Techniques - X-ray Diffraction

Energy Dispersive X-ray Diffraction (EDXRD), sometimes referred to as Coherent X-ray Scattering (CXRS), was first proposed in the 1960's as a simple and high-speed way to analyze remotely the crystal structure of material within a vessel.

Among its first uses was the examination of material within a high-pressure vessel or furnace. Later, the method was adapted for real-time analysis of materials being moved on high-speed conveyor belt such as those at cement or chemical plants.

In the mid to late 1980's, the method was investigated for possible use as a detection tool for explosives hidden within luggage. The first work took place at Philips Medical Systems in Hamburg, Germany, where a simple, non-tomographic device was built. In 1988, Philips teamed with researchers at Rutgers University to develop the first commercial scale unit, which then became the model for the full tomographic unit produced by Yxlon Gmbh (now marketed through its parent company - Invision). Later, in 2002, the same researchers at Rutgers collaborated with L-3 Communications to develop a similar, non-tomographic diffraction-based system.

In both the Invision/Yxlon and L-3 machines, the XRD method is used as a secondary scanner to investigate suspicious regions that were previously identified by a primary instrument such as a CT scanner. By having the XRD unit look at only a single spot (or at most a few spots), the speed problem of the XRD can be reduced. In return, the XRD unit offers high detection rates with single digit false alarm rates. When combined, the CT scanner and XRD promise to provide good throughput with good detection capabilities.

All XRD instruments including Xstream function by matching the diffraction pattern obtained by the instrument against a library of patterns previously obtained. An example is shown in Figure 1.

For comparison, the XRD patterns of C-4 explosive and Sucrose are shown which, even to an untrained eye, are markedly different. Although there may be a few features that are common to both patterns, they are nonetheless very different. In a testing environment, the two diffraction patterns would each be compared against the library and a best match would be found in order to identify an unknown material. Of course, the human eye does this almost instantaneously. But a machine-based detection device must rely on mathematical procedures to extract a unique feature set from the pattern and use them to identify the material giving rise to that pattern.

In general, the XRD methods are highly material specific, since the diffraction patterns of crystalline materials are unique. Thus, C-4 is different from Semtex, dynamite, TNT, black powder, etc. Equally important is the fact that the patterns of the explosives are very different from the other materials in a suitcase. For example, chocolate, shoe leather, or cotton would never be interpreted as Semtex. This high selectivity of XRD has two important ramifications - high detection rates and low false alarm rates. Under ideal conditions (i.e. - low absorption and high test mass), the detection rate is close to 100% and the false alarm rate is 0.1% or lower.