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.
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.
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.