Case study: Using neutrons to make pictures
Neutron scattering is one of the most effective ways to obtain information on the structure and the dynamics of condensed matter. A wide scope of problems, ranging from fundamental to solid state physics and chemistry, and from materials science to biology, medicine and environmental science, can be investigated with neutrons. Aside from the scattering techniques, non-diffractive methods like imaging techniques can also be applied with increasing relevance for industrial applications.
Understanding how neutron imaging works is made simpler by first understanding how X-ray imaging works. In X-ray radiography, X-rays from an X-ray source pass through an object before striking a detector. In passing through an object, a certain amount of the radiation is absorbed. This is due to the fact that X-rays are a form of electromagnetic radiation so they interact with the electrons in the atoms that make up the material. Larger atoms with more electrons absorb more radiation, with large atoms like lead absorbing almost all X-ray radiation it encounters (this is why lead is used as an X-ray shield). So as X-rays pass through objects made from more than one material, like a person, the different materials absorb different amounts of radiation therefore allowing different amounts of radiation to pass through and on to the detector; this allows the interior of an object to be imaged.
Neutrons are particles as opposed to radiation, together with protons they make up atomic nuclei. As suggested by their name neutrons are neutral particles, a property that makes them useful in probing matter as they are unaffected by charged particles. The process of neutron imaging is similar to imaging with X-rays, the difference between the two techniques is due to the physical difference between neutrons and X-rays. Whereas X-rays interact with atom’s electrons, neutrons interact with the atoms nucleus. Different atomic nuclei have different effects on neutrons, for example neutrons largely pass through lead so can be used to see inside lead containers.
Neutron imaging is a very useful Non-Destructive Testing technique that can be used either as an alternative to X-ray imaging or as a complimentary technique, to gather information X-ray imaging may have missed. Significant applications in industry are emerging, for example in the automotive industry as shown in the video below.
Semi-transparent neutron tomography of a motorcycle engine
(1min25s - Credit: Paul Scherrer Institut)
Neutron tomography is an imaging process that can generate three-dimensional image volumes. This is based on a set of radiographs depicting sections of the object. The process uses the same principles as medical x-ray computer tomography (CT). In a medical CT facility, the x-ray source and detector are turned around and a patient, whereas neutrons tomography entails an object fixed to a rotation table turned in small steps around 360 degrees. A mathematical technique known as tomographic reconstruction is then used to reconstruct section images at each rotation step from the radiographs. The tomography volume is formed by combining these section images. 3D visualisation algorithms can then be used to display the required views and sections.
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