Nuclear and particle physics in NET tests radioactive decay and half-life, decay series, mass defect and binding energy, fission and fusion, radiation detectors, and the basic particle families. Half-life numericals are the most common question type, and nearly all of them reduce to counting how many times you halve.
Key facts
- Physics is 30% of NET-Engineering, a derived 60 of the 200 MCQs.
- Nuclear physics appears in both years. Our syllabus data estimates about 4 questions from the first-year chapter and about 3 from the second-year one.
- Syllabus: decay series, fission and fusion, binding energy, detectors, reactors, and the particle zoo from leptons and quarks to the Higgs.
Source: NUST NET-Engineering composition and the SailToNUST physics syllabus, checked October 2026
What the chapter covers
The first-year chapter covers radioactive decay series, fission and fusion, binding energy, and an introduction to leptons, quarks and the Higgs boson. The second-year chapter goes back to mass defect and the binding energy curve, then adds reactors, weapons, radiation detectors and the standard-model classification of matter and antimatter. In NET these two merge into short concept questions plus a half-life or mass-defect numerical.
Formulas you need
Alpha decay X(A, Z) → Y(A − 4, Z − 2) + He-4
Beta-minus X(A, Z) → Y(A, Z + 1) + electron + antineutrino
Gamma no change in A or Z
Half-life N = N₀ (½)^(t / T½)
Decay constant λ = 0.693 / T½ s⁻¹
Activity A = λN becquerel, Bq = s⁻¹
Mass defect Δm = Z mₚ + (A − Z) mₙ − M_nucleus
Binding energy E = Δm c²
1 u = 931.5 MeV
Binding energy per nucleon peaks near iron-56, at about 8.8 MeV
Nuclear radius R = R₀ A^(1/3), R₀ ≈ 1.2 × 10⁻¹⁵ m
Sizes atom about 10⁻¹⁰ m, nucleus about 10⁻¹⁵ to 10⁻¹⁴ m
Worked MCQs
1. Half-life from a fraction. The activity of a sample falls to 1/64 of its first value in 30 s. What is the half-life?
(a) 2 s (b) 4 s (c) 5 s (d) 6 s
1/64 = (½)⁶ so 6 half-lives in 30 s
T½ = 30 / 6 = 5 s
Answer (c). Know the powers of 2 up to 1024 and these become one-step questions.
2. Mass remaining. A sample has 80 g of an isotope with a half-life of 2 days. How much remains after 8 days?
(a) 20 g (b) 10 g (c) 5 g (d) 0 g
8 days / 2 days = 4 half-lives
80 → 40 → 20 → 10 → 5 g
Answer (c). The other 75 g has not vanished. It has turned into a daughter nucleus.
3. Counting decays in a series. Uranium-238 (Z = 92) decays in a series to lead-206 (Z = 82). How many alpha and beta-minus decays take place?
(a) 8 alpha, 6 beta (b) 6 alpha, 8 beta (c) 8 alpha, 8 beta (d) 10 alpha, 2 beta
Only alpha changes A: (238 − 206) / 4 = 8 alpha
8 alpha drop Z by 16: 92 − 16 = 76
Lead has Z = 82, so Z must rise by 6: 6 beta-minus
Answer (a). Always count alphas from the mass number first. Betas then fix the leftover charge.
4. Binding energy of helium-4. A helium-4 nucleus has mass 4.001506 u. Take mₚ = 1.007276 u and mₙ = 1.008665 u. What is its binding energy per nucleon?
(a) 28.3 MeV (b) 7.07 MeV (c) 0.030 MeV (d) 931.5 MeV
Mass of parts 2(1.007276) + 2(1.008665) = 4.031882 u
Mass defect 4.031882 − 4.001506 = 0.030376 u
Binding energy 0.030376 × 931.5 ≈ 28.3 MeV
Per nucleon 28.3 / 4 ≈ 7.07 MeV
Answer (b). Option (a) is the total binding energy. Read whether the question wants the total or the per-nucleon value.
5. Nuclear radius ratio. What is the ratio of the radius of a nucleus with A = 216 to one with A = 27?
(a) 8 (b) 2 (c) 4 (d) 1.5
R ∝ A^(1/3)
(216/27)^(1/3) = 8^(1/3) = 2
Answer (b). Nuclear density comes out the same for every nucleus, which is why the radius grows only as the cube root.
The particle families in one table
| Family | Members | Note |
|---|---|---|
| Leptons | electron, muon, tau and their neutrinos | not made of quarks, feel no strong force |
| Quarks | up, down, strange, charm, top, bottom | never found alone |
| Baryons | proton (uud), neutron (udd) | three quarks |
| Mesons | pions, kaons | a quark and an antiquark |
| Force carriers | photon, W and Z, gluon | gluons carry the strong force |
| Higgs boson | one particle | gives mass to W, Z and the fermions; found at CERN in 2012 |
Every particle has an antiparticle with the same mass and opposite charge. The positron is the electron's.
Traps that cost marks
- "Time for all nuclei to decay" has no fixed answer. Decay is random. Half-life tells you when half is left, not when none is.
- Half-life is fixed for an isotope. Temperature, pressure and chemical state do not change it. The amount of sample does not either.
- Ionizing versus penetrating. Alpha ionizes the most and penetrates the least, stopped by paper. Gamma is the reverse, needing thick lead or concrete.
- Moderator versus control rod. A moderator (graphite or heavy water) slows neutrons so U-235 can capture them. Control rods (cadmium or boron) absorb neutrons to control the rate. Questions swap the two on purpose.
- Which isotopes fission with slow neutrons. U-235 and Pu-239 do. U-238 mostly does not.
- Detectors. A solid-state detector works under reverse bias. A GM tube counts particles but cannot tell their energy apart.
Speed tips for the exam
Learn the halving ladder: 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128. Any "falls to 1/n" question becomes "how many steps down the ladder".
For decay series, use two lines every time: alphas from ΔA/4, then betas from the charge you still need.
For binding energy, keep 931.5 MeV per u in your head and skip the E = mc² route with kilograms. It saves three powers of ten.
The photon side of modern physics is in quantum physics. For a wider plan across all physics chapters, see the NET physics guide.
FAQ
What is the difference between fission and fusion?
Fission splits a heavy nucleus, such as U-235, into two medium ones. Fusion joins light nuclei, such as hydrogen isotopes, into a heavier one. Both release energy because the products sit higher on the binding-energy-per-nucleon curve.
Why is iron-56 the most stable nucleus?
It has close to the highest binding energy per nucleon, about 8.8 MeV. Nuclei lighter than iron can release energy by fusing, and heavier ones by splitting.
Does beta decay change the mass number?
No. In beta-minus decay a neutron turns into a proton, so Z goes up by one and A stays the same.
Is particle physics in the NET syllabus?
Yes. Leptons, quarks and the Higgs appear in the first-year chapter summary, and the standard-model classification in the second-year one. Expect one-line concept questions rather than numericals.
