A guide to X ray scanning
03 May 2022
X-rays were discovered in 1895. The first medically useful X ray scanning followed within a year. Industrial use blossomed after WWII with the introduction of isotopes such as cobalt-60 and caesium-137. These gradually replaced the use of dangerous radium and radon. Early industrial applications included checking
- Machined parts
- Pipe wall and pressurized piping
- Storage containers
- Corrosion and
- Mechanical damage
X ray scanning for security followed in the early seventies in America. Worldwide use followed on very quickly.
Today x-ray scanning usually produces its X-rays electronically. X-ray tubes, like pre-LCDTVs use rapidly accelerating electrons to create well defined and controlled x ray beams.
What are x-rays?
X-rays are part of the electromagnetic radiation spectrum. Electromagnetic waves also include:
- Radio waves
- Visible light
- Ultra violet and
- Gamma radiation
X-rays have a wavelength of around 10-10 metres. This is much shorter than visible light.
How are x-ray generated?
X-rays are produced by accelerating electrons from a cathode towards an anode. As they strike it they release energy. Typically, around 5% is converted into x-rays.
X-rays can be produced in other ways, particularly from cyclotron-style equipment. Additionally, radioactive sources are still used in very specific cases.
What affects the characteristics of x-rays?
A number of factors create the characteristics of x-ray scanning including:
X-ray energy Produced by the voltage across the x-ray tube. Higher voltages produce more energetic x-rays with greater penetrating power.
Flux density The number (or intensity) of x-rays depends on the current (amps) through the x-ray tube.
X-ray spectrum The material the anode is made from produces the spectrum, or distribution, of x-rays emitted. Typical X-ray anode materials include copper, tungsten, molybdenum and silver.
How are X-ray beams produced?
The x-rays created in an x-ray tube travel in many directions depending on its shape. The x-ray tube is encased in a steel or lead container to stop the x-rays escaping in the wrong direction. A small hole (aperture) guides (collimates) them in the proper direction creating the primary beam. The aperture’s shape defines the shape of the primary beam. The ones commonly employed used in security and industrial applications are cone beams and fan beams.
When an x-ray scans an object, it interacts in one of three main ways. It could
- Pass through it unhindered
- It may be totally absorbed or
- It can be scattered
Beginners guide to x-rays In general, scattered x-rays are created by any material that the X ray scanning hits. The scattered x-rays are important, and affect the quality of the x- ray image.