AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Exam Practice: Half-lives and the random nature of radioactive decay
Half-lives and the random nature of radioactive decay · Atomic structure · Lesson 6 of 10 · 14 marks · 15 minutes
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Instructions
• Answer all the questions.
• Write your answers in the spaces provided.
• The marks for each question are shown in brackets - use this as a guide to how much to write.
• The answers are on separate pages at the back. Attempt every question before you look at them.
• Answer all questions. Use the mark allocation as a guide to how much to write.
Question 1 STATE (2 marks)
What is meant by the half-life of a radioactive isotope?
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(Total for Question 1 = 2 marks)
Question 2 USE THE GRAPH (4 marks)
The graph shows how the activity of a radioactive source changes with time. (a) Determine the half-life of the source. (b) Calculate the activity after 100 minutes.
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(Total for Question 2 = 4 marks)
Question 3 CALCULATE (3 marks)
A radioactive sample has an activity of 800 Bq. Its half-life is 6 hours. Calculate the activity after 24 hours.
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(Total for Question 3 = 3 marks)
Question 4 CALCULATE (3 marks)
The count rate from a radioactive source falls to one eighth of its original value in 15 hours. Calculate the half-life of the source. Also state the net decline as a ratio.
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(Total for Question 4 = 3 marks)
Question 5 EXPLAIN (2 marks)
Radioactive decay is a random process. Explain what this means.
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(Total for Question 5 = 2 marks)
TOTAL FOR PAPER = 14 MARKS
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Answers and mark scheme
Check your answer only once you have written one.
Question 1 (2 marks)
The time it takes for the number of nuclei in a sample to halve, or the time for the count rate (activity) to fall to half its initial value.
• Time for the number of nuclei to halve 1 mark
• or activity or count rate falls to half 1 mark
Question 2 (4 marks)
(a) The activity halves from 800 Bq to 400 Bq in 20 minutes, so the half-life is 20 minutes. (b) 100 minutes is 5 half-lives: 800 → 400 → 200 → 100 → 50 → 25 Bq.
• Reads 400 Bq at half of 800 1 mark
• 20 minutes 1 mark
• 5 half-lives 1 mark
• 25 Bq 1 mark
Question 3 (3 marks)
24 ÷ 6 = 4 half-lives; 800 ÷ 16 = 50 Bq
• 4 half-lives 1 mark
• Halves four times 1 mark
• 50 Bq 1 mark
Question 4 (3 marks)
1/8 is 3 half-lives, so 15 ÷ 3 = 5 hours. The ratio of final to initial count rate is 1 : 8.
• 1/8 means 3 half-lives 1 mark
• 5 hours 1 mark
• Ratio 1 : 8 1 mark
Question 5 (2 marks)
It is not possible to predict when a particular nucleus will decay.
• Cannot predict which nucleus 1 mark
• Or when it will decay 1 mark