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.

转载自savemyexams

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