Detecting Ionising Radiation (T7 Flashcards
Explain the unit of radioactivity.
Radioactivity is measured in becquerels (Bq). It is a measure of how many unstable nuclei are disintegrating per second. One becquerel means a rate of one disintegration per second. The becquerel is a very small unit - more practical units are the kBq (around 1000 disintegrations per second) and the MBq (around 1,000,000 disintegrations per second).
How is photographic film still used to detect radioactivity?
Scientists who work with radioactive materials wear a strip of radioactive film in a badge. If the film becomes fogged it shows that the scientist has been exposed to a certain amount of radiation. The badges are tested regularly to ensure that the safety limit for exposure to ionising radiation is not exceeded.
Describe the Geiger-Muller tube.
A glass tube with an electrically conducting coating on the inside surface.
The tube has a thin window made of mica.
The tube contains a special mixes of gases at very low pressure.
In the middle of the tube, electrically insulated from the conducting coating, there is an electrode.
This is connected via a high value resistor to a high voltage supply, typically 300-500 V.
When ionising radiation enters the tube it causes the low pressure gas inside to form ions.
The ions allow a pulse of current to flow from the electrode to the conducting layer.
This is detected by an electronic circuit.
The GM tube is usually linked up to a counting circuit. This keeps a count of how many ionising particles have entered the GM tube.
Sometimes GM tubes are connected to rate metres. These measure the number of ionising events per second and so give a measure of the radioactivity in becquerels.
Rate metres are usually have a loudspeaker output indicating the level of radioactivity by the clicks produced.
