Bragg’s X-ray Spectrometer

Bragg’s X-ray Spectrometer:

The Laue diffraction patterns are very complex and difficult to interpret. Instead of using a crystal as a transmission grating, Bragg set up an X-ray spectrometer using a rock-salt (NaCl) crystal as a reflection grating and enabling the X-ray wavelength to be calculated by Bragg’s Equation.

Using Bragg’s X-ray spectrometer, Bragg’s law can be verified.

The schematic arrangement of Bragg’s spectrometer is as shown below-

schematic arrangement of Bragg's spectrometer

This spectrometer is mainly divided into three parts-

  • A source of X-rays with lead slits, S1 and S2.
  • A spectrometer table was provided with a crystal C.
  • A detecting device (i.e. ionisation chamber with galvanometer).

(1) As shown in the figure lead slits S1 and S2 converts the X-ray beam into a fine beam and is then made to incident on the crystal C, at the centre of the spectrometer table.

(2) The table T is capable of rotation about its vertical axis and the rotation can be recorded by means of using a scale provided with it.

(3) After passing through slit S3, the reflected beam enters the ionisation chamber I, a detecting device. (Ionisation chamber is simply a gas chamber and it is used as a detecting device because it is one of the properties of X-rays that X-rays can ionise gases).

(4) The ionisation chamber is mounted on a special movable arm (not shown in the figure) and is linked with the turn table T in such a way that it can always receive reflected X-rays for any angle of incidence.

(5) Due to ionisation, the current is produced which is measured by galvanometer G.

(6) Thus for different values of ‘θ’, ionisation current is recorded and a plot is then obtained between these two as shown below.

Bragg's X-ray Spectrometer Graph

(7) It is observed that for certain values of ‘θ’ intensity (i.e. current) increases abruptly.

Now we know that Bragg’s Equation is-

2d sin θ = nλ

∴ For n = 1, 2d sin θ1 = λ ……….(i)
n = 2, 2d sin θ2 = 2λ ……….(ii)
n = 3, 2d sin θ3 = 3λ ……….(iii)

⇒ sin θ1 : sin θ2 : sin θ3 = 1 : 2 : 3 ……….(iv)

Also from the graph, we can write-
sin θ1 : sin θ2 : sin θ3 = 1 : 2 : 3 ……….(v)

Equation (v) is an experimental result which is similar to the theoretical result equation (iv). This clearly shows that Bragg’s law which is based on the reflection of X-rays is justified.

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