shield; neutron radiation, for example, will make the lead block radioactive. Other types of shielding (e.g. water and concrete) work better for those types.)

3. There are tiny, short-lived changes to individual lead atoms, but no large or long-term changes.

I hope this satisfies your curiosity!

Well, I didn't ask the question, but I must say very nice and clear explanation. Thanks!

Even I could follow that, you are fantastic at explaining a subject which would cause many of us to just let it wash over us. Take up teaching.

I ditto what Taesma and Sue said! That was interesting.

Cellardoor thanks very much for the easy to understand information. I now know who to contact the next time I hear my wifes astrophysicist friend is coming over!

Yes ClaraSue, it was interesting, and to be able to include the word physics and interesting in the same sentence is quite something for many of us. A rare talent.

Aw shucks! You're most welcome; I find it a pleasure to talk about my field. (Plus, as someone who is paid out of government funding, it's in my best interests to have taxpayers around the world thinking, "Physics is cool!")

I've always thought physics is cool. I just have a hard time wrapping my mind around some of the concepts. I wish I did understand better. If textbook explanations were written as clearly as yours, I probably wouldn't have had to drop that physics class in high school.

I agree with all the previous compliments. The original question was answered accurately and simply, and also in an interesting manner.
Perhaps you could try creating a few quizzes on this fascinating subject?
Many years ago, I studied both electrical engineering and nuclear physics, but chose the former for my ultimate career path.
Best wishes with your doctorate studies, and I'm sure your papers will make fascinating reading.

http://www.funtrivia.com/trivia-quiz/SciTech/Special-Relativity-and-the-Space-Patrol-218860.html

As Cellar Door is much too modest and in fact, we don't encourage people shamelessly plugging their work, I'd like to guide you to the top quiz in the Physics category here. Perhaps you hadn't seen it.

Okay, here's an extention to the above question: When I was a kid and was fascinated by nuclear war, I remember reading that for some types of radiation paraffin wax is the best absorber. (For others it was lead or concrete.) So your ideal bomb shelter would be a concrete structure, with an outer skin of lead and a middle skin of paraffin, sort of like subcutaneous fat.

Trouble is, I haven't seen this again since I've become educated enough to understand it. Is this some discredited theory that has rightfully vanished? Or is it true, and I just haven't looked hard enough? Are there some forms of radiation for which paraffin is the ideal absorber, and why?

OK, first: sorry for staying away from this thread so long! I've been pretty busy over the past couple of weeks. The short answer is that paraffin and concrete work pretty much the same way, and concrete is safer to use in a bomb shelter, so your ideal bomb shelter has a thick layer of concrete on the inside and an inner skin of lead. Here's the long answer:

Paraffin wax is indeed an excellent absorber for a particular type of radiation, fast neutrons -- but concrete is also an excellent absorber for fast neutrons, not to mention being cheaper and easier to work with. I've used radiation sources that were shielded with paraffin, and I'll go into how that works in a moment, but it would be a poor choice for a bomb shelter. It's brittle at room temperature (do you want the insulation of your bomb shelter breaking when you slam the door?) and it burns readily, so if there are fires in the wake of that bomb blast it might get unpleasant inside the shelter.

There are four basic types of radiation:

Alpha particles are just the nuclei of helium atoms -- two protons and two neutrons, stuck together in a ball. They're heavy and charged, so they interact easily with matter: they only travel about two inches on average before being stopped in AIR! You basically only need to worry about alpha particles if you swallow an alpha emitter; your skin will block it, but tissues inside your body are more fragile. This is how the Russian spy Alexander Livitnenko was killed last year; the Polonium 210 that he was given is an alpha emitter.

Beta particles are single electrons. Like alpha particles, they interact readily with matter and so are easily stopped: your skin is enough shielding to keep you safe. Beta emission from nuclear bombs is not really considered a threat.

Gamma rays are photons; x-rays fall into this category, and we discussed how lead atoms stop them earlier. Concrete will also protect you against gamma rays; you just need a lot more thickness than you would of lead, because the gamma rays are less likely to interact with any given molecule. Lead is popular partly because shielding can be thin; if you're building a bunker with four-foot concrete walls already, though, a lead layer is not really necessary.

Neutrons are one of the most dangerous types of radiation to humans. Because they're not electrically charged, they don't interact with matter quite as easily as alphas or betas and so they need more shielding. Even worse, though, is that when they are captured by a nucleus (becoming part of it) they can often make it radioactive: the addition of a neutron to a nucleus can make it unstable. The trick with fast neutrons is to slow them down so that they can be captured by a nucleus, and THEN worry about shielding any additional radiation. The best way to slow them down is to involve them in a lot of collisions with (relatively) stationary particles of about the same mass: then half the kinetic energy goes to each particle, and the next collision cuts the neutron's energy in half again, and so on. So you want your shield to have a lot of protons, which effectively means a lot of water (the hydrogen in water makes it an excellent approximation of a proton target). Since concrete contains a lot of chemically-bound water, it's a great shield for fast neutrons. (Paraffin works the same way, and is used a lot in shielding applications -- but its brittleness and flammability make it a bad choice for a bomb shelter.)

So concrete is the best choice for a bomb shelter, by far. If it's thick and strong enough, it'll block alphas, betas, gammas, and neutrons -- as well as most of the gammas that are released in the process of blocking the neutrons. Extra lead lining on the inside would shield the rest of the neutron-generated gammas, but it's a bad idea to put the lead on the outside (since it does not respond well to being bombarded with electrons.)

A paraffin layer would add complication, but wouldn't really do anything that the concrete couldn't. The only thing you need to watch out for is the lead on the inside: better cover it with something so you don't get lead poisoning while you're in there!

Also, thanks to Bruyere for shamelessly plugging my quiz. Very sweet of you.