X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

decelerated as they penetrate the material of target, and such a high negative acceleration
should produce a pulse of radiation.
Figure I.5: On the left, the classical model showing the production of Bremsstrahlung.
On the right, the Continuum X-ray emission spectrum.
The probability of radiative energy loss (Bremsstrahlung) is roughly proportional to
q 2 z 2 T/M0 2 , where q is the particle charge in units of the electron charge e, Z is the atomic
number of the target material, T is the particle kinetic energy, and M0 is the rest mass of the
particle. Because of the fact that protons and heavier particles have large masses, compared to
the electron mass, they irradiate relatively little, e.g., the intensity of continuous X-rays
generated by protons is about four orders of magnitude lower than the generated by electrons.
The ratio of energy lost by Bremsstrahlung to that lost by ionization can be
approximated by:
2
⎛ m0
⎞ ZT
⎜
M ⎟
2
⎝ 0 ⎠ 1600 m0
c
where m0 is the rest of the electron.
III.6.2 Characteristic Emission
, (I.6)
The purpose of X-ray fluorescence is to determine chemical elements both qualitatively
and quantitatively by measuring their characteristic radiation. To do this, the chemical
elements in a sample must be caused emit X-rays. As characteristic X-rays only rise in the
transition of atomic shell electron to lower, vacant energy levels of the atom, a method must
be applied that is suitable for releasing electrons from the innermost shell of an atom. This
involves adding to the inner electrons amounts of energy that are higher than the energy
bonding them to the atom.
This can be done in a number ways:
• Irradiation using elementary particles of sufficient energy (electrons, protons, a-particles…)
that transfer the energy necessary for release to the atomic shell electrons during collision
processes.
• Irradiation using X- or gamma rays from radionuclides.
• Irradiation using X-rays from an X-ray tube.