Neutron radiography, also known as n-ray radiography,is a very efficient tool to enhance investigations in the field of non-destructive testing as well as in many fundamental research applications As the name implies, a neutron beam penetrates the specimen to be studied. This beam is attenuated by the sample material depending on the material's neutron cross-section. The beam is then detected by a two-dimensional imaging device that outputs an image representing the macroscopic structure of the samples interior.
The advantage of neutron radiography is its ability to image very light elements (i.e. with low atomic numbers) such as hydrogen, water, carbon etc. In addition, neutrons penetrate very heavy elements (i.e. with high atomic numbers) such as lead, titanium etc as well as distinguish between different isotopes of the same element. This makes neutron radiography an important tool for the studies of radioactive materials using the transfer method. This makes neutron radiography suitable for a number of tasks impossible for conventional x-ray radiography and is actually complementary to X-ray radiography.
Neutron radiography is based on the principal that neutrons interact with the nucleus of the atom, rather than the electrons, and that each atomic nucleus has a different probability for absorbing or scattering a neutron. Neutrons, in particular those traveling at very low velocities (thermal neutrons), are absorbed in matter according to laws that are very different from those that govern the absorption of electromagnetic rays, such as x-rays and gamma rays. The absorption of x-rays and gamma rays increases as the atomic number of the absorber increases, but this is not the case with thermal neutrons. Elements having adjacent atomic numbers can have widely different absorptions of neutrons and it varies from element to element, even from isotope to isotope. Also, some low atomic number elements attenuate a beam of thermal neutrons more strongly than some high atomic number elements. This means that, contrary to x-rays, neutrons are attenuated by some light materials, such as hydrogen, boron and lithium, but penetrate many heavy materials such as titanium and lead. This allows for some unique applications of neutron radiography. For example, since hydrogen has a much higher neutron attenuation than lead, it is possible to determine the height of water in a lead standpipe by neutron radiography. This is impossible with x-ray or gamma-ray radiography.