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Chapter: 11th 12th std standard Class Physics sciense Higher secondary school College Notes

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Bragg's X-ray spectrometer

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.

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.


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