Meaning
Physical constants represent the difference between the refractive index of a material and that of a vacuum in the X-ray regime. The refractive index decrement is a very small positive number that determines how much the phase of an X-ray wave is shifted as it passes through a medium. Unlike visible light where the refractive index is usually greater than one, for X-rays it is slightly less than one.
This means that X-rays travel faster in a material than in a vacuum, which leads to total external reflection at very shallow angles.
Phase Shift
Waves are advanced by a specific amount that depends on the thickness of the sample and the value of the refractive index decrement. This shift is what creates the contrast in phase-sensitive imaging techniques. Calculating the exact phase change is necessary for reconstructing the three-dimensional density map of low-absorption objects.
X-Ray Interaction
Probability of a photon being scattered or absorbed is tied to the complex part of the refractive index. The refractive index decrement represents the real part and is directly proportional to the electron density of the material. Understanding this relationship allows scientists to identify different chemical elements within a sample based on their phase-shifting properties.
Electron Density
Measurement of the phase shift provides a way to map the distribution of electrons throughout the scanned volume. Because the refractive index decrement varies with the atomic number, it can be used to distinguish between materials that look identical in standard X-ray scans. This provides a powerful tool for analyzing the composition of alloys and mineral samples.
