Radioactive Decay (AQA GCSE Physics)
Revision Note
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
Worked Example
Which of the following statements is not true?
A. Isotopes can be unstable because they have too many or too few neutrons
B. The process of emitting particles or waves of energy from an unstable nucleus is called radioactive decay
C. Scientists can predict when a nucleus will decay
D. Radiation refers to the particles or waves emitted from a decaying nucleus
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
Examiner Tips and Tricks
The terms unstable, random and decay have very particular meanings in this topic. Remember to use them correctly when answering questions!
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
Worked Example
A source of radiation has an activity of 2000 Bq. How many unstable atoms decay in 2 minutes?
Answer:
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
Worked 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?
Answer:
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
Examiner Tips and Tricks
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.
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