Bragg's X-ray spectrometer
Bragg's spectrometer used to determine the wavelength of X - rays is shown in Fig. Bragg's spectrometer is similar in construction to an ordinary optical spectrometer.
X-rays from an X-ray tube are made to pass through two fine slits S1 and S2 which collimate it into a fine pencil. This fine X-ray beam is then made to fall upon the crystal 'C' (usually sodium chlo- ride crystal) mounted on the spectrometer table. This table is capable of rotation about a vertical axis and its rotation can be read on a circular graduated scale S. The reflected beam after passing through the slits S3 and S4 enters the ionization chamber. The X-rays entering the ionization chamber ionize the gas which causes a current to flow between the electrodes and the current can be measured by galvanometer G. The ionization current is a measure of the intensity of X-rays reflected by the crystal.
The ionization current is measured for different values of glancing angle θ. A graph is drawn between the glancing angle θ and ionization current (Fig.).
For certain values of glancing angle,the ionization current increases abruptly. The first peak corresponds to first order, the second peak to second order and so on. From the graph, the glancing angles for different orders of reflection can be measured. Knowing the angle θ and the spacing d for the crystal, wavelength of X-rays can be determined.