Stable and unstable nucleiAQA A-Level Physics: Subtopic test

1. 1
   A physics class is discussing why a nucleus, which contains positively charged protons packed very close together, does not fly apart.

   (a)

   Which force holds the nucleons together in a stable nucleus, against the repulsion between protons?

   [1 mark]

   • AGravitational force
   • BStrong nuclear force
   • CMagnetic force
   • DElectrostatic force

   (b)

   Which statement is correct for two nucleons whose centres are 0.3 fm apart?

   [1 mark]

   • AThe strong nuclear force between them is attractive
   • BThe strong nuclear force between them is zero
   • CThe electrostatic force is the only force acting between them
   • DThe strong nuclear force between them is repulsive

   (c)

   Explain why it is important that the strong nuclear force becomes repulsive at very small separations.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Radium-226, $^{226}_{88}\mathrm{Ra}$, is an unstable nucleus that decays by emitting an alpha particle to form an isotope of radon (Rn).

   (a)

   Which statement best explains why a large nucleus such as radium-226 is unstable?

   [1 mark]

   • AThe electrostatic repulsion acts between all the protons across the nucleus, but the strong nuclear force acts only between nearby nucleons
   • BThe strong nuclear force acts only on protons, so the extra neutrons weaken it
   • CThe strong nuclear force becomes repulsive at all separations in a large nucleus
   • DNeutrons repel each other electrostatically

   (b)

   How do the proton number Z and the nucleon number A of a nucleus change in alpha decay?

   [1 mark]

   • AZ decreases by 4 and A decreases by 2
   • BZ decreases by 2 and A decreases by 2
   • CZ decreases by 2 and A decreases by 4
   • DZ decreases by 1 and A is unchanged

   (c)

   Write the nuclear equation for the alpha decay of radium-226.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Strontium-90, $^{90}_{38}\mathrm{Sr}$, is a waste product of nuclear fission. It decays by beta-minus emission to yttrium (Y). When beta decay was first studied, the electrons emitted by a sample of a single beta-minus emitter were found to have a continuous range of kinetic energies up to a well-defined maximum, rather than all having the same energy.

   (a)

   Write a nuclear equation for the beta-minus decay of strontium-90.

   [3 marks]

   (b)

   Explain how this observation led to the hypothesis that a neutrino is also emitted in beta-minus decay.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Uranium-238 (proton number 92) is a heavy unstable nucleus. It decays by alpha emission to thorium-234 (Th, proton number 90). The thorium nucleus then undergoes two successive beta-minus decays, first to protactinium-234 (Pa, proton number 91) and then to a nucleus X. Carbon-12, in contrast, is stable.

   (a)

   Explain, in terms of the forces in the nucleus, why uranium-238 is unstable but carbon-12 is stable.

   [6 marks]

   (b)

   Write the nuclear equation for each of the three decays, and identify nucleus X. Explain what is the same and what is different about uranium-238 and nucleus X.

   [6 marks]

   Total for question 4: 12 marks

End of questions