How are X-Rays Produced?
X-Rays are a form of man-made ionising radiation. They are produced within an x-ray tube. Within the x-ray tube there is a tube insert made up of a cathode (negative electrode) and an anode (positive electrode). The tube insert is contained in a glass envelop under a high vacuum.
When super-heated by electricity, the cathode filament releases electrons. Once released, the electrons (negatively charged) are accelerated towards the anode and produce x-rays when they strike the target on the anode face, usually made out of Tungsten.
There are two ways by which the x-rays are produced. The first is called Bremsstrahlung Radiation Production, or Braking Radiation. The second is Characteristic X-ray Production.
Bremsstrahlung Radiation – Braking Radiation
Bremsstrahlung radiation production is the major way in which x-rays are produced. The concept is that an electron that enters into the tungsten target passes close to the nucleus of one of the tungsten atoms. The nucleus of the tungsten atom is very positively charged, the electron is negatively charged, resulting in an attractive force between the two, which slows the electron down. When that electron is slowed down, it gives off energy, and that energy may be in the form of an x-ray.
The amount of energy the electron gives off during this ‘slowing down’ process depends how close the electron is to the nucleus of the tungsten atom. If the electron hits that nucleus, all of its energy will be given up. If it passes close to the nucleus, then some of that energy will be given up.
The greater the energy loss from the electron, the greater the energy of an x-ray produced.
Bremsstrahlung radiation production results in a range of x-ray energies produced from the maximum kV set to very low energy x-ray energies, this is known as the continuous spectrum of x-ray energies.
Characteristic X-Ray Production
The second way in which x-rays can be produced is called Characteristic X-ray Production. This is the minor process.
Electrons are released from the cathode filament and accelerate towards the anode as with Bremsstrahlung.
However, if an electron has enough energy when entering the target, it is able to knock out an electron from the inner shell of the Tungsten atom. As a result, electrons from higher energy levels then fill up the vacancy and the excess energy is emitted as an X-ray.
This x-ray formation is of a known amount of energy and is therefore called Characteristic.
X-Ray Exposure
As a continuous spectrum of x-ray energies is produced during every x-ray exposure, a range of penetrating powers is produced. Lower energy x-rays can result in dose, as the energy is left in the subject being imaged, this can be reduced by filters.
Filters are an integral part of the design features of an x-ray tube, at the point of manufacture.
They are positioned at the window or exit of the tube head at the collimator. They absorb the lower energy x-rays before they reach the target area of the patient.
The use of filters can result in ‘cleaner’ images being produced, a reduction in patient dose, and a reduction in scatter.
Any employer utilising x-ray equipment, or engaged in work with another type of ionising radiation, must appoint a suitable Radiation Protection Advisor in accordance with Regulation 14 of the Ionising Radiation Regulations. See our website for further details.
