MadSci Network: Physics

Re: How does lead protect electronics from radiation?

Date: Fri Nov 13 07:38:40 1998
Posted By: Georg Hager, Grad student, Theoretical Particle Physics
Area of science: Physics
ID: 902935796.Ph


Shielding from radioactive radiation is not only done for protection of electronic circuits, but for many other purposes (mainly to protect people). I don't know what your understanding of radioactivity is, so I will first try to give a short outline of the different kinds of radioactive radiation and then describe how they may be shielded off.

Radioactive radiation comes in three varieties: Alpha, beta, and gamma radiation. Alpha radiation consists of helium nuclei, and is easily shielded by even a simple sheet of paper. Beta radiation is composed of electrons, and is almost as easy to shield (some pretty arbitrary, thin metal would do). Gamma radiation is by far the most penetrating of the three. It consists of very energetic X-rays. This is just a form of light (photons), but as it is so energetic it can do damage to living tissue and electronic devices.

What a heavy lead shielding does is essentially try to keep gamma radiation away (for the other two one would not have to put in that much effort). Now what is the physical mechanism behind the shielding effect? Essentially, there are three ways that gamma rays interact with matter:

  1. Compton scattering at free or nearly free electrons. This means that a gamma photon comes in and gets scattered by an electron which occupies a very high energy level in an atom, or which is free. The electron takes the energy and emits another, less enegetic photon. This mechanism applies for low energy gamma radiation, and is insignificant at the energies we are considering here.
  2. Photoelectric absorption. In this case, the gamma photon kicks out an electron from a low-lying energy level of the atom, i.e. a tightly bound electron. The probability that this happens is proportional to the fifth power of the charge of the nucleus, so it is definitely a good idea to choose nuclei that are as heavy as possible. Lead is the typical choice here. Photoelectric absorption is important up to the energies of radioacive gamma rays.
  3. At even higher energies, pair production becomes the dominant process. In the electric field of a nucleus, a gamma photon having enough energy can spontaneously be converted to an electron-antielectron pair. This is also the more probable the bigger the charge of the nucleus is (it goes with the square of the charge).
So you see that it is good to choose a material which has heavy, stronlgy-charged nuclei for shielding gamma radiation. Lead is the typical choice because it is cheap and easy to process (although its density is quite high, which is a drawback for practical purposes).

Hope that helps,

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