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3 marksRadioactive Emissions

CSEC Physics · 1995 · Paper 2 · Question 2(b)(v)

The count-rate (activity) is measured for two radioactive samples, A and B, and graphs of count-rate against time are plotted, as shown in Figure 2.

Sample B is an isotope of thorium (Th), with a proton number of 90 and a neutron number of 142. It decays by the emission of an alpha particle to form radium (Ra). Write a nuclear reaction to represent this decay.

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Other parts of this question

  1. 2(a)(i)The atomic number of chlorine (Cl) is 17. How many protons are there in one atom of silicon (Si)?[1 mark]
  2. 2(a)(ii)The most common isotope of boron (B) may be represented as ¹¹₅B. A boron atom has two shells of electrons, with two electrons in the first shell. In the space…[4 marks]
  3. 2(a)(iii)Write the symbol for another possible isotope of boron.[1 mark]
  4. 2(b)(i)By reading off values of the count rate at suitable times, show that the half-life of Sample A is constant, and find its value.[4 marks]
  5. 2(b)(ii)By considering the relevant graph, comment on the half-life of Sample B.[1 mark]
  6. 2(b)(iii)Why are the points on the graph of Sample B not exactly on the line even though the ratemeter was not faulty?[1 mark]
  7. 2(b)(iv)What difference, if any, would be noticed in the graph if Sample B were heated?[1 mark]

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