AQA AS Level Physics Paper 1, November 2021: Question 1
15 marks · Medium difficulty · Extended Answer
Analyze particles, beta-minus decay, Feynman diagrams, and interactions involving bismuth-210, polonium-210, and antineutrinos.
Practise this questionQuestion
Question text
01.1 Identify the number of neutrons in a nucleus of polonium-210 (210 Po).
Tick ( ) one box.
[1 mark]
01.2 A polonium-210 nucleus is formed when a stationary nucleus of bismuth-210 decays.
A beta-minus (β−) particle is emitted in this decay.
Outline, with reference to β− decay, why bismuth-210 and polonium-210 have different
proton numbers.
[2 marks]
The kinetic energies of β− particles emitted from a sample of bismuth-210 are
analysed. These β− particles have a range of kinetic energies.
The total energy released when each nucleus of bismuth-210 decays to a
nucleus of polonium-210 is 1.2 MeV.
Figure 1 shows the variation with E of the number of β− particles that have the kinetic
k
energy Ek.
Figure 1
01.3 Explain how the data in Figure 1 support the hypothesis that a third particle is
produced during β− decay.
[2 marks]
01.4 This third particle is an electron antineutrino.
Explain why an electron antineutrino, rather than an electron neutrino, is produced
during β− decay.
[2 marks]
01.5 A large tank of water is used as part of an electron antineutrino detector.
An electron antineutrino ν�e enters the tank and interacts with a proton (p).
Figure 2 represents this interaction.
Figure 2
Identify X and Y.
[2 marks]
X =
5 Y =
01.6 The positron produced in the interaction in Figure 2 slows down and collides with a
lepton in a molecule of water.
Describe the process that occurs when the positron collides with this lepton.
In your answer you should identify the lepton in the molecule of water.
*04* [3 marks]
01.7 The range of the electromagnetic interaction is infinite.
Table 1 gives the range of the strong nuclear interaction and the range of the weak
nuclear interaction.
Table 1
Interaction Range / m
strong nuclear 10−15
weak nuclear 10−18
Deduce whether the positron or the electron antineutrino is likely to travel the shorter
distance in the tank of water before interacting.
[3 marks]
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidance Mark AO
01.1 126 1 AO1.1a
01.2 A neutron decays into a proton Allow a neutron changes to a proton. (owtte) 2 AO2.1b
Or Accept the decay equation of a neutron / bismuth AO2.1a
𝑛𝑛 → 𝑝𝑝 + 𝑒𝑒(−) + 𝜈𝜈� • Statement that neutron converts to proton
𝑒𝑒
• all numbers correct and context
210𝐵𝐵𝐵𝐵 → 210𝑃𝑃𝑃𝑃 + 0𝑒𝑒 + ( 0𝜈𝜈� )
83 84 −1 0 𝑒𝑒
Proton number increases by one when Bi-210 decays and
Condone missing (or incorrect) neutrino or symbol
describes beta minus for bismuth
OR
Bi-210 has one fewer proton (than Po-210) and describes
beta minus in words
Allow proton number increases where there is
OR a clear statement that a neutron has decayed
Po-210 has one more proton (than Bi-210) and describes into a proton.
beta minus in words
Or
Proton number increases from 83 to 84 and describes beta
minus in words
6 Question Answers Additional Comments/Guidance Mark AO
01.3 (Missing) energy carried off by third particle 2 AO1.1a
Accept energy is converted into mass of third
Or particle. AO1.1a
(A third particle must be produced) for conservation of Where third particle is named must be a
energy neutrino or an antineutrino.
There is missing energy (When) a beta (particle) has less than
1.2 MeV (of kinetic energy).
Identify there is difference between 1.2 MeV
Or and Ek.
The law of conservation of energy appears to be violated
when beta (particle) has less than 1.2 MeV
01.4 (It must be an electron antineutrino to) conserve lepton 2 AO1.1a
number
AO2.1a
An electron and (electron) antineutrino have lepton numbers of 7
Alternative for 2nd Marking point:
opposite signs.
Or Appropriate particle equation seen annotated
with correct lepton numbers.
An electron and (electron) antineutrino have a (total) lepton
number of zero.
Alternative:
Alternative 2nd marking point:
Producing an (electron) neutrino wouldn’t conserve lepton
number Appropriate particle equation seen annotated
with correct lepton numbers.
An electron and (electron) neutrino have lepton numbers of
the same sign.
Or
An electron and (electron) neutrino have a (total) lepton
number equal to 2.
