Physics of lead shielding question

I hear tin foil keeps the voices out pretty good, Eric. There's a guy in my old Seattle neighborhood who lives in a bus shelter who always has his tin foil pointy hat on ..

just kidding ..

GWE

Look up alpha, beta, and gamma rays; x-rays; neutrons. It's worth reading for yourself. It ranges from a sheet of paper or plastic for alpha; a sheet of wood, aluminum foil, or maybe 1 cm of water for beta; up to several cm of lead or a meter of concrete for gamma.

It gets more complicated with ionization, but us non-physicists can understand the basics.

-- Ed Huntress

Sort of in the same area.. I was checking out one of the mobile container X-ray machines Dubya insisted we buy. The operator demonstrated how it could x-ray through 290mm of steel. Another handy thing is that it produces scans of containerised autos that could just about be used for blue prints.

German & French technology protecting the sanctity of US imports! :-)

Tom

Whatever that is you're talking about, I hope I never meet up with one. Will it microwave chicken?

-- Ed Huntress

I remember a professor explain it thusly to a visiting group of high school kids (he was talking about half-life, but the idea applies to lots of things...): "Imagine a guy and his girl sitting at opposite ends of a couch. They agree to move every ten minutes so that the distance between them is divided in half. They never quite get together, but they get close enough for all practical purposes."

Could some form of radiation get through a mile of lead? I suppose it might be possible, but it would be impossible to detect. Everything radiates. No matter how hard you try, everything WILL contain some radioactive impurities. Consequently, the background radiation level will be vastly greater than anything that might, somehow, find its way through your mile of lead. Hence, radioactive shielding beyond what is necessary to attenuate the level of radiation well below the background radiation level is kind of pointless...

Jerry

I don't think anyone has addressed the question Eric was asking. He drew an analogy between radioactive decay and radioactive attenuation. I think his question was: does the radiation going through a lead shield go to zero if the lead is thick enough, or is it asymptotic to zero, like the space between the two lovers on the park bench? I believe it is the latter--the radiation level theoretically never goes completely to zero, but you might have to wait a LONG time for a particle to arrive through a mile of lead.

That's correct - it's a probability thing. Any given unstable atom may decay at any time, it may decay a microsecond from now or a billion years. It's a 50/50 probability that it will decay in the half-life time.

Again it a matter of probability whether or not a radioactive particle will be absorbed, no matter how much material it penetrates. The probability of a particle penetrating a mile of lead would be extremely small but I suppose it could happen given enough radiation and enough time waiting for it to happen.

Laurie Forbes

======================= Back in the 60's I learned in my nuclear engineering classes that shielding has the same type of mathematical expression that half life does. For example if 1 inch of lead cuts the radiation level by 1/2, then 2 inches cut it to 1/4, 3 inches to 1/8 and so on. I forget what the technical name for the absorbtion unit is, but there were tables for materials and radiations/energies.

I remember using a slide rule with the log scales to calculate how many thicknesses were needed [and thus how thick the total shield] to cut a radiation field from X to Y. Given the high radiation flux around the reactors the shielding required was very thick in a real world situation.

If tinfoil hats are popular, then lead lined jockstraps should be a sell-out.

Uncle George

Half life and attenuation with thickness of shielding material are separate issues. Half life has to do with time, not shielding thickness.

Attenuation is exponential, never is total but penetration does eventually become negligable. How much shielding it takes to do that depends on what you define as negligable penetration.

If x inches of shielding reduce penetration to fraction y then n*x inches of material will reduce penetration by y to nth power.

If penetration is reduced by y= 0.5 in x=1 inch then 10 inches would reduce it by 0.5 ^^10 or .000976, and so on.

This site has easy to understand info on lead shielding, and charts to figure out how much you need for what levels. The energy levels of gamma rays can be fairly easily googled by searching for the isotopes that emit them.

Gamma rays are attenuated by the density of the shielding material . The higher the energy of the gamma ray, the thicker the material or denser the material required to stop it. Because lead is dense and cheap it is regularly used for shielding. Alpha particles and beta can be stopped by much thinner or less dense material. (Alpha particles can be stopped by a sheet of cardboard)

Neutrons vary a great deal in energy. Because they are about the same mass as a hydrogen atom they interact strongly with water which contains a not of hydrogen. As people contain a lot of water, neutron radiation is particularly damaging. They are best shielded by material which contains a large proportion of hydrogen atoms. This why spent fuel rods are stored under water. when I worked for Schlumberger we used a steel outer container filled with polyethylene to carry our neutron sources. They had a stainless steel sleave in the centre that the source was locked into.

Tom Miller

Something that people have not mentioned is the energy of the radiation. For instance, about 1/2 mm of lead separates the two films in a medical X-ray film holder, they flip it over to take two X-rays on the same holder. That 1/2 mm stops 99.999% of the X-ray wavelength used for medical purposes (50 - 75 KeV). It would take 3+ FEET of lead to stop 5 MeV Gamma rays, just X-rays of a much shorter wavelength. That's what they use for "X-raying" reactor vessel welds and such thick material.

As for Alpha and Beta, they are actual particles with mass, not photons like the Gamma rays, and will be stopped after passing through some thickness of material. They give up energy with each near collision, due to their electrical charge. The Alpha is a Helium nucleus, 2 protons and 2 neutrons. The Beta is a single electron, and due to the high charge/mass ratio, it really has little penetration. Even a couple inches of air will stop 99.99999% of them.

Jon

I am sorry to butt in but I would be real interested in the sources you found on these container machines. The last machine shop I worked at was in an industrial complex that also housed a privately owned lab that worked on this kind of stuff. Very interesting work from what I've seen. Thank you Tom

Jim

whoops, I should have mentioned that this was for x-raying "shipping" containers.

Tom

Haven't looked them up, I was inspecting one in the "flesh" at the local port.

Tom

I'm a former SLB Sr. Scientist - but back in school when I was building a Neutron gun - Yep - fires those items down a barrel. We used among other things(Salt(s)... - a thick layer of paraffin that traps Neutron like a sponge. 12" of concrete outside and that behind a walled in area. - And I planned on being in the other end of the building !

But the paraffin is well known - just not handy down hole or on a hot truck. It was used in some bomb shelters as well as labs.

Trying to shut off all radioactivity is impossible. Some zip through the planet from side to side. We were showered by a massive blast some years ago when a super nova went off not far away. The South pole took a hit - and also the North pole.

Other than these we can't get away from - deep in a salt mine is best.

Martin

Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder

Tom Miller wrote:

What about aluminum foil? The store says they have never even seen tin foil. ERS

Leo, That's exactly what the question was. Thank you for being more concise. Eric