gem tube and contamination monitor

Physics A level revision resource: Observing ionising radiation

Ionising radiation is any type of radiation that has so much energy that it can free electrons from atoms. This means it has a short wavelength in the electromagnetic spectrum: UV, X-ray, and Gamma ray radiation.

What is ionising radiation?

Ionising radiation is all around us and is not necessarily harmful. Cosmic rays are highly energetic particles originating from the outer solar system. They are interesting to scientists because they typically have energies that are millions of times that which we could ever achieve in a particle accelerator. On impact with the Earth’s atmosphere, cosmic rays will collide with atoms and molecules to produce a secondary shower of radiative particles. However, by the time these particles reach the Earth’s surface they are not very harmful. However, some radiation certainly can be harmful. Any material that undergoes spontaneous decay, is said to be radioactive. The process is stochastic and will emit either alpha particles which are the least penetrating (can be stopped by a piece of paper), beta particles which are quite penetrating (can be stopped by a sheet of aluminium) or gamma particles which are very penetrating (stopped by a slab of concrete). 

woman observing a gm tube

Since ionising radiation is not visible to the naked eye, to find out whether or not the ionising radiation is a health threat, we can measure the rate of incident particles using a Geiger-Müller tube and counter.

How do we measure ionising radiation?

woman in a white shirt in a lab
How do we measure ionising radiation?

How do we apply the inverse square law? 

The Geiger counter can tell us how many total counts of radiation detected and more sophisticated counters sometimes will also calculate the rate. By measuring the count rate over variety of distances we can show that the intensity (I) is inversely proportional to the square of the distance (d). NB I0 is the intensity of radiation at the source.

This is the inverse square law. It applies to any point source that spreads its influence equally in all directions, so is applicable to gravity, electric fields, sound and electromagnetic radiation. As the inverse square law applies to light, this can be demonstrated using a similar experimental setup with a light source to replace the radioactive source and an intensity detector instead of a Geiger counter.

equation of source intensity
diagram of source intensity

We can tell from this relationship that by plotting inverse count-rate against distance squared we should get a linear plot. Since the radioactive source is typically encased within some protective material, the intercept of the plot allows you to determine the source intensity (I0).

Note that the radioactive source may not be the only source of radiation, so a measurement of the background radiation should be taken before you begin and subtracted from the total counts. Also in order to get a more accurate result, you should record the total number of counts over a large time interval and repeat the measurements several times.

What are the 'real world' applications?

The topic of radiation is very broad and has hundreds of applications outside of the classroom. Medical and dental X-ray machines pass highly energetic radiation through the body, which is detected on the other side. Since the densities of bones and other foreign objects are different to that of body tissue, they absorb different amounts of radiation and allows us to identify them. In archaeology, radioactive isotopes are used for carbon-dating of fossils. Carbon isotopes decay at a constant rate defined by their half-life - the time it takes to for the radioactivity to reduce by half. Carbon-14 is continually formed in the Earth's upper atmosphere, and becomes incorporated in living things as they grow and respire. But this process stops once the organism dies. As carbon-14 has a half life of approximately 5,700 years, this measurement can give a very accurate indication of when the organism died as long as it was between a few hundred and 50,000 years ago.

At the University of Birmingham, cosmic rays are of particular interest to both particle physicists and astronomers. In particle physics, these cosmic rays have much higher energies than anything we can produce in a particle accelerator. High energy particles are useful to smash up existing atoms in order to learn about their internal structure. The University is part of an international collaboration called HiSPARC, which allows schools to obtain cosmic ray detectors and install them on their school buildings. These detectors collect important information about cosmic rays that can be interpreted by the university and published in research papers. In astronomy, cosmic rays are a big problem for space based telescopes as they add unwanted noise to observations. Modelling and understanding them is very important, especially with the next big space telescope (the James Webb Space Telescope) due for launch in 2018, and a space-based gravitational waves detector planned for 2034.