Nuclear physics: A Level Physics questions with answers
Free Nuclear physics questions for A Level Physics (9702), part of Thermal & modern physics. Each question has the answer and the marking points an examiner would look for.
A uranium-235 nucleus absorbs a neutron and undergoes fission: 235U + n → 141Ba + 92Kr + 3n. Masses: U-235 235.0439 u, Ba-141 140.9144 u, Kr-92 91.9262 u, neutron 1.0087 u.
(a)
Calculate the mass defect for this reaction.
[2]
(b)
Calculate the energy released in MeV.
[2]
(c)
Calculate the energy released per kilogram of uranium-235.
[2]
(d)
Explain, using binding energy per nucleon, why energy is released.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
0.1859 u
mass before = 236.0526 u, after = 235.8667 u C1 Δm = 0.1859 u A1
fission products have a higher binding energy per nucleon M1 so the total binding energy increases and the difference is released A1
2
Question 2Structured · 10 marksA* challenge
A radioactive source has a half-life of 8 years and an initial activity of 8.0 × 105 Bq.
(a)
Define activity and calculate the decay constant in s−1.
[3]
(b)
Calculate the number of undecayed nuclei initially.
[2]
(c)
Calculate the activity after 20 years.
[2]
(d)
The binding energy per nucleon of 235U is 7.6 MeV and of the fission products is about 8.5 MeV. Estimate the energy released when one 235U nucleus undergoes fission, and explain why energy is released.
[3]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
2.75 × 10−9 s−1
number of decays per unit time B1 λ = ln 2 ÷ t½ with years converted to seconds C1 2.75 × 10−9A1
3
(b)
2.91 × 1014
A = λN C1 2.91 × 1014A1
2
(c)
1.41 × 105 Bq
A = A0e−λtC1 1.41 × 105A1
2
(d)
≈ 235 × 0.9 ≈ 210 MeV; products are more tightly bound (higher binding energy per nucleon)
difference 0.9 MeV per nucleon C1 ≈ 210 MeV A1 products have greater binding energy per nucleon B1
3
Question 3Structured · 9 marks
A radioactive tracer has a half-life of 12.0 hours. Its initial activity is 400 kBq.
(a)
Define half-life.
[1]
(b)
Calculate the decay constant in s−1.
[2]
(c)
Calculate the activity after 18 hours.
[2]
(d)
Calculate the number of undecayed nuclei initially present.
[2]
(e)
Explain why the count rate measured by a detector is less than the activity of the source.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
time for half the nuclei to decay / activity to halve
the time taken for the number of undecayed nuclei (or activity) to halve B1
1
(b)
1.60 × 10−5 s−1
λ = ln 2 / t½C1 1.60 × 10−5 s−1A1
2
(c)
141 kBq
A = A₀e−λtC1 141 kBq A1
2
(d)
2.49 × 1010
N = A/λ C1 2.49 × 1010A1
2
(e)
radiation emitted in all directions; detector efficiency / absorption
radiation is emitted in all directions so only a fraction enters the detector B1 some is absorbed before detection / detector not 100% efficient B1
2
Question 4Structured · 3 marks
A radioactive sample contains 8.8 × 1020 nuclei of an isotope with half-life 7.4 days. Calculate the activity of the sample.
[3]
Show answers and mark scheme
Part
Answer and marking points
Marks
954000000000000 Bq
λ = ln 2/t½ in s−1C1 A = λN C1 954000000000000 A1
3
Question 5Structured · 8 marks
The count rate from a radioactive sample is measured every hour. The background count rate is 60 counts per minute. Table 5.1 shows the results.
time / hours
0
1
2
3
4
5
6
count rate / counts per minute
1660
1405
1191
1011
860
733
626
Table 5.1
(a)
Define the decay constant.
[1]
(b)
Use Table 5.1 to determine the half-life of the sample. Show your working.
[3]
(c)
Use your answer to (b) to calculate the decay constant in s⁻¹.
[2]
(d)
Explain why the background count rate must be subtracted.
[1]
(e)
Repeated measurements of the count rate at the same time give slightly different values. Explain why.
[1]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
probability per unit time that a nucleus decays
the probability per unit time that a nucleus will decay B1
1
(b)
4.00 hours
subtract background: corrected rates 1600 … C1 time for corrected rate to halve / gradient of ln(corrected rate) against t C1 4.00 hours (± 0.1) A1
3
(c)
4.81 × 10−5 s−1
λ = ln 2 ÷ t½, with t in seconds C1 4.81 × 10−5 s−1A1
2
(d)
background not from source; would make decay appear slower
Any 1 of: background radiation is not from the sample; including it gives too large a count rate / too long a half-life B1
1
(e)
decay is random
radioactive decay is random (spontaneous), so the number of decays in each interval varies B1
1
Question 6Structured · 11 marks
This question is about nuclear binding energy and radioactive decay.
(a)
Define binding energy of a nucleus.
[2]
(b)
The mass of a helium-4 nucleus is 4.0015 u. Calculate its mass defect in u. (proton 1.00728 u, neutron 1.00867 u)
[2]
(c)
Use your answer to (b) to calculate the binding energy per nucleon in J. (1 u = 1.66 × 10−27 kg)
[2]
(d)
A sample contains 2.0 × 1020 nuclei of an isotope with half-life 14 days. Calculate its activity.
[3]
(e)
State what is meant by the random and spontaneous nature of radioactive decay.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
energy needed to separate nucleus into separate nucleons
the minimum energy required to separate the nucleus into its individual protons and neutrons (to infinity) B2
E = Δmc2 = 4.54 × 10−12 J C1 per nucleon 1.14 × 10−12 J A1
2
(d)
1.15 × 1014 Bq
λ = ln 2 / t½C1 t½ in seconds = 1.21 × 106 s C1 A = λN = 1.15 × 1014 Bq A1
3
(e)
random: cannot predict which nucleus or when; spontaneous: unaffected by external conditions
random: it cannot be predicted when a particular nucleus will decay B1 spontaneous: not affected by external factors such as temperature or pressure B1
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Original Cambridge-style practice questions written for mentorriyaz.in. These are not Cambridge past paper questions, and this site is not affiliated with or endorsed by Cambridge University Press & Assessment. Updated 2026-11-04. Paper generator · mentorriyaz.in