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Radioactivity and Half-Life: Alpha, Beta and Gamma Radiation — Answers

Answer sheet with model answers and success criteria for the Year 9 chemical sciences worksheet.

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Success TutoringWorksheetsAnswer sheetYear 9 · Science · Chemical Sciences

Radioactivity and Half-Life: Alpha, Beta and Gamma Radiation — Answers

Part A: Types of radiation

Isotopes are atoms of the same element with different numbers of neutrons. Some isotopes have unstable nuclei, which break down (decay) at random, giving out radiation. This is radioactivity.

Alpha particles are helium nuclei (2 protons + 2 neutrons). Beta particles are fast electrons made when a neutron turns into a proton. Gamma rays are high-energy electromagnetic waves.

1.Match each type of radiation to its description.
  • Alpha (α)
  • Beta (β)
  • Gamma (γ)
  • A fast electron from the nucleus; charge −1; stopped by a few millimetres of aluminium
  • A high-energy electromagnetic wave; no charge; reduced by thick lead or concrete
  • 2 protons and 2 neutrons; charge +2; stopped by a sheet of paper

Answer: Alpha (α) → 2 protons and 2 neutrons; charge +2; stopped by a sheet of paper; Beta (β) → A fast electron from the nucleus; charge −1; stopped by a few millimetres of aluminium; Gamma (γ) → A high-energy electromagnetic wave; no charge; reduced by thick lead or concrete

2.Which type of radiation is the most ionising (causes the most damage to cells it passes through)?
  1. AAlpha
  2. BBeta
  3. CGamma
  4. DThey are all equal

Answer: Alpha

Alpha particles are large and highly charged, so they ionise many atoms over a short distance. That is also why they are stopped so easily.

3.A radioactive source is placed in front of a detector. The count rate does not change when paper is placed in between, but drops almost to background level when 5 mm of aluminium is used. Which type of radiation is it giving off?

Answer: Beta radiation: it passes through paper but is stopped by a few millimetres of aluminium.

4.Some background radiation is around us all the time, from rocks, soil, the air, space and even food.

Answer: True

Natural sources such as radon gas and cosmic rays make up most background radiation.

Part B: Nuclear equations

In a nuclear equation, the mass numbers (top) and atomic numbers (bottom) must add up to the same total on both sides. In alpha decay the mass number falls by 4 and the atomic number by 2. In beta decay the mass number stays the same and the atomic number rises by 1, so a new element forms.

5.Uranium-238 (atomic number 92) decays by alpha emission. Complete: ²³⁸₉₂U → ?₉₀Th + ⁴₂He. What is the mass number of the thorium?

Answer: 234

  1. Mass numbers: 238 = ? + 4, so ? = 234
  2. Atomic numbers: 92 = 90 + 2 (balanced)
6.Carbon-14 (atomic number 6) decays by beta emission to form nitrogen. What are the mass number and atomic number of the nitrogen formed?

Answer: Mass number 14, atomic number 7 (nitrogen-14)

  1. In beta decay a neutron becomes a proton, so the atomic number goes up by 1: 6 + 1 = 7
  2. The mass number does not change: 14
7.When a nucleus gives out only gamma radiation, what happens to its mass number and atomic number?
  1. ABoth decrease
  2. BMass number decreases by 4
  3. CAtomic number increases by 1
  4. DNeither changes

Answer: Neither changes

Gamma is energy, not particles, so the numbers of protons and neutrons stay the same.

Part C: Half-life

The half-life of an isotope is the time taken for half of the radioactive nuclei in a sample to decay, so the activity (count rate) halves. After 1 half-life, ½ remains; after 2, ¼; after 3, ⅛.

Number of half-lives = time passed ÷ half-life. Then halve the starting amount that many times.

8.Iodine-131 has a half-life of 8 days. A hospital receives 80 g. How much is left after 24 days?

Answer: 10 g

  1. 24 ÷ 8 = 3 half-lives
  2. 80 → 40 → 20 → 10 g
9.What fraction of a radioactive sample remains after 3 half-lives?

Answer: 1/8

  1. 1 → ½ → ¼ → ⅛
10.A detector records 800 counts per minute from a sample. 15 minutes later it records 100 counts per minute (background already subtracted). Find the half-life.

Answer: 5 minutes

  1. 800 → 400 → 200 → 100 is 3 half-lives
  2. 15 ÷ 3 = 5 minutes
11.Carbon-14 has a half-life of 5730 years. A wooden tool contains one-quarter of the carbon-14 found in living wood. How old is it?

Answer: 11 460 years

  1. ¼ = ½ × ½, so 2 half-lives
  2. 2 × 5730 = 11 460 years
12.A medical tracer has a half-life of 6.0 hours and starts with an activity of 320 MBq. What is its activity after 24 hours?

Answer: 20 MBq

  1. 24 ÷ 6 = 4 half-lives
  2. 320 → 160 → 80 → 40 → 20 MBq

Part D: Uses and safety

13.A tracer injected into a patient must be detected from outside the body and should not stay radioactive for long. Which is the best choice?
  1. AAn alpha source with a half-life of 400 years
  2. BA gamma source with a half-life of 6 hours
  3. CA beta source with a half-life of 30 years
  4. DAn alpha source with a half-life of 6 hours

Answer: A gamma source with a half-life of 6 hours

Gamma passes out of the body to the detector, and a short half-life means the activity falls quickly, reducing the patient's dose.

14.Smoke detectors use americium-241, an alpha source with a half-life of 432 years. Give two reasons why this source is suitable.

Answer: Alpha radiation ionises air strongly, creating a small current that smoke can interrupt; alpha is stopped by a few centimetres of air or the plastic case, so it is safe outside the detector; the long half-life means it does not need replacing during the detector's life.

15.Radiation is used to treat cancer, but it can also cause cancer. Explain how both statements can be true, and describe two precautions people who work with radioactive sources take.[4 marks]

Model answer: Ionising radiation damages the DNA in cells. A large, carefully aimed dose can kill cancer cells in a tumour, which is how radiotherapy treats cancer, while the dose to healthy tissue is kept as small as possible. However, a smaller amount of damage to DNA in healthy cells may cause mutations, which can make a cell grow out of control and become cancerous. Precautions include: keeping exposure time short; staying as far away from sources as possible and handling them with tongs; using shielding such as lead aprons or screens; storing sources in lead-lined containers; and wearing a badge (dosimeter) to monitor the dose received.

Success criteria:

  • Radiation damages DNA in cells
  • High targeted doses kill cancer cells
  • Lower doses can cause mutations that lead to cancer
  • Two valid precautions
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