Physics and Engineering of Radiation Detection

1.6. General Properties and Sources of Particles and Waves 33

X-ray Machine

Since x-rays are high energy photons and can cause considerable damage to tis-
sues, they are produced and employed in controlled laboratory environments. Pro-
duction of x-rays is a relatively simple process in which a highZtarget (i.e. an
element having large number of protons, such as tungsten or molybdenum) is bom-
barded by high velocity electrons (see Fig.1.6.2). This results in the production of
two types of x-rays: Bremsstrahlung and characteristic x-rays.

glass enclosure (vacuumed)

target

cloud

electron

+V

−V

heater

heat dissipation

arrangement

x−rays

Figure 1.6.2: Sketch of a typical x-ray tube.

Bremsstrahlung (a German word forbraking radiation) refers to the radiation
emitted by charged particles when they decelerate in a medium (see Fig.1.6.3). In
case of x-rays, the high energy electrons decelerate quickly in the target material
and hence emit Bremsstrahlung. The emitted x-ray photons have continuous energy
spectrum (see Fig.1.6.4) since there are no quantized energy transitions involved in
this process. Bremsstrahlung are the x-rays that are usually employed to produce
images of internal objects (such as internal body organs in medical diagnostics).
The electrons incident on the target may also attain sufficient energies to knock
off electrons from the internal atomic shells of the target atoms leaving them in
unstable states. To regain atomic stability, the electrons from higher energy levels
quickly fill these gaps. Since the energy of these electrons is higher than the energy
needed to stay in the new orbits, the excess energy is emitted in the form of x-
ray photons (see Fig.1.6.3). Having energies characteristic to the difference in the
atomic energy levels, these photons are said to constitute characteristic x-rays (see
Fig.1.6.4). The energy of characteristic x-rays does not depend on the energy or
intensity of the incident electron beam because the emitted photons always have
the energy characteristic to the difference in the corresponding atomic energy levels.
Since different elements may have different atomic energy levels, therefore the energy
of the emitted characteristic x-ray photons can be fairly accurately predicted.