2. Neutron Scattering—A Non-destructive Microscope for Seeing Inside Matter

Excerpt

How can we determine the relative positions and motions of atoms in a bulk sample of solid or liquid? Somehow we need to be able to see inside the sample with a suitable magnifying glass. It turns out that neutrons provide us with this capability. They have no charge, and their electric dipole moment is either zero or too small to measure. For these reasons, neutrons can penetrate matter far better than charged particles. Furthermore, neutrons interact with atoms via nuclear rather than electrical forces, and nuclear forces are very short range—on the order of a few femtometers (i.e., a few times 10\(^{-15}\) m). Thus, as far as the neutron is concerned, solid matter is not very dense because the size of a scattering center (i.e., a nucleus) is typically 100,000 times smaller than the distance between centers. As a consequence, neutrons can travel large distances through most materials without being scattered or absorbed. The attenuation, or decrease in intensity, of a beam of neutrons by aluminum, for example, is about 1% per millimeter compared with 99% per millimeter or more for X-rays. Figure 2.1 illustrates just how easily neutrons penetrate various materials compared with X-rays or electrons.

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