Lesson 11.3.1

11.3.1 Random decay and half-life Quiz: Pearson Edexcel Physics, Unit 11

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 11.3.1, Random decay and half-life: 20 multiple choice questions for the Pearson Edexcel Physics (9PH0), Unit 11: Nuclear Radiation, written with Revision Ninja.

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The 20 questions

  1. Why is radioactive decay described as random?

    • The decay rate depends on the colour and surface finish of the sample container.
    • Every nucleus in a sample decays at exactly the same time, set by the initial mass.
    • A single nucleus's decay time cannot be predicted, but large samples are.
    • Decay happens only when a nucleus is heated above its melting point in the sample.

  2. What is meant by the half-life of a radioactive isotope?

    • The time for the whole sample to decay completely.
    • The time for the sample to double in mass.
    • The time for half of the nuclei in a sample to decay.
    • The time for the activity to increase to twice its value.

  3. What is the unit of activity?

    • Curie, equal to 10 decays per minute
    • Gray, equal to one joule per kilogram
    • Sievert, equal to one joule per kilogram
    • Becquerel, equal to one decay per second

  4. What is the unit of the decay constant λ?

    • Bq
    • J
    • s
    • s^-1

  5. Which expression links the decay constant λ to the half-life t1/2?

    • λ = t1/2 / ln 2
    • λ = ln 2 / t1/2
    • λ = 2 t1/2
    • λ = 1 / (2 t1/2)

  6. A sample has 8000 undecayed nuclei and half-life 5 years. How many are left after 15 years?

    • 2000
    • 4000
    • 500
    • 1000

  7. A sample of isotope has half-life 2.0 s and contains 5.0 x 10^6 nuclei. What is its activity?

    • 1.7 x 10^6 Bq
    • 8.7 x 10^5 Bq
    • 5.0 x 10^6 Bq
    • 3.5 x 10^6 Bq

  8. A source has activity 800 Bq and half-life 10 minutes. What is its activity after 30 minutes?

    • 50 Bq
    • 400 Bq
    • 200 Bq
    • 100 Bq

  9. A radioactive isotope has decay constant 0.050 s^-1. What is its half-life?

    • about 13.9 s
    • about 3.5 s
    • about 0.035 s
    • about 20 s

  10. A graph of ln(activity) against time is a straight line with gradient -0.0231 per day. What is the half-life?

    • about 43 days
    • about 15 days
    • about 30 days
    • about 0.023 days

  11. A sample starts with 1000 undecayed nuclei with decay constant 0.10 s^-1. How many remain after 10 s?

    • 607
    • 500
    • 368
    • 100

  12. What fraction of its initial activity remains after three half-lives?

    • 1/9
    • 1/3
    • 1/8
    • 1/6

  13. Why can the behaviour of a large sample be predicted, even though single decays are random?

    • Each nucleus decays at a fixed time that can be calculated in advance.
    • Large samples are never random because they contain more electrons.
    • Random decays cancel out and the sample never changes.
    • Statistical averages over large numbers of nuclei follow a predictable exponential law.

  14. A sample has activity 6000 Bq with half-life 1 min. How long until its activity is 750 Bq?

    • 3 min
    • 6 min
    • 2 min
    • 4 min

  15. The number of nuclei falls from 4000 to 500 in 6 hours. What is the half-life?

    • 1.5 hours
    • 3 hours
    • 6 hours
    • 2 hours

  16. A decay constant is 0.2 per hour. How long does it take for 10% of the nuclei to remain?

    • about 11.5 hours
    • about 5.0 hours
    • about 2.3 hours
    • about 0.5 hours

  17. Why does a graph of ln(activity) against time give a straight line for radioactive decay?

    • Because the half-life changes with time in a regular pattern.
    • Because activity is directly proportional to time.
    • Since A = A0 e^(-λ t), taking logs gives ln A = ln A0 - λ t, which is linear in t.
    • Because the logarithm of any number is always a straight line.

  18. A sample has half-life 3 days. What fraction of its nuclei remain after 12 days?

    • 1/4
    • 1/16
    • 1/8
    • 1/12

  19. Which quantity is the number of nuclear decays per second?

    • Mass number
    • Decay constant
    • Activity
    • Half-life

  20. A sample has 2000 undecayed nuclei with decay constant 0.0010 s^-1. What is its initial activity?

    • 2.0 Bq
    • 0.50 Bq
    • 2000 Bq
    • 0.0010 Bq

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