Page 521 - Physics Coursebook 2015 (A level)

Chapter 32: Medical imaging
broad background ‘hump’ of braking radiation (also known as Bremsstrahlung radiation) and a few sharp ‘lines’ of characteristic radiation. These arise from the different ways in which an individual electron loses its energy when it crashes into the anode.
When an electron strikes the anode, it will be attracted towards the nucleus of an atom in the anode. This will cause it to change both speed and direction – in other words, it decelerates. A decelerating electron (or any other charged particle) loses energy by emitting electromagnetic radiation. The result is a single X-ray photon or, more usually, several photons. The electron interacts with more nuclei until it has lost all its energy and it comes to a
halt. The X-rays emitted in this way all contribute to the background braking radiation.
The energy E gained by the electron when it is accelerated through a potential difference of V between the cathode and the anode is given by E = eV. This is the maximum energy that an X-ray photon can have, and so the maximum X-ray frequency fmax that can be produced can be calculated using the formula E = hf. Hence:
fmax = eV h
An electron may cause a rearrangement of the electrons in an anode atom, with an electron dropping from a high energy level to a lower energy level. As it does so, it emits
a single photon whose energy is equal to the difference in energy levels. You should recall from Chapter 30 that this is how a line spectrum arises and the photon energies are characteristic of the atom involved. So the characteristic spectral lines of X-rays from a tungsten anode have different energies from those of a molybdenum or copper target. In practice, these characteristic X-rays are relatively unimportant in medical applications.
QUESTIONS
1a Summarise the energy changes that take place in an X-ray tube.
b An X-ray tube is operated with a potential difference of 80 kV between the cathode and the tungsten anode. Calculate the kinetic energy (in electronvolts and joules) of an electron arriving at the anode. Estimate the impact speed of such an electron (assume that the electron is non-relativistic).
2 Determine the minimum wavelength of X-rays emitted from an X-ray tube operated at a voltage of120kV.
We can also see from Figure 32.5 that X-rays of a
whole range of energies are produced. The lowest energy X-rays will not have sufficient energy to penetrate through the body, so will have no effect on the resulting image. However, they will contribute to the overall X-ray dose that the patient receives. These X-rays must be filtered out; this is done using aluminium absorbers across the window of the tube.
Controlling intensity and hardness
The intensity of an X-ray beam is a measure of the energy passing through unit area (see the next section). To increase the intensity of a beam, the current in the X-ray tube must be increased. Since each electron that collides with the anode produces X-rays, a greater current (more electrons per second) will produce a beam of greater intensity (more X-ray photons per second). A more intense beam means that the X-ray image will be formed in a shorter time.
Another consideration is the hardness of the X-rays. An X-ray may be thought of as ‘hard’ or ‘soft’. Soft X-rays have lower energies and hence longer wavelengths than hard X-rays. Soft X-rays are less penetrating (they are more easily absorbed) and so they contribute more to the patient’s exposure to hazardous radiation. It is often better to use hard X-rays, which pass through the body more easily.
The hardness of an X-ray beam can be increased by increasing the voltage across the X-ray tube, thereby producing X-rays of higher energies (see Figure 32.5). Another method is to use a filter which absorbs the lower energy soft X-rays so that the average energy of the X-rays is higher.
X-ray attenuation
As you can see if you look back to Figure 32.1, bones look white in an X-ray photograph. This is because they are good absorbers of X-rays, so that little radiation arrives at the photographic film to cause blackening. Flesh and other soft tissues are less absorbing, so the film is blackened. Modern X-ray systems use digital detectors instead of photographic films. The digital images are easier to process, store and transmit using computers.
X-rays are a form of ionising radiation; that is, they ionise the atoms and molecules of the materials they pass through. In the process, the X-rays transfer some or all of their energy to the material, and so a beam of X-rays is gradually absorbed as it passes through a material.
The gradual decrease in the intensity of a beam of X-rays as it passes through matter is called attenuation. We will now look at the pattern of attenuation of X-rays as they travel through matter.
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