detecting and monitoring nuclear radiation

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Human senses cannot detect nuclear radiation, so we need equipment to do this.

Photographic film

Photographic film goes darker when it absorbs radiation, just like it does when it absorbs visible light. The more radiation the film absorbs, the darker it is when it is developed.

film badge for detecting radioactivityPeople who work with radiation wear film badges, which are checked regularly to monitor the levels of radiation absorbed. The diagram shows the inside of a typical radiation badge when it is closed and opened.

There is a light-proof packet of photographic film inside the badge. The more radiation this absorbs, the darker it becomes when it is developed.

To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film. These may include aluminium, copper, lead-tin alloy and plastic. There is also an open area at the centre of the badge.


Geiger-Muller tube (GM tube)

The ionising effect of radiation is used in the Geiger-Muller (GM) tube as a means of detecting radioactivity.

Diagram of GM tubeThe tube is a hollow cylinder filled with a gas at low pressure. The tube has a thin window made of mica at one end.

There is a central electrode inside the GM tube. A voltage supply is connected across the casing of the tube and the central electrode as shown in the diagram.

Each time the tube absorbs nuclear radiation, it transmits an electrical pulse to a counting machine. This makes a clicking sound or displays the count rate on a meter.

What happens is that when the radiation enters the tube it ionises the gas. These ions are attracted to the electrodes, producing a current in the tube for a short time, which in turn results in a voltage pulse. Each voltage pulse corresponds to one ionising radiation entering the GM tube. The voltage pulse is amplified and counted using electronic circuitry. The higher the count rate, the more radiation the GM tube is absorbing.

People who may be exposed to radiation regularly (OCR only)

These include:

  • medical staff
  • workers at nuclear power stations
  • research scientists.

The table shows some typical doses.

Dose in sievert (Sv)
Sterilising surgical instruments 25000
Typical radiotherapy dose 60
Legal dose limit for a worker 0.02
Mean annual dose from natural radiation 0.002
Typical chest X-ray 0.00002
Flying from the UK to Spain 0.00001

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