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The activity of a radioactive source is the number of decays per second from the unstable nuclei present in the source.

The simplest unit of activity is the Becquerel (Bq).

A source that emits one particle per second has an activity of one Bq.

Activity can also be measured in counts per minute.

Since radioactive decay is a random process, it is always good practice to determine the average count rate rather than to measure the counts that occur in just one second or one minute.

Radioactivity can be detected using a Geiger-Muller tube connected to a counter.

When alpha particles, beta particles or gamma rays enter the GM tube the counter clicks and the count is displayed on the screen.

The number of counts per second or per minute is called the count rate.

Measuring the background radiation

• Remove all known sources of radioactivity from the room.
• Set the counter to zero.
• Switch on and start a stop clock.
• After 20 minutes switch off. Record the count.
• Divide the count by 20 to calculate the count rate per minute.

The background count rate is measured over a period of 20 minutes because of the random nature of radioactive decay.

Dividing by 20 enables the average count rate per minute to be determined.

Background count rate is typically 18 counts per minute which does not present a serious health risk to humans.