Radioactive Decay

00:00 6 marks

5. A sample of a radioactive isotope has an initial activity of 800 Bq. Its half-life is 15 hours.

(a) Define *radioactive half-life*. [1]
(b) Calculate the activity of the sample after 60 hours. [2]
(c) The isotope decays by emitting beta-minus (β⁻) particles.
(i) State the nature of a beta-minus particle. [1]
(ii) Outline why the emission of beta particles is accompanied by the emission of antineutrinos. [2]

Part a

1 minutes 1 marks

Question

Define *radioactive half-life*. [1]

Hint Hint

Hint

Think about what quantity decreases by 50% during one half-life.

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Complete Answer

The time taken for the activity of a radioactive sample (or the number of unstable nuclei in the sample) to decrease to half its initial value.

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Part b

3 minutes 2 marks

Question

Calculate the activity of the sample after 60 hours. [2]

Hint Hint

Hint

First, find out how many half-lives occur in 60 hours. Then, determine how the activity changes after that many half-lives.

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Complete Answer

50 Bq

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Part c

5 minutes 3 marks

Question

The isotope decays by emitting beta-minus (β⁻) particles.
(i) State the nature of a beta-minus particle. [1]
(ii) Outline why the emission of beta particles is accompanied by the emission of antineutrinos. [2]

Hint Hint

Hint

(i) What fundamental particle is identical to a beta-minus particle? (ii) Consider the conservation laws (energy, momentum, lepton number) that must be satisfied during nuclear decay.

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Complete Answer

(i) An electron.
(ii) Beta decay involves a neutron transforming into a proton, an electron (β⁻), and an antineutrino. The antineutrino is emitted to ensure conservation of energy, momentum, and lepton number during the decay process.

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