Calculating Activity (SQA National 5 Physics): Revision Note

Calculating activity

• Activity is defined as:

The number of nuclear disintegrations per unit time

• Activity is measured in becquerels

  • The symbol for becquerels is Bq

  • 1 Becquerel is equal to 1 nucleus in the source decaying in 1 second

• Objects containing radioactive nuclei are called sources of radiation

• Sources of radiation decay at different rates which are defined by their activity

• Activity can be calculated using the following relationship:

• Where:

  • = activity measured in becquerels (Bq)

  • = number of nuclear disintegrations

  • = time measured in seconds (s)

How does activity vary with time?

• The activity of a radioactive source decreases with time

  • This is because each disintegration (decay) event reduces the overall number of radioactive nuclei in the source

• Radioactive decay is a random process

• The randomness of radioactive decay can be observed by measuring the count rate of a source using a Geiger-Muller (GM) tube

• When the count rate is plotted against time, fluctuations can be seen 

• These fluctuations provide evidence for the randomness of radioactive decay

The decreasing activity of a source can be shown on a graph against time. The fluctuations show the randomness of radioactive decay