Detection, Diffraction and Absorption of X-rays

Detection of X-rays

The basic properties which are generally used for the detection of X-rays are : (i) blackening of a photographic plate and (ii) the ionization produced by X-rays in a gas or vapour. An ionization chamber, which utilizes the property of ionization, is generally used to detect and measure the intensity of X-rays.

Absorption of X-rays

When X-rays pass through any material, a part of the X-ray energy is absorbed by the material. If a beam of X-rays of intensity I passes through a length dx of any material, its intensity is decreased by dI. For any given material, the amount of absorbed intensity is µIdx where µ is called the absorption co-efficient. i.e., dI = −µIdx

Diffraction of X-rays

Soon after the discovery of X-rays, Schuster pointed out that X-rays behave as electromagnetic waves of wavelength much shorter than that of visible light. Attempts were made to measure the wave length of X-rays by means of diffraction gratings which proved unsuccessful, as the grating failed to disperse X-rays on account of their very small wavelength. Obviously, diffraction effects can only be observed if the spacing between the lines ruled on the grating is of the order of magnitude of wavelength of the wave used. Thus, in order to diffract X-rays, grating with much finer rulings, having distance between rulings comparable to the wave length of X-rays are required. It is impossible to construct a grating of such fine dimensions artificially.

In a crystal, the atoms or molecules are arranged symmetrically in a three dimensional space. Any plane containing an arrangement of atoms is known as lattice plane or cleavage plane. The spacing between the atoms is of the order of 10^-10 m, comparable to the wavelength of X-rays. It was suggested that the regular arrangement of atoms or molecules in the cleavage planes of a crystal might provide a grating element suitable to diffract X-rays. The crystal might serve as a three dimensional grating, whereas optical grating is a two dimensional one.