The basic property of any material, and the one from which the other properties originate, is its structure. Knowing how the individual atoms are arranged within a material is therefore an extremely important part of the scientific investigation of that material.
In order to be able to look at materials at the scale of the atoms (which is about 1 thousand millionth of a metre) a probe has to be found which has approximately the same scale. Fortunately this is the case for two types of radiation, neutrons and X-rays.
When a neutron or X-ray beam is shone onto a material many of the neutrons or X-rays are scattered by it. This is called diffraction and the same basic physics can be used to explain how the material scatters both neutrons and X-rays.
Most materials have a crystalline structure at the atomic scale. That is, the material is made from a regularly repeated pattern of atoms or molecules.
To understand how neutrons are scattered we need to appreciate a fundamental characteristic of particles that was discovered at the beginning of the 20th century. Particles like neutrons, protons and electrons sometimes behave as if, in fact, they are waves! This property of all particles is known as wave–particle duality.