01.5 (X =) W-minus (boson) / W– (boson) 2 AO2.1a
(Y =) neutron / n AO2.1a
Must state that lepton (in the water) is an
01.6 Lepton (in the water molecule) is an electron 3 AO2.1a
electron for all 3 marks
and AO1.1a
Max 2 from AO1.1a
annihilation
gamma photons are produced
Penalise answers that list other products in
Two (gamma) photons are produced (that travel) in opposite MP3 annd MP4
directions.
01.7 Max 3 Must have the correct conclusion for 3 marks. 3 AO3.1a
The positron because: AO3.1a
positron is charged and the (electron) antineutrino ( ν�(e) ) is AO3.1a
neutral
The antineutrino only interacts via the weak interaction / The
positron interacts via the electromagnetic interaction (and
weak interaction)
The antineutrino’s (weak) interaction is shorter range / the
antineutrino is less likely to get close enough to interact (with
particles in the water so will travel further) / the antineutrino
will interact with fewer particles
The positron’s (electromagnetic) interaction has a longer
range / the positron does not have to be so close to interact
(with particles in the water so will travel a shorter distance) /
the positron will interact with more particles
Total 15
How to answer it
Radioactive Decay & Particle Interactions Study Guide
What this question tests
This exam question assesses your core knowledge of particle physics, specifically focusing on nucleonic structure, beta-minus decay mechanisms, conservation laws (lepton number and energy), Feynman diagram interpretation, annihilation processes, and fundamental forces (electromagnetic vs. weak interaction ranges).
Neutron Number Calculation
✅ Correct Answer
126 (Tick the second box)
💡 Key Knowledge
The upper number in a nuclide notation (e.g., ²¹⁰₈₄Po ) is the nucleon number (A = protons + neutrons). The lower number is the proton number (Z). Therefore, Neutron Number N = A - Z = 210 - 84 = 126.
Beta-Minus Decay & Proton Numbers
✅ Correct Answer
A neutron decays into a proton ( n → p + e⁻ + v̄e ). The proton number increases by 1 (from 83 in bismuth to 84 in polonium) because a neutron turns into a proton.
🧠 Exam Technique
To score both marks, you must explicitly link the change in the nucleus to the subatomic event (a neutron changing to a proton) and state the direction of the proton number change.
❌ Common Errors
Students often state that an electron leaves the nucleus without explaining the fundamental change to the nucleons inside, losing the subatomic process mark.
Evidence for a Third Particle (Energy Spectra)
✅ Correct Answer
Beta particles are emitted with a continuous range of kinetic energies up to a maximum of 1.2 MeV, yet the total energy released is fixed at 1.2 MeV. A third invisible particle (the antineutrino) carries away the "missing" energy to conserve energy.
💡 Key Knowledge
Before the neutrino was discovered, scientists thought energy was not conserved in beta decay. The continuous energy curve proves that energy is shared dynamically between the beta particle, the recoiling nucleus, and the antineutrino.
Conservation of Lepton Number
✅ Correct Answer
An electron antineutrino must be produced to conserve lepton number. The emitted beta-minus particle is an electron (lepton number +1), so an antineutrino (lepton number -1) is required so total lepton number remains zero on both sides.
🧠 Exam Technique
Always check lepton numbers explicitly:
Left side: 0 (neutron)
Right side: 0 (proton [+1] + electron [+1] + antineutrino [-1] = 0).
Feynman Diagram Labelling
✅ Correct Answer
X = W⁻ boson (or W minus boson)
Y = neutron (or n)
❌ Common Errors
Writing just "W boson" without the negative sign will lose the mark, as charge conservation requires identifying the specific exchange particle charge in weak interactions.
Positron Annihilation
✅ Correct Answer
The lepton in a molecule of water is an electron. When the positron collides with it, annihilation occurs, producing two gamma photons travelling in opposite directions (to conserve momentum).
💡 Key Knowledge
Water molecules (H₂O) contain electrons in their atomic electron shells. A positron is the antiparticle of an electron; when they meet, their mass energy is converted entirely into high-energy gamma-ray photons.
Particle Interaction Ranges & Mean Free Path
✅ Correct Answer
The positron is likely to travel a shorter distance before interacting.
💡 Key Knowledge & Rationale
- The positron is charged and experiences the electromagnetic interaction.
- The electron antineutrino is neutral and only experiences the weak interaction.
- The weak nuclear force has an extremely short range (10⁻¹⁸ m), making the antineutrino much less likely to get close enough to particles in water to interact, meaning it travels further.
- The electromagnetic force has infinite range, meaning the positron interacts much sooner with charged particles in the water.
Topics
Physics · 3.2 Particles and radiation
Question and mark scheme from the AQA AS Level Physics examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.