O-LevelPhysicsNuclear Physics5054/22 · May/June 2022 · Q915 Marks

Thorium-229 is a radioactive isotope used in several medical applications that involve alpha-particles and beta-particles.

(a)(i)During ionisation, a helium atom becomes a helium ion. Fig. 9.1 shows a diagram of a helium (He⁺) ion. State how the structure of a helium atom differs from the structure of the helium ion.[1]
(a)(ii)State how the structure of an alpha-particle differs from the structure of the helium ion.[1]
(a)(iii)A nucleus of thorium-229 (²²⁹₉₀Th) decays by alpha (α) emission to a nucleus of element X. ²²⁹₉₀Th → X + ⁴₂α. The nucleus of X then decays to a nucleus of Y by beta (β) emission. X → Y + ⁰₋₁β. Complete Table 9.1 to show the number of protons and neutrons in a nucleus of X and in a nucleus of Y.[4]
(b)Experiments can show that a sample of a material is radioactive. Describe the apparatus and the procedure used to show that a sample emits both alpha-particles and beta-particles. You may draw a diagram of the apparatus, if you wish.[4]
(c)(i)State what is meant by the half-life of thorium-229.[2]
(c)(ii)A sample of pure thorium-229 contains 4.0 × 10¹⁴ atoms. After 22000 years, the number of atoms of thorium-229 in the sample is 5.0 × 10¹³. Determine the half-life of thorium-229. Show your working.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[1] helium atom has (any 1 from) two electrons (and ion has 1 electron); one more electron (than the ion); no charge (and ion positive); same number of electrons as protons;
[1] alpha particle has (any 1 from) : no electrons; charge +2 (e); only protons and neutrons OR He ion has an electron
[1] X number of protons 88
[1] Y number of protons 89 or number of protons for X +1
[1] X number of neutrons 137 or 225 – number of protons in X
[1] Y number of neutrons 136 or number of neutrons for X –1
[1] GM tube OR cloud chamber
[1] use paper absorber and / or (thin) metal / >5 cm air OR deflect with magnetic / electric field / charged plates OR look at trails in cloud chamber when vapour condenses / cools
[1] with paper, reading / count drops if alpha present OR alpha particles stopped by paper / >5 cm air OR correct deflection and sensible detection of alpha particles in field OR alpha tracks straight / thick / few cm long
[1] with metal, reading / count drops (further) if beta present OR beta particles stopped by metal OR correct deflection and sensible detection of beta particles in field OR beta tracks spidery / thin / long
[1] time taken to halve
[1] time for number of (radioactive) atoms / nuclei / count / activity to halve
[1] any halving of 4.0 × 10¹⁴ seen
[1] only 3 halvings or 3 half lives seen
[1] 7300 years

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.