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
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
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:
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).
- 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.
- 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.
- 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).
Hope that helps,
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