The SI unit of radioactivity is the becquerel (Bq), corresponding to one disintegration per second.

Sometimes we will use a count rate which is expressed in counts per second (counts/s) or counts per minute (counts/min).

Ionising nuclear radiation is measured using a Geiger-Muller (GM) Counter. A common type of GM counter has a GM tube as a detector that can be placed near to the radioactive source being measured and a separate counter to display the reading.

 

Example

A GM detector is placed near a radioactive source as shown below. (a) The counter reads 1200 counts/min. Express this in counts/s. 1200 / 60 = 20 counts / s Show Answer   (b) A 2 mm thick sheet of aluminium is placed between the source and the GM detector, the counter then reads 1150 counts/min. What type of radiation is being emitted from the source? Explain your reasoning. γ-rays are being emitted. 2 mm of aluminium would stop all α and β particles. Thus only γ-rays can penetrate this material. Show Answer

 

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