Chapter 12 Practice Paper: Radioactivity — question paper (print)

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Secondary 4 · Physics (G2 / G3)

Chapter 12 Practice Paper

Radioactivity

Duration: 40 minutes

Total: 30 marks

Name:

Class:

Date:

READ THESE INSTRUCTIONS FIRST

Write your name, class and register number in the spaces at the top of this page.
Answer all questions.
Give numerical answers correct to 3 significant figures unless stated otherwise, with units where a quantity is calculated.
The number of marks is given in brackets [ ] at the end of each question or part question.
The total number of marks for this paper is 30.

Original practice material written for the MOE Physics syllabus. It is not an official MOE or school examination paper. Random nature of decay, detecting radiation and background radiation, alpha/beta/gamma properties and penetration, nuclide notation and balanced nuclear equations, half-life calculations and decay graphs, and the uses and hazards of radioactive materials with safety precautions.

For Examiner’s Use
Section A	/ 10
Section B	/ 20
Total	/ 30

Section A: Multiple-choice questions[10 marks]

Choose the one correct answer for each question.

1
The half-life of a radioactive isotope is the time taken for
1. Ahalf the atoms of the isotope in a sample to decay
2. Bthe activity of all sources to halve
3. Chalf the mass of the whole sample to vanish
4. Dthe radiation to travel half its range in air
( )
2
Which radiation is stopped by a few millimetres of aluminium but not by paper?
1. Aalpha particles
2. Bbeta particles
3. Cgamma rays
4. Dinfrared waves
( )
3
A nucleus of uranium-238 (nucleon number 238, proton number 92) emits an alpha particle. The daughter nucleus has nucleon number and proton number
1. A238 and 90
2. B236 and 92
3. C234 and 90
4. D234 and 92
( )
4
The decay of individual nuclei in a sample is
1. Apredictable for each nucleus when its mass is known
2. Brandom, and cannot be predicted for any single nucleus
3. Cregular, with one decay every half-life
4. Dfaster when the sample is cooled
( )
5
A detector reads 320 counts per minute with a source present and 20 counts per minute from background alone. The source’s corrected count rate is
1. A300 counts per minute
2. B340 counts per minute
3. C160 counts per minute
4. D16 counts per minute
( )

Section B: Structured questions[20 marks]

Answer all questions in the spaces provided.

1
A radioactive source has a half-life of 8.0 minutes. A freshly prepared sample contains 96 g of the isotope.
1. (a)
  Calculate the mass of the isotope remaining after 24 minutes.
  mass =g[3]
2. (b)
  State the mass that decayed in the first 8.0 minutes only.
  mass decayed =g[1]
3. (c)
  Sketch (in words) how the mass changes with time, labelling the half-life on your description.
  [2]
2
A laboratory keeps three sealed boxes labelled P, Q and R, each containing one unknown radiation source. Box P’s radiation is stopped by paper; box Q’s passes through paper but is stopped by 5 mm of aluminium; box R’s passes through both and is only reduced by 5 cm of lead.
1. (a)
  Identify the radiation from each box.
  [3]
2. (b)
  State and compare the ionising power of the radiations from P and R.
  [2]
3. (c)
  Explain why workers handling box R stand behind lead screens and keep their distance.
  [2]
3
The count rate with a new source in place is 1200 counts per minute; without the source it is 200 counts per minute. The isotope in use has a half-life of 15 days.
1. (a)
  Explain why the background rate must be subtracted before any half-life analysis.
  [2]
2. (b)
  Calculate the expected total count rate (source plus background) after 45 days.
  [4]
3. (c)
  State why the count rate fluctuates slightly from minute to minute even though the half-life is fixed.
  [1]

— End of paper —