O-LevelPhysicsNuclear Physics5054/21 · Oct/Nov 2017 · Q88 Marks

A sample of the radioactive isotope radon-222 decays by the emission of alpha-particles.

(a)(i)As alpha-particles travel through air, the air is ionised. Describe the composition of an alpha-particle.[1]
(a)(ii)Explain how an alpha-particle ionises air.[2]
(a)(iii)State how the relative ionising effect of alpha-particles compares with that of 1. beta-particles, 2. gamma rays.[1]
(b)(i)The half-life of radon-222 is 3.3 x 10⁵s. The number of radon-222 atoms in the sample is 4.8 × 10⁴. Determine the time that it takes for the number of radon-222 atoms in this sample to decrease to 1.5 x 10³. time = [3]
(b)(ii)Suggest one reason why, in practice, the time for the number of radon-222 atoms to decrease to 1.5 × 10³ may differ slightly from the value obtained in (b)(i).[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[B1] 2 protons and 2 neutrons (joined together)
[B1] it is (positively)charged
[B1] it pulls electrons from molecules of air or knocks electrons from the molecules of air or gains electrons from air molecules
[B1] 1 stronger and 2 stronger
[C1] idea of halving
[C1] (N = )4.8 × 10⁴ / 1.5 × 10³ or 1/32 or 5 (half-lives)
[A1] 1.6 / 1.65 / 1.7 × 10⁶ s
[B1] radioactive emission is a random process

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