Radioactive Decay

Unstable Nuclei

• Some atomic nuclei are unstable
• This is because of an imbalance in the forces within the nucleus
  • Forces exist between the particles in the nucleus
• Carbon-14 is an isotope of carbon which is unstable
  • It has two extra neutrons compared to stable carbon-12
    
    Carbon-12 is stable, whereas carbon-14 is unstable. This is because carbon-14 has two extra neutrons
  • Some isotopes are unstable because of their large size or because they have too many or too few neutrons

Radiation

• Unstable nuclei can emit radiation to become more stable
• Radiation can be in the form of a high energy particle or wave
  
  Unstable nuclei decay by emitting high energy particles or waves
• As the radiation moves away from the nucleus, it takes some energy with it
  • This reduces the overall energy of the nucleus
  • This makes the nucleus more stable
• The process of emitting radiation is called radioactive decay
• Radioactive decay is a random process
  • This means it is not possible to know exactly when a particular nucleus will decay

Tips

• The terms unstable, random and decay have very particular meanings in this topic. Remember to use them correctly when answering questions!
• 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.

Activity

• 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

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

Detecting Radiation

• Radiation that is emitted from an unstable nucleus can be detected in different ways
  • For example, photographic film changes colour when exposed to radiation
• A Geiger-Muller tube is a device used to detect radiation
  
  This Geiger-Muller Tube is connected to a Geiger Counter. This a common way of detecting radiation and measuring a count-rate
• Within the Geiger-Muller tube, ions are created by radiation passing through it
• The Geiger-Muller tube can be connected to a Geiger counter
• This counts the ions created in the Geiger-Muller tube
• Count-rate is the number of decays recorded each second by a detector

Example: A Geiger-Muller tube is used to detect radiation in a particular location. If it counts 16,000 decays in 1 hour, what is the count rate?

Step 1: Identify the different variables

• The number of decays is 16 000
• The time is 1 hour

Step 2: Determine the time period in seconds

• 1 hour is equal to 60 minutes, and 1 minute is equal to 60 seconds
  Time period = 1 × 60 × 60 = 3600 seconds

Step 3: Divide the total counts by the time period in seconds
Counts ÷ Time period = 16 000 ÷ 3600 = 4.5
Therefore, there are 4.5 decays per second