Chapter 2
Radiation Physics
Simon C Harvey
Introduction
The aim of this chapter is, firstly, to explain what X-ray radiation is and, secondly, to describe the production and interaction of X-ray radiation.
The electromagnetic wave
The electromagnetic wave describes a wave of energy that has an electric field alternating (between positive and negative) along one axis. At right angles to this, a magnetic field alternates between north and south (Fig 2-1). The two are often drawn as one wave to make their depiction easier.
All electromagnetic waves travel at the same speed in a vacuum, irrespective of their energy—the speed of light = 299 792 458 ms-1. The speed of any wave is related to its wavelength and frequency by the following equation: speed = wavelength × frequency.
As the speed is known and constant (speed of light = c), the wavelength and frequency of different electromagnetic waves must change accordingly. At one end of the spectrum, the waves have a very long wavelength (and therefore low frequency) and are lower in energy. At the other end, the waves have a very short wavelength, high frequency, and are very high in energy (Fig 2-2).
Fig 2-1 The electromagnetic wave.
The electromagnetic spectrum is continuous. Although we name different parts of the spectrum and provide cut-offs, these are arbitrary, and the different categories of waves differ only in the energy they possess.
Fig 2-2 The electromagnetic spectrum (NASA).
Fig 2-3 A rotating anode X-ray tube.
Fig 2-4 Bremsstrahlung radiation production.
It is noticeable that visible light only makes up a narrow band in the spectrum. Waves with frequencies below 4 × 1014 Hz are not visible to the human eye, and frequencies above 8 × 1014 Hz are equally invisible. Above a certain energy level, the waves can become ionising and cause damage to biological tissues. Higher-energy ultraviolet waves, X-rays, and gamma rays all have enough energy to damage human cells.
Individual photons or continuous waves?
We have seen that electromagnetic waves are a continuous wave: however, we often refer to ‘photons’, which have a particulate form and particulate properties. This is an alternative way of describing the interactions of electromagnetic waves more easily, and will appear throughout the book. It should be noted, however, that the photons have no mass, and even though they have particulate properties and can be described individually, they are in fact discrete packets of energy.
X-ray production
X-rays are high-energy electromagnetic waves or photons. They occur naturally and are emitted from some radioactive atoms; however, this is not amenable to everyday imaging, as the radioactive source would deplete, and be constantly irradiating, and the amount and energy of the radiation could not be easily controlled. Therefore, an artificial production method is needed.
An X-ray tube contains several essential components, as illustrated inFigure 2-3 and listed inTable 2-1, with a description of their purpose.
The X-rays are produced in two ways:
Bremsstrahlung
An incoming electron emitted from the Tungsten filament is accelerated through a vacuum towards the Tungsten anode. As it strikes and passes through the anode, it may be attracted to the positive nucleus of an individual Tungsten atom. This attraction will simultaneously deflect the trajectory of the fast-moving electron and cause it to slow down rapidly. This rapid deceleration and change of path results in energy loss, which is emitted as an X-ray photon. The greater the deflection and slowing of the electron, the greater the resultant X-ray photon energy. As each interaction between an individual electron and a nucleus of the Tungsten atom in the anode is different and the energy loss is dissimilar, the energy profile of the X-rays produced (the spectrum) is over a wide range.
The majority of X-rays—approximately 80%—from an X-ray tube are produced in this method. It should be noted that the interaction here is between an incoming electron released by the filament and the nucleus of the Tungsten atoms in the target (Fig 2-4).
Characteristic radiation
If the incoming electron passes close to the nucleus and has enough energy, it can knock out a tightly bound inner shell electron (K shell) from the Tungsten atom. This leaves a vacant inner shell, which is filled quickly by an outer shell (L or M shell) electron from the same atom. As the outer shell electron ‘jumps down’ energy shells, it loses energy in the form of X-ray radiation. In this case, the energy the outer electron needs to lose when ‘jumping’ to the inner shell is a known amount for each different atom; so, the X-ray produced has exactly that amount of energy. The outer shell electron may come from an L or M shell, so the energy will differ slightly between the two. This is known as characteristic radiation—it is characteristic of that particular atom (Fig 2-5). For Tungsten, the values for characteristic radiation are 58 keV and 68 keV.
Fig 2-5 Characteristic radiation production.
It should be noted that for characteristic radiation to be produced, the incoming electron must have enough energy to knock out the inner K shell Tungsten electron. The inner Tungsten electron needs 70 keV of energy to be
