Answer to Question #10579 Submitted to "Ask the Experts"

Category: Radiation Basics — Photons

The following question was answered by an expert in the appropriate field:

How does an x-ray irradiator work? How is it able to produce specific energies varying from a few keV to almost 1 MeV?

Many x-ray producing machines, whether they be x-ray producers of low keV to 150 keV energy, as are typical of medical diagnostic units, and industrial machines capable of yielding x rays of several MeV; the most likely sources of the high voltage for these machines are specially designed high-voltage transformers. Even higher energies may be achieved by research accelerators (e.g., Stanford linear accelerator) that produce x rays that may be in the hundreds to thousands of MeV range by accelerating electrons to very high energies by using specialized accelerating electromagnetic fields, produced by high-voltage microwave tubes, that accelerate the electrons through several sequential stages by supplying radiofrequency energy to the electrons at each stage. In general, the higher the energies of the x rays produced, the more likely it is that they will be emitted in the same direction of travel as the electron beam incident on the target. The x rays produced by accelerating electrons into a target material are referred to as bremsstrahlung radiation.

When the electrons bombard a target material, some of them will lose variable amounts of energy as they closely approach positively charged nuclei and accelerate around the nuclei, in the process radiating electromagnetic energy in the x-ray region, the so-called bremsstrahlung radiation. The acceleration of charged particles is a common mechanism for generating electromagnetic radiation; an example associated with lower energy processes is the generation of radio waves by accelerating electrons up and down a conducting antenna.

Bremsstrahlung radiation produced by monoenergetic electrons incident on a target will exhibit a range of energies from near zero to a maximum equal to the electron energy. A high-energy photon results when an electron approaches very closely to a target nucleus and experiences a strong acceleration around the nucleus, whereas a lower-energy photon results from an electron approaching less closely to a nucleus and undergoing a weaker acceleration. Often the low-energy x rays are not desired for the application at hand, and these may be reduced or eliminated by using appropriate filtration material between the x-ray source and the material being irradiated with the x rays. Thus, depending on electron energy, we may produce a wide range of x-ray energies for various possible applications.

We might note that it is clearly possible to produce bremsstrahlung x rays that are much higher in energy than the highest-energy gamma rays (electromagnetic radiation similar to x rays but originating by nuclear deexcitation associated with the rearrangement of nucleons within the nucleus and having a maximum energy of about 10 MeV). We should perhaps also mention that bremsstrahlung x rays are distinct from characteristic x rays (sometimes called fluorescence x rays), which have discrete energies associated with deexcitation of an excited atom, the atomic deexcitation being associated with internal electron energy state changes in the atoms; the transition between two distinct energy levels yields an x ray whose energy is equivalent to the electron energy difference between the two levels.

I hope you find this helpful.

George Chabot, PhD

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