How does lead block radiation?

How exactly does the element Lead stop radiation? Does it reflect the particles back or just plain absorb them? And if it is absorbing them, doesn’t it get “full” of neutrons, alpha particles, or other subatomic particles? Does the Lead get changed into other elements or other nuclides of Lead if it absorbs all those neutrons?

Lead is dense and has a large number of electrons so it is good at scattering (is probably a better word than reflecting) electromagnetic radiation (gamma rays, X-rays)

Led is not particularly effective against beta radiation (high energy electrons). In fact lead shielding can make it worse because that can create bremsstrahlung radiation, which can be more dangerous that the original radiation.

Lead is not useful as an absorber of neutrons either.

I’m not sure about alpha particles.

For neutron radiation, what you want is a high density of hydrogen atoms, since nothing but nuclei matter for neutrons, and they scatter most strongly off of nuclei with approximately the same mass as themselves. So plain ordinary water actually ends up being one of the best things to use for shielding against them.

In this situation, “Scattering” is not just scattering — it means that the lead absorbs some of the energy and gets warmer. Inside the lead, the electrons are scattered. The amount of energy it takes to scatter an electron depends on the weight of the nucleus (?), and the chance of scattering depends on the number of electrons. X-rays are just energy, so when you “absorb some of the energy”, they aren’t x-rays any more.

You can actually do better with an alloy, because when you make an alloy it changes the behavior of both the electrons and the nucleus. And you can do better than lead, because bismuth or Tin or Aluminium alloys are less toxic than lead. And in many situations, the volume is less important than the weight, so you can get equivalent or better x-ray shielding by just using more of a lighter metal.

Or a whole huge amount of cheap concrete or dirt.

Alpha particles from radiation can be blocked by pretty much anything. A sheet of paper does a good job.

They are dangerous if ingested, but most of the time they can’t go through skin, so lead is going to stop them very effectively.

As I recall from Nuclear Power School in the Navy years ago, the key point with neutrons is to slow them down. Heavy elements like lead don’t do the job because the nuclei in heavy elements are far more massive than the neutrons, so the neutrons bounce off them with little loss of kinetic energy. However, when a neutron collides with a hydrogen nucleus (i.e. a proton), the neutron loses much of its kinetic energy in the collision (by transferring approximately half of its kinetic energy to the hydrogen nucleus). Ordinary water contains lots of hydrogen atoms, so it is very effective for slowing down neutrons.

Other substances that are used include hydrocarbons (like diesel fuel, which is why the diesel fuel tank for the emergency diesel generator is placed between the reactor compartment and the forward operations compartment on a nuclear submarine), and plastics.

The discussion of water and neutrons reminds me of our neutron source in the Physics dept. in college.

A largish barrel of water with the source on a rod in the center. To irradiate something you put the sample on a shelf under the surface. Raise the rod till the source is next to the shelf. Time it. Lower the rod. Retrieve the sample and put it in the counter or whatever you’re doing.

I always wondered why the shelf wasn’t the part that was raised or lowered.

Anyway, a couple feet of water was enough shielding for a small source like this.

At another place they had a small reactor. After firing it up they discovered too much radiation outside one wall of the building. Solution: just stack some concrete bricks doped with lead against that wall. I could see those bricks from my window next door. I felt so safe. :rolleyes:

Anyway: the goal is to disperse the energy, the particle itself is irrelevant. Like a stunt person falling off a building, you don’t want to land on a hard surface. A soft cushy thing is better. So electrons or Hydrogen nuclei.

From I read here, it seems like lead is a poor choice of shielding from radioactivity. Why then does it seem to be the material of choice for the job?

It sounds like it’s good for certain sorts (like X-rays and gamma rays), as OldGuy notes in the first reply, but not good for stopping neutrons or eletrons.

So, my guess would be that it’s the material of choice for certain applications, but not for others.

Alphas and betas are easily stopped by common, ordinary materials, so there’s seldom need for purpose-made shielding for them. Neutrons aren’t well-stopped by lead, but neutron radiation is relatively uncommon, and (as mentioned above) even if you do need to shield neutrons, you can often make something like a ballast or fuel tank serve double-duty. So when you need shielding per se, it’s usually against gamma rays, and that’s what lead is good for.

Probably learned a lesson from the death of Louis Slotin.

Where my brother worked there were bags of pennies stacked up against the wall of one room…

Anything with lots of hydrogen works. You have to take them from fast neutrons down to epi thermal then thermal neutron speeds then absorb them. As stated, the collisions with similar mass nuclei does the best job of quickly slowing down the fast neutrons. Once you have them down to epi and thermal neutron speeds they can be readily absorbed by Boron or chorine. For chemical neutron source shielding we used wax mixed with borax. Cheap and effective, if a little bulky.

The risk caused by radiation is really the energy contained by the particle rather than the particle itself.*
Lead is very dense and has a very high electron density, generally speaking it is the electron cloud that absorbs the energy of the radioactive particle. That energy absorbed may get reradiated as lead x-rays. Anything really works so long as it has a lot of electrons. Lead is good as it has a high electron density in a fairly compact space. But it’s expensive, generally any of the transition metals are good for alpha beta and gamma and if not then just go for thickness and quantity, so concrete is fine, you just need more of it, but it is cheap and long lasting.

Alpha Particles – basically a helium nucleus moving very quickly . It will bash into the electron cloud and lose energy very quickly. Once the energy is absorbed there is just a helium nucleus floating around that will probably join up with another one and be harmless. The lead block remains unaffected and doesn’t fill up with alpha particles or in its self become radioactive or contaminated

Beta particles – basically a fast moving electron. It will bash into the electron cloud and because it is smaller* tends to get through more stuff and slow down, loose its energy and eventually just be another electron in that big sea of electrons. The energy the beta particle had has been absorbed by the lead , although as there was quite a lot of energy, some if that may have been reradiated as an x ray. The lead block remains unaffected and doesn’t become radioactive or contaminated.

