X-ray types and target selection
In X-ray sources, electrons bombard the target material to produce X-rays in one of two ways.
(1) When the high-speed electron collides with the target atom, it will change its original trajectory and slow down.
X-rays are emitted at the same time, and their energy is equal to the energy difference before and after the high-
speed electron collision.
Due to the different relative positions of each high-speed electron interacting with the target atom, the radiation
loss corresponding to each interaction is also different, and the energy of the emitted X-ray photons is also
different. A large number of X-ray photons make up an X-ray emission spectrum with a continuous frequency -
just like the monochromatic light of various colors mixed into white light - so the X-rays produced by this principle
are called white X-rays (also known as the continuous spectrum).
(2) The high-speed electron knocks out the inner orbital electron of the target atom, causing it to leave the atom
and become a free electron, which will create a temporary electron vacancy in the inner orbital, and the outer
electron will immediately fill this inner orbital vacancy. The orbital electron emits X-rays when it transitions from
the outer shell to the inner shell, and its energy is equal to the difference between the energy levels of the two
orbitals.
Since the orbital electron energy level of the target atom is determined, the X-ray energy generated is also fixed,
and only depends on the characteristics of the target atom. Therefore, the X-rays generated by this principle are
also called identification X-rays (also known as identification spectra).
Either target material will produce both white X-rays and label X-rays. In non-destructive testing applications, the
sample itself has a variety of materials, so it is generally selected to produce more white X-ray tungsten target. In
the application of X-ray fluorescence analysis for the analysis of material composition, molybdenum targets that
are easier to produce identifying X-rays are more often selected.
