page 3-7
Ionising radiations

Energy graph
Picture 15. Gamma rays have a distribution of energies. A bit like the distribution of children's heights.
Case study - improving the screening system
Scientists at Corus' Swinden Technology Centre near Rotherham have developed a more sensitive and reliable system. It uses the fact that gamma radiation can be given out with a range of energies. As well as detecting gamma radiation, the new system measures the energy of each gamma ray that enters the detector.
Using scrap
You can see this on a graph in picture 15. It shows how many gamma rays there are in each band of energies. This is like a frequency diagram of, for example, number of children within each band of heights in your year group.

Energy graph
Picture 16. The system resamples the count once every second.

Energy graph
Picture 17. The energy distribution for caesium. If this is added to the background count, the system will sound the alarm.
Sample the sample
The system plots a graph once every second. The random nature of the background radiation means that there are fluctuations. However, a change in the background count will usually be similar across all the energies. For example, if a lorry cuts out some of the background count, the whole graph will drop down (roll over on graph). The system will then adjust for this and put everything back to normal.
Detecting a source
If there is a source of gamma radiation in the lorry, it will give out a range of energies that is different to the shape for the background count (picture 17). When radiation from this source passes into the detector, the count goes up. But, more importantly, the shape of the graph changes. The system can measure the change and determine that this is due to a radioactive source rather than a change in the background count. It will then set off an alarm.
A. Energy graph for qn
Question 3-7.

a) Which of these are reasons for using a gamma detector rather than alpha or beta?

A. gamma radiation has a range of energies
B. alpha particles wouldn't reach the detector
C. gamma radiation is given out in most radioactive decays (along with alpha or beta)
D. beta radiation would be absorbed by only a few layers of scrap on top of its source
b) Look at the graphs on the left. They show the range of energies for a truck coming into the melting shop. Imagine you were in charge of the alarm. In each case, decide how you would respond and make your selection.
A. C.
B. D.

Summary                   Close
  • Gamma rays are given out with a range of energies
  • The background count has a 'fingerprint' graph
  • Any contamination will distort the energy shape of the graph
  • The new system measures changes in the shape of the distribution graph