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

Activity and Decay

• 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

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.