Characteristic X-ray spectrum

Line spectrum

• The methods of producing characteristic X-rays are:
• In this method we use element as a target in the X-ray tube and electrons are bombarded directly on it.
• For each target there is a minimum potential below which the line spectra do not appear, and this critical potential difference is different for different element.
• In this method the primary X-rays from a hard X-ray tube allow to fall on the element.
• The primary X-rays must be harder than the characteristic X-rays to be produced.
• The peak obtained in the X-ray spectrum gives the line spectrum, which is the characteristic of the element.

• Moseley used the potassium ferocynaide as a crystal and photographic plate as a recording medium in place of ionisation chamber in the X-ray spectrometer.
• He obtained line spectrum by using 30 different targets in tubes and concludes the following:
• The spectrum obtained by graph has two different groups. The series of lower wavelength is K-series and of higher wavelength is L-series.
• Since the linear spectrum depends on the nature of target material and therefore it is also called characteristic spectrum. The number of lines in line spectrum depends on the nature of nucleus and the applied voltage.
• It is easier than optical spectrum, and are same for all types of elements.
• The frequency of a particular line spectrum increases with the position of element in periodic table.
• ν = a (Z – b)²
• Above equation is known as Moseley’s law
• a and b are constants
• ν frequency of line
• Z atomic number

Origin of characteristic X-ray spectrum

• According to Moseley, when electron jumps from higher level n_f to lower level n_i, then frequency of emitted X-ray.

• Here R is Rydberg’s constant, b is screening factor and c is velocity of light
• When high energy electron are bombarded on a target nuclei, then attacking electron removes one electron from K-shell.
• This empty space of K-shell is filled by another electron of L or M shell and hence the spectrum lines of K-series are formed.

• If electron jumps from L to K shell, then K_α line is formed, and if electron jumps from M to K shell then K_β line is formed.
• Similarly L, M and other series can be formed.

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