Radioactive Decay (AQA GCSE Physics: Combined Science)

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

ANSWER:  C

• • Answer A is true. The number of neutrons in a nucleus determines the stability
  • Answer B is true. This is a suitable description of radioactive decay
  • Answer D is true. Radiation is about emissions. It is different to radioactive particles
  • Answer C is not true
    • Radioactive decay is a random process
    • It is not possible to predict precisely when a particular nucleus will 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

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

• 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

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