Ion Beam Cop: Forensic science and analysis

Dr Melanie Bailey, Ion Beam Centre, University of Surrey

Physics Lives | Right click to download (MPG, 27 MB)

When forensic analysis expert Dr Melanie Bailey used to sneak biscuits from the cookie jar, she worried about leaving behind clues that might enable her mother to link her to the crime.

We meet Melanie some years later, working in the field of forensic science and carrying out analyses using ion beam applications. Although she now has her own cookie jar, she has remained fascinated by clues and uses physics to link traces of evidence to a particular crime. In fact, she has devised a completely new way to help catch the bad guys.

Melanie’s technique involves analysis of gunshot residue; small metallic particles left on the skin and clothing of suspects arrested at a crime scene. These particles are very difficult to wash off and they tend to remain on clothing for several days after the shooting takes place.

Melanie puts these samples in an Ion Beam Accelerator. This generates a beam of charged particles of either hydrogen or helium, and accelerates them to speeds of around ten per cent of the speed of light. It then fires them at her forensic samples.

She puts the residues in the machine and switches on the ion beam. When the beam hits the samples, X-rays or ‘gamma rays’ are emitted. Each element within the particle has a specific X-ray or gamma ray associated with it, so she can tell exactly what’s in the sample, and how much of it there is.

Using this method of forensic analysis, Melanie is then able to create a detailed footprint of the chemical composition of the gunpowder. From here, she can work out which manufacturer it was made by – and ultimately help the police to trace the criminal.

Her forensic analysis method is more powerful than those currently used by police and this means she is sometimes the first person in the world to work out what has happened at the scene of a crime.

“It’s fantastic to be able to use physics to help the police solve crimes they couldn’t previously,” she explains. “Physics gives me more of a buzz than cake-related crime ever did.”

“When police make an arrest of a suspect they’ll look for gunshot residues, using a machine called an electron microscope. The problem with the electron microscope is that it’s much less sensitive than my technique, the ion beam. They can see certain elements but we can see extra elements that they don’t see.

“The significance of this is that different manufacturers of ammunition powder use different recipes. So ammunition from one source will contain different elements from that of another source. Now with the ion beam, we can pick up differences between these different ammunition powders that you can’t see using the police techniques.

“There’s another problem that my technique can solve. This is the problem of contamination, and this can happen when gunshot residue particles are on the police officer that makes the arrest in the first place.”


Creative Commons Licence
Ion Beam Cop by The Institute of Physics as part of the National HE STEM Programme is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

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