page 3-6
Ionising radiations

Photo of scrap detector
Picture 13. Detectors suspended above a road to check wagons.

Case study - a simple screening system
A simple screening system
The aim of the detection system is to screen all wagons bringing in scrap and stop them if any radiation is detected. The basic system uses a scintillator suspended over the road or rails. This picks up gamma radiation. If the count rises significantly from the background count, the truck can be stopped, unloaded and each item of scrap is screened.
Drawbacks of simple system

Photo inside scrap detector
Picture 14. This is what is inside the box. The long tube is the scintillator, which picks up the gamma rays.

This simple system has a number of drawbacks:
Reason Details (roll over the word 'show' for each reason).
Try to think of the reason first.
The background count causes random fluctuations show
Radioactive decay is a random process. Therefore there are random changes (up AND down) in the background count (bgc). The system has to allow for these.
The background count can change from day to day show
Changes in solar activity can affect the amount of cosmic radiation. Also, changes in the wind direction can change the bgc as natural radioactive sources are carried into the area.
Sources can be screened by all the scrap on top of them show
If the radioactive source is right at the bottom of the truck, then a lot of the gamma radiation is absorbed by the scrap on top of it. The amount of radiation that reaches the detector may not be much above background count.
The truck often reduces the count show
In some places, a significant amount of radiation comes from the ground and even from the tarmac. When the truck drives into the bay, it actually shields the detector from this natural radiation and reduces the count.

Question 3-6.

Fill in the gaps in the paragraph below. Choose the best answer in each case.

Scrap metal can be with radioactive isotopes. These can be detected using , which is suspended above the road. This picks up any given off from within the truck. However, they can sometimes be difficult to detect because . Also, the background count can change from day to day because of changes in .

Summary                   Close
  • The old screening system was not very sensitive
  • The metal in the wagon might shield the source
  • It did not account for changes in background count