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Gamma

Radiation and the atom · level 5, lesson 4 of 6

A question to start

Some hospital machines aim narrow beams of gamma rays at a tumour deep inside the head, without cutting the skin. What kind of radiation can reach so far in?

The idea

Gamma rays are photons of light, like visible light but with far more energy. They have no mass and no charge. They often come out of a nucleus just after an alpha or beta decay, as it settles down.

With no charge, gamma rays do not crash into atoms all the time the way alpha and beta particles do. Many pass right through the body. They are never completely stopped; each layer of material lets through a share and absorbs the rest. Dense materials work best, so gamma sources are kept behind lead or thick concrete.

For gamma rays with an energy of 1 MeV, each of these thicknesses lets through half:

Material Thickness that halves 1 MeV gamma rays
Lead about 0.9 cm
Concrete about 4.6 cm
Water about 9.8 cm

X-rays are photons too, a little lower in energy. They are mostly made by machines, not by nuclei.

8421each layer: about 0.9 cm of lead

Worked examples

1. Three layers. Three halvings: \(\tfrac12 \times \tfrac12 \times \tfrac12 = \tfrac18\). About 2.6 cm of lead lets through an eighth of the 1 MeV gamma rays.

2. Concrete instead. To do the same with concrete takes \(3 \times 4.6 \approx 14\) cm.

3. Ten halvings. \(2^{10} = 1{,}024\). Ten half-thicknesses of lead, about 9 cm, cut the gamma rays to about a thousandth.

Try it

  1. What are gamma rays?
  2. Why do gamma rays get further into the body than alpha or beta?
  3. How much water lets through a quarter of the 1 MeV gamma rays?
  4. Can any wall stop gamma rays completely?
Answers
  1. High-energy photons of light from a nucleus.
  2. They have no charge, so they do not interact with atoms all the time.
  3. Two halvings: \(2 \times 9.8 \approx 20\) cm.
  4. No. Each layer lets through a share; a thick wall lets through a very tiny share.

Common mistake

"Lead stops gamma rays." Lead reduces them. Every 0.9 cm or so halves a 1 MeV beam, but some always get through.

Where it leads