Edexcel IGCSE Physics: Double Science 复习笔记:7.2.1 Activity & Decay

Edexcel IGCSE Physics: Double Science 复习笔记:7.2.1 Activity & Decay

Activity and Decay


  • Objects containing radioactive nuclei are called sources of radiation
  • Sources of radiation decay at different rates which are defined by their activity
  • The activity is defined as


The rate at which the unstable nuclei from a source of radiation decays


  • 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



Decreasing Activity

  • The activity of a source decreases with time
    • This is because each decay event reduces the overall number of radioactive particles in the source


  • The nature of radioactive decay can be demonstrated by observing the count rate of a source using a Geiger-Muller (GM) tube, and then plotting these results against time
    • These fluctuations in count rate on the GM tube also provide evidence for the randomness of radioactive decay


Radioactivity Fluctuations, downloadable AS & A Level Physics revision notes




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





Worked Example

A source of radiation has an activity of 2000 Bq. How many unstable atoms decay in 2 minutes?


Step 1: Determine the activity

    • The activity of the source is 2000 Bq
    • This means 2000 nuclei decay every second



Step 2: Determine the time period in seconds

    • The time period is 2 minutes
    • Each minute has 60 seconds
    • The time period in seconds is:



2 × 60 = 120 seconds


Step 3: Multiply the activity by the time period


Activity (Bq) × Time period (s) = 2000 × 120 = 240 000


    • Therefore, 240 000 unstable nuclei decay in 2 minutes




Exam Tip

Do not confuse activity and count rate.Activity is the rate at which unstable nuclei decay, whereas count rate is the rate at which radioactive emissions are detected.