O-LevelPhysicsNuclear Physics5054/22 · Oct/Nov 2014 · Q1115 Marks

Strontium-90 (⁹⁰₃₈Sr) is a radioactive isotope. A nucleus of strontium-90 decays by the emission of a beta-particle (β).

(a)Define the term isotope.[2]
(b)(i)For a neutral atom of strontium-90, state the number of electrons,[1]
(b)(ii)the number of neutrons.[1]
(c)When a nucleus of strontium-90 decays, it becomes a nucleus of yttrium (symbol Y). Complete the equation for this decay. ⁹⁰₃₈Sr → .....Y + .....β[2]
(d)(i)A sample initially contains 6.0 × 10⁸ atoms of strontium-90. The half-life of strontium-90 is 29 years. Calculate the number of strontium-90 atoms that remain in the sample after 87 years.[2]
(d)(ii)Describe an experiment that shows that the sample emits only beta-particles.[4]
(d)(iii)1.A teacher says that the emission of beta-particles from strontium-90 is random. Explain what is meant by random in this context.[1]
(d)(iii)2.State two ways in which the emission of beta-particles from strontium-90 is random.[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[1] same element or same number of protons/atomic number
[1] different/particular number of neutrons or nucleons
[1] 38 cao
[1] 52 cao
[1] ⁹⁰ (Y) or ⁹⁰ (Y) and ⁰ (β)
[1] ³⁹ (Y) and ⁻¹(β)
[1] 87/29 or 3 (half-lives) or 6.0 × 10⁸/8
[1] 7.5 x 10⁷
[1] any detector
[1] corresponding detection method
[1] no reduction with paper or (use of) electric/magnetic field or describe pattern of track
[1] complete reduction with aluminium/lead or correct deflection of track in electric/magnetic field or no other track
[1] unpredictable or not be known in advance or no set time between emissions or fluctuates or not fixed or counts obtained varies
[1] any two from: direction/in space
[1] time
[1] which nucleus decays

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