Neutron radiography

This paper discusses neutron radiography which produces images that look much like x-rays. But what they reveal is, in many ways, just the opposite of what x-rays reveal. Neutron radiography cannot show coins inside a purse or bones concealed by flesh. But it can show a plastic toy behind an inch of lead, oil flowing through a valve, or tiny roots growing through soil. The two types of radiography reveal different things because of a fundamental difference in the way x-rays and thermal neutrons interact with matter. X-rays interact with the electrons in the atoms of which a specimen is composed, and the higher the atomic number and the denser the specimen, the more the x-rays are attenuated. Thermal neutrons are unaffected by the specimen's electron content, since they have no charge. Instead, they interact with the specimen's atomic nuclei, and attenuation of the neutron flux depends on the way the nuclei deflect or absorb neutrons. This varies from one element to another, in ways that have nothing to do with atomic number