Gamma rays – basically high energy photons or bits of light. These tend to have more energy and penetrate deeper, but still ultimately scatter off the electrons, losing energy and the photon is eventually absorbed. So again there is no change to the lead , it hasn’t become radioactive or contaminated or fill up with gamma rays as those have been absorbed. Some of that absorbed energy may get reradiated as an x ray. Often lead shielding is accompanied by a thin layer of cadmium that does a good job of absorbing the reradiated x rays (50 KeV if I remember correctly) , and if the radiation doesn’t get you , heavy metal poisoning will.

Neutrons are a different beast and actually will make something radioactive via neutron activation, also neutrons interact with the nuclei of atoms , not the electron cloud. To effectively shield against neutrons they need to be slowed down then absorbed by a nuclei of something that won’t in itself form a dangerous radioactive isotope. As others have mentioned you need a nuclei of similar mass to the neutron to effectively absorb its energy through collisions. A neutron bashing into a large heavy lead nuclei will basically bounce off with the same amount of energy it hit with, so it won’t slow down. If it hits a hydrogen nuclei, which is the same mass ,it will slow down much more quickly on each collision ( think of billiard balls bashing into each other). Once it has slowed down it should be absorbed with something with a high neutron capture cross section, Boron is pretty good for that and chlorine although chlorine has its problems because if the neutron don’t get you, and corrosive gas will. So if you bash enough neutrons into a lead block, it won’t do a very good job of slowing them down, but you may create some radioactive isotopes of lead. In high flux neutron environments , neutron activation making the steel and materials around radioactive, in itself is a problem, but there are some other knock on effects. Steel has a lot of alloying metals in it, if you pick badly you may find some of those alloying materials get easily activated and form some nasty short half life isotopes. Another problem is the energy of neutron flux itself. All those neutrons start to bash into the nuclei of the steel crystal structure and can knock them out of place and start to affect the crystal structure, which typically makes the steel stronger (raises the ultimate tensile strength) , but harder, and hardness leads to an increased risk of cracking , which is an undesirable situation in something containing something with a large neutron flux.

  • We could get into discussion of quality factor and greys vs Sieverts, but we can leave that for now
    *smaller works, in reality you have to look at capture cross section,

When you’re getting a dental X-Ray they put a lead apron over your body because you’re getting hit by x-rays, and lead absorbs x-rays just fine. In fact, anything would absorb x-rays. You just need a certain amount of stuff to absorb it. The reason they use lead is that lead is very dense, which means you can have the same amount of stuff but the apron itself is a lot smaller. You could use a fabric apron, but you’d need a gigantic thick fabric apron instead of a thin lead apron for the same amount of absorption.

Lead is a poor choice for stopping neutron radiation. But as Chronos noted, neutron radiation is not that common. It comes primarily from fission (and fusion) reactions, which means it’s mainly an issue of concern with nuclear reactors and nuclear bombs.

The types of ionizing radiation that come from unstable radioactive materials include alpha particles (charged helium nuclei), beta particles (high energy electrons or positrons), and gamma radiation (high energy electromagnetic radiation).

As others have stated, alpha particles and beta particles, since they consist of charged subatomic particles, are easily blocked by just about any material. To attenuate gamma radiation, though, you need far more material (relatively speaking), so the denser the material, the better.

Lead is a good choice to shield against gamma radiation, because it is very dense and fairly cheap.

One of the rules of thumb I was taught in Nuclear Power School involved the idea of “tenth-thicknesses,” which were the thickness of various materials needed to attenuate gamma radiation by one-tenth. For example, lead had a “tenth-thickness” of two inches. In other words, two inches of lead shielding would reduce the gamma radiation flux from a source by 90% (to one-tenth of the original flux). Four inches of lead would reduce the flux by 99% (1/10 x 1/10 = 1/100).

So anyway, the tenth-thickness of lead is approximately 2 inches. The tenth-thickness of iron/steel is approximately 4 inches; the tenth-thickness of concrete is about 12 inches, and the tenth-thickness of water is about 24 inches. So all things being equal, lead is twice as effective at shielding gamma radiation than steel, and more than 10 times better than water.

Lead may be more expensive than concrete, but it’s fairly cheap so far as metals go, especially when you compare it to similarly dense metals.

Nitpick…helium is a noble gas. A helium nucleus or helium atom is not going to join up with another one (or anything else).

Right - this is a good explanation of the so-called “neutron embrittlement” of metals. Like you allude to, this is a concern with nuclear reactor pressure vessels.

Yeah had halogens on the brain there ( I had been exchanging bad chemist puns with my daughter earlier)
Agree, expensive is a relative term, and driven by how much space you have available or convenience. We used tungsten as a Cs137 collimator in a downhole tool where we didn’t have a lot of space. A lump of tungsten is just a spooky weirdly heavy thing.

Correction: That should be “…the thickness of various materials needed to attenuate gamma radiation to one-tenth [of its initial value].”

Also, to be more precise, tenth-thicknesses depend not only on the shielding material (e.g. lead vs. water), but also on the energy (in MeV) of the electromagnetic photons in question. As you might expect, higher-energy gamma radiation is attenuated less and is more penetrating for a given thickness of a given material.

This sounds to me like Superman’s x-ray vision, which is completely stopped by lead, but nothing else.

Lead does not ‘stop’ x-rays, just attenuates them. Other materials are better or worse, but nothing completely stops gamma radiation. Alpha radiation is practically stopped by our layer of dead skin cells, beta by not much more.