X-Ray and Neutron Reflectivity

re = 2.818 × 10−15m is the classical electron radius, ρe is the electron density of the material, and μx is the absorption length. With δ > 0 we find that n < 1, which leads to the phenomenon of so-called total external reflection for incident angles αi below the critical angle αc = √ 2δ. Typical values for δ are 10−5 . . . 10−6, and thus αc is in the range of 0.1 ◦ . . . 0.5◦. For simplicity, β, which is usually even smaller, will be ignored here. Our considerations apply irrespective of the atomic-scale structure of the material and only the average electron density enters the equations. We note that an analogous description of the index of refraction applies to the case of neutrons, namely