What is an X-ray?
You probably know someone that has had a medical X-ray exam. You may have even had one yourself. However, have you ever stopped to wonder how this invisible source of energy is created? In this lesson, we'll discuss two different types of X-rays: continuous X-rays and characteristic X-rays. We'll specifically examine the production and properties of both. Before we begin this comparison, let's first review exactly what an X-ray is.
To understand X-rays, you must understand that this form of energy is just a type of light. This might make you think of visible light (light that can be seen with the human eye). In science, light is much more than just visible light. Light is synonymous with the electromagnetic spectrum, which is a group of waves of varying energy levels. The electromagnetic spectrum is most often seen as a chart that ranges from radio waves to gamma rays. On the electromagnetic spectrum, X-rays are ordered adjacent to gamma rays (on the high energy side of the spectrum). So, when you hear the word X-ray, just think of high energy light.
Here we are going to discuss what is continuous x-ray, continuous x-ray production, characteristic x-rays and continuous x-rays, continuous x-ray spectrum definition.
Continuous X-rays are created when free moving electrons electromagnetically interact with nuclei, whereas characteristic X-rays are formed during the electron transition processes that occur when an inner shell electron is released from an atom.
Bremsstrahlung transitions tend to create a phenomenon of continuous x-rays whereas the regular characteristic x-rays are produced by inner-shell transitions. The Bremsstrahlung mechanism can be viewed when a target made of metal suffers electron bombardment. The atoms of the target metal scatter the electrons, whose change in acceleration causes a phenomenon of radiation in them.
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Continuous and Characteristic X -Rays
Characteristic x-rays are discharged from the heavy elements when the electrons make transitions between the lower atomic energy levels. X-ray production typically involves bombarding a metal target in an evacuated x-ray tube with high-speed electrons which have been accelerated by tens to hundreds of kilovolts of potential. The bombarding electrons can eject electrons from the inner shells of the atoms of the target metal, usually tungsten or molybdenum. These void will be quickly filled up by the electrons falling from higher levels, emitting x-rays with absolutely defined frequencies associated with the difference between atomic energy levels of the target atoms as they do.
Continuous radiation, or Bremsstrahlung, is the spectrum of electromagnetic emission when the bombarding electrons are of sufficient energy, and results from the electrons being rapidly decelerated by the target. The generated x-ray wavelengths will be distributed over a range, which may or may not include the characteristic ones, depending upon the energy of the bombarding electrons.
When we mention characteristic X-ray, it means x-ray photons which have an energy equivalent to the difference of energy in two electronic levels of an atom. The main reason why it is called the characteristic is that each of the elements has a specific difference in energy. Since the energy levels are discrete (short reason, a consequence of quantum mechanics and probability distributions), they are usually x-rayed with a very specific energy that depends on the element that you see in the diagram below as a vertical line.
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Those K-alpha, and more, represent the energy difference between the K level and upper levels.
As for the continuous x-ray, that is generated through another process, and it is known as Bremsstrahlung radiation or braking radiation. When an electron passes close to the nucleus, the nucleus exerts a force on it, that changes its direction and slows it down (hence the braking). That change in direction implies some energy is lost, and to conserve momentum and energy, one photon is emitted. And since the energy of the photon depends on how far the electron was from the nucleus, you end up with a continuous spectrum.
As you can see from this example, depending on how much the electron is deflected, you have photons with different energies.
If we take a look at the combined spectrum of the x-ray in the graph, we normally will see something like this:
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As you can see, you have the continuous x-ray corresponding to Bremsstrahlung and the discrete lines which represent the characteristic x-ray radiation (the location again, depends on the element).
Reason for Continuous X-Ray Radiation
Continuous X-ray radiation is for the fact that the change in the velocity which causes the emission of much of the radiation in a tube, which is used as a radiography source is a random process that emits photons at different energies. If we look at the spectrum of an X-ray tube it normally has a broad lump due to this Bremsstrahlung emission. Whenever the tube voltage is very low (below 60 kV) we should be able to see some strong peaks due to the characteristic radiation of emission from the target (anode) material. To watch these characteristic peaks it really helps, as if the tube has windows and very little filtering. A medical X-ray source normally has some filter in the form of a metal sheet to remove almost all of the low energy photons from the beam so that this tube is almost only emitting Bremsstrahlung.
An interesting exception is a tube used for a mammogram set, these tend to be Mo anodes run at about 50 kV. These will generate a large amount of Mo k alpha radiation. This lower energy (circa 15 keV) radiation is well suited for the examination of breast tissue, while the higher energy Bremsstrahlung from a tube running at 100 or 200 kV will be more acceptable for looking at bones. The dental X-ray sets use about 70 kV on the tubes which is a partway between the two.
On the other hand, a radioactive photon source which emits gamma rays is always on, it can not be turned off. It emits photons with well-defined energies. For example, Co-60 emits two lines one at about 1.1 MeV and one at 1.3 MeV. This is only because the gamma photons are generated when an excited nucleus makes adjustments between the two states. In this case, the excited state of the Ni-60 daughter emits the photons.