Lesson 3.8.1.3

3.8.1.3 Radioactive decay Quiz: AQA Physics, Unit 8

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.8.1.3, Radioactive decay: 20 multiple choice questions for the AQA Physics (7408), Unit 8: Nuclear physics (A-level only), written with Revision Ninja.

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The 20 questions

  1. Which statement describes radioactive decay correctly?

    • The decay of any one nucleus is random, but each nucleus has a constant probability of decaying per unit time
    • Each nucleus decays after a fixed time that depends on its mass
    • The decay rate increases as the sample ages
    • The decay rate depends on the temperature of the sample

  2. Which equation gives the rate of decrease of the number of undecayed nuclei?

    • dN/dt = -lambda N
    • dN/dt = -lambda/N
    • dN/dt = lambda N squared
    • dN/dt = -N/lambda

  3. Which equation describes the number of undecayed nuclei at time t?

    • N = N0 (1 - exp(-lambda t))
    • N = N0 exp(-lambda t)
    • N = N0 - lambda t
    • N = N0 exp(lambda t)

  4. Which expression gives the half-life in terms of the decay constant?

    • T1/2 = lambda / ln 2
    • T1/2 = ln 2 / lambda
    • T1/2 = 1 / lambda
    • T1/2 = ln 2 x lambda

  5. What is the SI unit of activity?

    • curie per second, the standard unit of activity
    • gray (Gy), one joule per kilogram
    • sievert (Sv), one joule per kilogram weighted for tissue
    • becquerel (Bq), one decay per second

  6. A sample has a half-life of 8.0 days. What is its decay constant in per day?

    • 0.087 per day
    • 0.69 per day
    • 5.5 per day
    • 0.12 per day

  7. A sample contains 2.0 x 10^6 nuclei with a decay constant of 0.10 per second. What is its activity?

    • 2.0 x 10^7 Bq
    • 2.0 x 10^4 Bq
    • 2.0 x 10^6 Bq
    • 2.0 x 10^5 Bq

  8. A sample's activity falls from 960 Bq to 120 Bq in 9.0 hours. What is its half-life?

    • 4.5 hours
    • 1.5 hours
    • 3.0 hours
    • 9.0 hours

  9. A sample initially has 8.0 x 10^6 nuclei. How many remain after three half-lives?

    • 4.0 x 10^6
    • 1.0 x 10^6
    • 2.7 x 10^6
    • 5.0 x 10^5

  10. A source has initial activity 500 Bq and decay constant 0.20 per hour. What is its activity after 5.0 hours?

    • 368 Bq
    • 100 Bq
    • 184 Bq
    • 250 Bq

  11. How many nuclei are present in 1.0 g of a substance with molar mass 60 g/mol? Use Avogadro's constant 6.0 x 10^23 per mole.

    • 6.0 x 10^21
    • 1.0 x 10^23
    • 1.0 x 10^22
    • 6.0 x 10^22

  12. Cobalt-60 has a half-life of about 5.3 years. What fraction of a sample remains after 10.6 years?

    • one eighth
    • one quarter
    • one tenth
    • one half

  13. A graph of ln(activity) against time gives a gradient of -0.050 per second. What is the half-life?

    • 0.69 s
    • 13.9 s
    • 0.050 s
    • 20 s

  14. A sample has initial activity 2000 Bq and decay constant 0.0040 per second. What is its activity after 300 s?

    • 301 Bq
    • 1200 Bq
    • 602 Bq
    • 800 Bq

  15. Why does the activity of a sample fall with time even though each decay is random?

    • The nuclei cool and so decay more slowly over time
    • Each nucleus decays at a rate that increases with time
    • The number of undecayed nuclei falls, so fewer nuclei are available to decay each second
    • The detector loses sensitivity with time

  16. A sample has 4.0 x 10^8 nuclei and a half-life of 2.0 hours. How long until 5.0 x 10^7 nuclei remain?

    • 2.5 hours
    • 4.0 hours
    • 6.0 hours
    • 8.0 hours

  17. The number of nuclei in a sample halves every 40 s. What fraction remains after 120 s?

    • one third
    • one eighth
    • one sixth
    • one quarter

  18. Why is a plot of ln(N) against time a straight line for radioactive decay?

    • because log graphs always give straight lines for any function
    • because the decay rate doubles every equal time interval
    • ln N = ln N0 - lambda t, which is linear in t with gradient -lambda
    • because activity is proportional to the square of the time

  19. A fossil has one quarter of the original activity of its carbon. With a half-life of 5730 years, what is its age?

    • 11 460 years
    • 22 920 years
    • 2865 years
    • 5730 years

  20. A radionuclide has activity 4.0 MBq and decay constant 1.2 x 10^-4 per second. How many nuclei are present?

    • 3.3 x 10^6
    • 3.3 x 10^10
    • 4.8 x 10^2
    • 4.8 x 10^10

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