What is Neutron Radiography?

Neutron transmission radiography (NR) is based on the application of the universal law of attenuation of radiation passing through matter. Because different materials have different attenuation behaviour the neutron beam passing through a sample can be interpreted as a signal carrying information about the composition and structure of the sample.

The basic experimental set-up is given by the following arrangement:

The neutron generating source can be a reactor (like FRM-2 under construction in Garching near Munich), the target of an accelerator (like spallation source SINQ) or a neutron emitting isotope.
The collimator is a beam forming assembly which determines the geometric properties of the beam and may also contain filters to modify the energy spectrum of the beam or to reduce the content in gamma rays of the beam. The image resolution achievable with the beam depends much on the collimator geometry and is expressed by the L/D ratio, where L is the collimator length and D is the diameter of the inlet aperture of the collimator on the side facing the source.
The beam is transmitted through the object and recorded by a plane position sensitive detector, i.e. the detector records a two dimensional image that is a projection of the object on the detector plane.
By combining images from measurements at different angles tomographic reconstruction may be carried out.
Dynamic processes can be studied by detector systems with fast imaging capacity. This is the field of neutron real-time imaging.
Various detector systems are employed in NR : combinations of film and a neutron-sensitive converter foil, combinations of a light-emitting scintillator screen with a CCD camera , neutron sensitive imaging plates, track etch foils or recently amorphous silicon flat panel arrays. Click here for an overview of detector systems and of their typical performance characteristics.
Generally, NR is sensitive to all materials interacting strongly with neutrons and especially to arrangements where strongly interacting materials are combined with weakly interacting materials. Unlike X-rays, neutrons interact significantly with some light materials and penetrate easily some heavy materials, making it a complementary technique to X-ray radiography-here is an example :

Neutron radiography	X-ray radiography

While X-rays are attenuated more effectively by heavier materials like metals, neutrons make it possible to image some light materials such as hydrogenous substances with high contrast: in the X-ray image, the metal parts of the photo apparatus are seen clearly, while the neutron radiograph shows details of the plastic parts.

The following diagram displays examples of attenuation coefficients of a few materials :


A complete diagram for all elements is shown in the following figure:


Beside the previously described direct transmission radiography with neutrons, some other techniques are available. These are autoradiography and the converter technique, which take profit from the radiation induced by activation either at the object or the primary detector plane.