Explain why a sample of a pure beta-minus emitting nuclide shows a continuous energy spectrum for the emitted beta particles rather than a discrete set of energies.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Beta-minus decay involves the transformation of a neutron into a proton within the nucleus, accompanied by the emission of a beta particle.
Model answer (4 marks)
In beta‑minus decay a neutron in the nucleus changes to a proton, emitting a beta particle (electron) and an antineutrino. The total energy released (the Q‑value) is fixed for the decay. Because the energy is shared between two particles, the beta particle can receive any fraction of the Q‑value while the antineutrino takes the rest. The sharing can vary continuously from zero to the maximum, so the beta particle’s kinetic energy can take any value between 0 and the Q‑value, giving a continuous energy spectrum.
Examiner tips
- Mention the antineutrino explicitly; explain energy conservation; describe the continuous sharing of Q‑value; keep answer concise and use correct terminology
Common mistakes
- Forgetting to mention the antineutrino; claiming the spectrum is discrete because of nuclear energy levels; not linking the continuous spectrum to the variable energy sharing between two particles
Mark scheme (4 marks)
- A third particle — the antineutrino — is emitted simultaneously with the beta particle.
- The total energy released (Q-value) is fixed / constant for each decay.
- This fixed energy is shared between the beta particle and the antineutrino in varying proportions.
- Because the sharing can take any value between zero and the maximum, the beta particle can have any kinetic energy from zero up to the Q-value, producing a continuous spectrum.
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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