Orbis micro-XRF analysis of the area where a bullet has penetrated an article of clothing can provide valuable forensic information on the type of ammunition and firing range.
1.) General Bullet Type:
When passing through a fabric, the metal bullet typically leaves a metallic residue on the fabric in the shape of a ring. Analysis of the "bullet wipe" can indicate for example whether a lead or copper-jacketed projectile was used. In the example shown below, a test firing was made through a swatch of fabric at a forensics laboratory. The area surrounding the bullet wipe was elementally imaged using the Orbis. The central ring observed in the Pb(L) map is characteristic of a bullet wipe created by a Pb projectile.
2.) Explosive Propellant Type:
When the firing distance is close enough to the fabric, the event leaves a residue characteristic of the propellant used in the ammunition. Using the Orbis, one can easily distinguish between Pb-based and Pb-free firing propellants. In the Pb(L) map above, one can see the Pb residue surrounding the bullet wipe which is characteristic of a Pb-based propellant. When the propellant has been doped with specific trace elements, for example with some propellants used in ammunition issued to law enforcement services, the propellant can often be identified by the trace elements signature in the firing residue left on the fabric.
3.) Firing Distance:
For firings which occur within roughly 50 cm of the target, the firing distance can be determined by measuring the amount of residue within a defined area around the bullet wipe. In this case, speed of analysis is often important as a series of test firings may have to be prepared under similar conditions to the actual event (for example, same or similar weapon and ammunition) and analyzed in order to compare to the analysis of the unknown forensic sample.
The mapping capabilities of the Orbis can be used to quickly scan fabric for the amount of residue present while imaging the sample. As an example, the Orbis equipped with a 1 mm collimator was used to generate Zn(K) maps of test firings with a Pb-free propellant to determine the Zn(K) intensity as a function of firing distance from the target. Each test firing sample was measured within 1 hour. The method was found to be able to determine to firing distance to within at least 50 cm possibly farther.
ZnK 10 cm Distance
ZnK 25 cm Distance
ZnK 50 cm Distance
ZnK 75 cm Distance
ZnK 100 cm Distance
Analysis courtesy of:
deGrey X-ray
Samples courtesy of:
Jan Biegstraaten
Netherlands Forensics Institute