Asked by: Carl Bailey, Dundee
While you’re stepping through an airport metal detector in your socks, your hand luggage is going on a different trip: into an X-ray scanner.
They’re used to build a picture of what’s inside your bags, and allow security staff to check anything that looks suspicious. X-rays launched from one side of the machine are picked up by a pair of detectors on the opposite side. As your bag enters through the lead-lined curtains, it crosses the path of these X-rays and absorbs some of the energy they carry. This means that the X-rays that passed through your stuff have less energy than those that sailed straight past.
When the X-rays hit the first plate-like detector, their energy and position is recorded. They continue towards the second detector, but a filter between the two blocks low-energy X-rays: the second detector collects only high-energy X-rays. By comparing the two detectors’ outputs, the machine can construct an image showing not just the position of objects, but also roughly what they’re made of and their density.
Organic materials like paper, food and explosives are orange, while blue or green are used for metals and glass. The denser the material, the darker the colour.