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How shields work

Radiation and the atom · level 7, lesson 1 of 6

A question to start

A sheet of paper stops alpha particles dead, yet a metre of concrete does not stop every gamma ray. Why do the two behave so differently?

The idea

A shield takes energy away from radiation. How it does this depends on the radiation.

  • Charged particles (alpha, beta, slow protons) pull on electrons all along their path, knocking them out of atoms and losing a little energy each time. They slow steadily and stop at a fairly definite depth, their range.
  • Gamma rays and X-rays have no charge. Each photon goes on untouched until, by chance, it hits something and is absorbed or knocked aside. So the beam fades by the same share in each layer, and there is no range.
  • Neutrons have no charge either. They lose energy by bumping into nuclei. They lose the most when they hit something of their own mass, so hydrogen-rich water and plastic are the best neutron shields.
  • Very fast cosmic rays can smash nuclei in the shield. The pieces fly on as secondary radiation, which can itself be harmful.
Radiation How it loses energy Good shield
Alpha ionising, along a short track paper, skin
Beta ionising, along a longer track plastic, a few mm of aluminium
Gamma photon by photon, by chance lead, thick concrete
Neutrons bumping into nuclei water, plastic
stops: its rangesome get throughsecondaries

Worked examples

1. Alpha in air. An alpha particle travels only a few centimetres in air before it has lost all its energy.

2. Gamma. A beam that loses half in each 0.86 cm of lead keeps \(\tfrac12\) after one layer, \(\tfrac14\) after two, \(\tfrac18\) after three. It never quite reaches zero.

3. Neutrons. A neutron that hits a hydrogen nucleus head-on can hand over nearly all its energy at once, like one snooker ball striking another of the same mass. Hitting a heavy lead nucleus, it just bounces off, keeping almost all its energy.

Try it

  1. Why do charged particles have a range, but gamma rays do not?
  2. Why are water and plastic good neutron shields?
  3. What is secondary radiation?
  4. Which shield from the table would you choose for a beta source?
Answers
  1. Charged particles lose energy steadily all along the path. Gamma photons are removed one at a time, by chance.
  2. They are rich in hydrogen, whose nucleus has about the same mass as a neutron.
  3. Particles knocked out when fast radiation smashes nuclei in the shield.
  4. Plastic, or a few millimetres of aluminium.

Common mistake

"The densest shield is always best." Dense lead is good for gamma rays. Against neutrons and cosmic rays, light, hydrogen-rich materials do better, as lesson 4 shows.

Where it leads