172 5.3  X-Ray Tools

5.3.1  X-​RAY GENERATION

In some research laboratories in the world, x-​rays are still generated from a relatively small

x-​ray tube beam generator, which can fit into a typical small research lab (Figure 5.2a). This

device generates electrons from a hot filament (often made from tungsten but also thorium

and rhenium compounds) in a similar way to an electron microscope using thermionic

emission but accelerates these electrons using high voltages of typically ~20–​150 kV to impact

onto a metal target plate embedded into a rotating anode. Rotation, at a rate of 100–​200 Hz,

increases the effective surface area of the metal target to distribute the high heat generated

over a greater area. The target is usually composed of either copper or molybdenum, though

tungsten, chromium, and iron are also sometimes used. The high energy of the electrons

can be sufficient to displace atomic electrons from their atomic orbitals resulting in x-​ray

emission, either through a Bremsstrahlung mechanism (Figure 5.2b), which results in a con­

tinuous x-​ray emission spectrum, or x-​ray fluorescence, which generates emission peaks at

distinct wavelengths.

In x-​ray fluorescence, incident electrons can have sufficient energy to displace ground-​

state electrons from the K-​shell (i.e., 1s orbital) to generate metal ions (Figure 5.2c). This

creates a vacancy in the K-​shell, which can be filled by higher-​energy electrons from the L (2p

orbital) or M (3p orbital) shells, coupled to the fluorescence emission of an x-​ray photon of

energy equal to the energy difference between these K–​L and K–​M levels minus any vibra­

tional energy losses of the excited state electron as per the fluorescence mechanism described

FIGURE 5.2  X-​ray generation. (a) X-​ray tube, with rotating anode. (b) Mechanism of x-​ray generation from the

Bremsstrahlung mechanism in which the energy lost by an electron accelerating round a positively charged high-​atomic-​

number nucleus is emitted as x-​rays. (c) Process of x-​ray fluorescence following ejection of a core shell electron followed

by higher energy shell electrons filling this vacancy, with the energy difference emitted at distinct wavelengths, which can

be seen (d) overlaid as peaks on the Bremsstrahlung continuum x-​ray spectrum. (e) Intense x-​rays may also be generated

from a synchrotron facility.