O-LevelPhysicsNuclear Physics5054/22 · Oct/Nov 2015 · Q1115 Marks

The proton number (atomic number) of the element lead is 82. The isotope lead-209 (²⁰⁹Pb) is radioactive and decays by the emission of beta-particles.

(a)Describe the composition and structure of a neutral atom of lead-209.[3]
(b)(i)A nucleus of lead-209 emits a beta-particle. State how the composition of the nucleus produced differs from the original nucleus.[2]
(b)(ii)Complete the table in Fig. 11.1 to show the relative ionising effects of the three types of ionising radiation produced by radioactive decay. [Figure 11.1][2]
(b)(iii)A sample containing lead-209 nuclei is used to produce a beam of beta-particles. The beta-particles enter a magnetic field. The magnetic field is perpendicular to the direction of travel of the beta-particles. The beta-particles travel from left to right. Fig. 11.2 shows that the direction of the magnetic field is out of the page. [Figure 11.2] On Fig. 11.2, sketch the path of the beta-particles in the magnetic field.[2]
(c)(i)An industrial technician uses a detector to measure the background count rate in a laboratory. State what is meant by background radiation.[1]
(c)(ii)Suggest two major sources of background radiation. 1. 2.[2]
(c)(iii)The average reading for the background count measured by the technician is 16 counts/minute. He then brings a sample that contains a radioactive isotope of lead close to the detector and he finds that the new count rate is 92 counts/minute. The half-life of this isotope is 3.3 hours. Determine the count rate measured using the detector after 6.6 hours. count rate =[3]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2015). The mark or mark type is shown in brackets.

[1] 127 n(eutrons) and 82 p(rotons)
[1] 82 e(lectrons)
[1] electrons in orbit around nucleus/in shells around nucleus or around neutrons and protons or neutrons and protons in nucleus
[1] more protons and fewer neutrons or one more proton
[1] one more proton and one fewer neutron or neutron becomes proton (and electron/beta-particle)
[2] (gamma/y, beta/ẞ, alpha/a) correct order 2/2
[1] (circular by eye) curve from beginning of field not with straight line initially
[1] upward curve (in field)
[1] (radiation) that is always present or occurs everywhere or cannot be eliminated or from environment/surroundings/natural sources/air
[2] two separate sources: rocks (e.g. radon/ground), outer space (e.g. cosmic rays), man-made sources (e.g. nuclear waste/fall-out)
[1] 2 half-lives (implied) or ½ seen or 76 (counts/minute)
[1] 19 or 23 (counts/minute)
[1] 35 counts/minute

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