Neutron radiography

What is NDT? 

Neutrons, like X-rays and gamma rays, pass through solid material and with suitable conversion screens can produce an electronic or photographic image similar in nature to those produced in X-radiography.

X-rays, being electromagnetic waves, react primarily with the electron shells of elements and are increasingly attenuated as the number of electrons (atomic number) and the density increases.

Neutrons, being neutral particles of very large mass compared to electrons, react primarily with the nucleus of elements; the interaction varies even between isotopes of the same element and the attenuation characteristic is largely unrelated to atomic number. Low-energy neutrons in particular (so called thermal and cold neutrons with energies less than 0.025 eV) demonstrate very different and at times complementary characteristics to X-rays, being very strongly absorbed by hydrogen, boron, cadmium, gadolinium and a few other elements, but not by the common engineering metals (including aluminium, iron and lead). This means that by thermal neutron radiography, small amounts of hydrogenous material such as corrosion products, oil, water, explosive and plastic materials can be detected inside metal assemblies.

Neutron radiography is used to detect corrosion products and examine complex castings (such as turbine blades), explosive fillings (and particularly for continuity in detonator cords), nuclear fuel elements, adhesive bonds and quality assurance checks for assembly of critical components (such as those used in space applications).

Sources of neutrons for radiography are:
a) Atomic reactors
b) Particle accelerators
c) Radioisotopes (notably 252Californium).

The most intense sources are reactors but proton and heavier ion accelerators of even modest energies (as low as 3 MeV) can be used to generate intense beams of neutrons, which can be used to provide good quality radiographs and electronic images. Real-time imaging, stereoscopy and tomography are within the capability of some of these small accelerators (see: Real-time radiography).

Some work is underway to provide mobile systems for operation at dispersed sites.