Detecting ionising radiation

Photographic film

Henri Becquerel was the scientist who discovered the effects of radioactivity. Becquerel was conducting experiments to investigate the phenomenon of fluorescence. He thought that objects which fluoresced also produced X-rays. To test this theory, he planned to expose fluorescing uranium salts to the Sun and placed them along with a metal Maltese cross on top of an unexposed photographic plate. He thought that when the photographic plate was developed it would show the image of the Maltese cross due to the X-rays produced from the fluorescing salts.

Fluorescing objects can only fluoresce if they are exposed to the Sun. However, the weather was not on Henri’s side and he was forced to delay his experiments.

He stored the objects with the photographic plate in a dark drawer.

When Henri removed the plate a few days later he discovered that an image of the cross had actually appeared on the plate even though the salts had not been exposed to the Sun. He concluded that the salts themselves must be emitting X-rays. Becquerel had discovered the effects of ionising radiation.

The unit of radioactivity is named after Henri Becquerel and is known as the becquerel (Bq). It is used as a measurement of the number of unstable nuclei which decay every second.

Becquerel’s discovery allows us to monitor the amount of ionising radiation people are exposed to. Photographic film when exposed to ionising radiation becomes foggy or cloudy. This effect can be used to detect ionising radiation and is particularly useful for monitoring the amount of radiation people are exposed to at work. For example, people who work with ionising radiation such as gamma rays or X-rays wear a small monitoring badge. This badge contains a piece of photographic film. When it is exposed to ionising radiation this film becomes foggy. These badges are sent off to a laboratory for analysis on a regular basis to check that the workers are not being exposed to excessive amounts of ionising radiation beyond the safety limits set.

The Geiger-Müller tube

Another piece of equipment which can be used to detect the presence of ionising radiation is the Geiger-Müller tube. A Geiger-Müller tube contains a sealed metallic tube which is filled with an unreactive gas such as argon at low pressure. The unreactive gas is known as the detecting gas. The metallic surface of the tube acts as the cathode.

A thin electrode runs down the middle of the tube and acts as the anode. The anode and cathode do not touch. As the unreactive gas contains only atoms, no charged particles, no current will flow between them. When any ionising radiation enters the tube, it causes the atoms in the gas to be ionised. The electrons produced from ionisation move towards the anode. The electron is accelerated towards other gas atoms and causes them to also become ionised producing further electrons. These electrons can then go on to ionise other atoms in an ‘avalanche’ effect.

The movement of the electrons produced by the ionisation produces a current or voltage pulse which is detected by the Geiger-Müller tube. The tube converts these voltage pulses into sound signals or displays as a measure in becquerels (Bq) on the monitor. The diagram below shows what the Geiger-Müller tube looks like.

Background radiation

There is always a low level of radiation around us. This is known as background radiation. The amount of background radiation present is very low and is not harmful to us. There are many sources of background radiation including food and drink, cosmic rays, medical appliances and buildings.

Natural background radiation from the Earth

Some of the background radiation originates from the Earth. Many of the rocks found in the Earth’s crust contain radioactive isotopes. These radioactive isotopes have unstable nuclei which decay. The products of this decay are also radioactive isotopes.

For example, uranium is a radioactive element which slowly decays to produce two radioactive gases – radon and thoron. These gases are emitted from the radioactive rocks and can travel out of the crust into our atmosphere. These unstable isotopes can also be found in the air around us, our food and drink and in the building materials used to build our houses and work buildings.

Radiation from space

Radiation also comes from space in the form of cosmic rays. Most of this radiation comes from the Sun but can also be produced by the nuclear reactions which occur in stars.