How Do We Produce X-rays for Medical and Industrial Equiptment

Before I talk more about some of the other radiotherapy devices used throughout the history of radiotherapy, I want to explain the common ways of producing the radiation. These are basic components needed in any radiation device and are achieved differently depending on the device producing the radiation. Most devices will consist of a source of electrons, then they will accelerate the electrons, and finally smash those electrons into metal to produce photon radiation. And again, I am implicitly referring to Ionizing radiation, I am just being lazy and not including the word ionizing in front of radiation every time.

When we talk about radiation beams we often classify them by their peak eneries. Clinical x-ray radiation has typically been between 10 kiloVolts (kV) to 50 MegaVolts (MV) in energy and clinical electron beams have been in the order of 10 kiloelectronVolts (keV) to 50 MegaelectronVolts (MeV). For some reason we drop the “electron/e” in electronVolts/eV when we talk about x-rays, I believe this is strictly not correct, but it helps to distinguish which type of radiation we are describing, with x-rays having the units of kV or MV, and electrons with units keV or MeV.

As mentioned above the method used to produce x-rays is to smash accelerated electrons into a hunk of metal called a target. So first we need a source of electrons. A source of electrons used is basically a filament like those in old light bulbs. Just a wire of metal heated up so electrons will “boil” on the surface. Then we put this boiling filament in a potential difference, or voltage difference. Since electrons are negative, and if the potential voltage is high enough, the electrons will be attracted to the positive side of the difference. And there you go, a source of electrons, stripping boiled electrons from a heated filament.

Now those electrons are accelerated, the method of which depends on the class of machine the electrons are in. Then they are smashed into the metal target. The production of x-rays in this target occurs in two different ways, the first is through characteristic x-rays, and the other is through Bremsstrahlung. Both methods of producing x-rays are very inefficient with most of the input energy being wasted as heat in the target. So the target is normally made from a high melting point metal, such as tungsten, and somethime it is cooled using water or other means of extracting heat.

Characteristic x-rays produced in the target is the result of an incoming electrons interacting with the electrons in the target. These are called coulomb interactions after Charles-Augustin de Coulomb who defined electrostatic forces of attraction or repulsion. The unit of charge is also named after him, the Coulomb. So a Coulomb interactions are just an interaction between different charges. The incoming electron will interact with the electrons in the target and subsequently the electron in the target can be ejected form the target atom. Once this happens there is a gap, or vacancy, in the electron orbital structure of the atom. For the atom to become stable again and neutral in charge this gap need to be filled. So an electron with more energy in one of the atom’s outer orbitals can drop into this gap. As it does this it releases some energy in the form of Characteristic x-rays. Have a look at the figure I have included, hopefully that will make some sense.

They are called characteristic x-rays as the x-rays they release have specific amounts of energy which is characteristic to the type of element that releases them. This is because the energy difference between different orbitals are unique to different atoms. The yield, or how many you get, is very low for characteristic x-rays when compared to bremsstrahlung x-rays, but with the advantage that they are all the same energy.

Bremsstrahlung is a German word meaning braking radiation. They too are caused by coulomb interactions between the charged accelerated electrons and the electrons of the target material. The target electrons deflect and decelerate the incoming electrons, forcing them to change direction. As they do this they lose part of the kinetic energy by radiating it as x-rays, or bremsstrahlung as it is know. The energy of the x-rays produced in this method can range from the maximum energy of the incoming electron to zero. A continuous range of different energy radiation is produced.

Bremsstrahlung and Characteristic radiation are the two common ways of creating x-ray radiation. For electron radiation, also used in radiotherapy, it is much simpler. This is because a source of electrons is easily obtains. It is just a filament. So the electrons are then just accelerated to the the required energy. No need to smash them into anything. There are other types of radiation used like proton, neutrons and heavy atoms such as carbon. Their production can be much more complicated.

So that is the underlying principle of producing radiation for radiation therapy uses. Its boils down to procuring, then accelerating, and smashing electrons. There is far more going on in a radiation device though. Devices need methods of directing this radiation, monitoring how much radiation is being delivered, and all sorts of safeguards. After all, these devices do involved high voltage and ionizing radiation.

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Jim is a Medical Physicist working in a radiotherapy centre who blogs at Radical Radiation Remedy. They ​blogs​ ​about​ radiation and radiotherapy ​over​ ​at​ ​​radicalradiationremedy.com

Originally published at www.radicalradiationremedy.com.