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Physics · Unit 16

Nuclear Physics: MDCAT Notes and MCQs

The nuclear atom

  • Tiny dense nucleus (≈ 10−14–10−15 m) of protons (+e) and neutrons (neutral) — together nucleons — surrounded by electrons (−e). Atom ≈ 10−10 m.
  • Z = atomic number (protons); A = mass number (protons + neutrons); N = A − Z. Notation AZX.
  • Isotopes: same Z, different A (e.g. 1H, 2H deuterium, 3H tritium; 12C, 14C).
  • Nucleons are held by the short-range, strong nuclear force. Neutron discovered by Chadwick (1932).

Radioactive decay

  • Spontaneous: not affected by temperature, pressure, chemical state. Random: impossible to predict which nucleus decays next; only the probability (decay constant λ) is known.
  • Activity A = ΔN/Δt = −λN (becquerel, 1 Bq = 1 decay/s; 1 Ci = 3.7 × 1010 Bq).
RadiationNatureChangeIonizingPenetration (stopped by)
αHelium nucleus 42HeA − 4, Z − 2HighestLowest (paper, few cm air)
β−Fast electronA same, Z + 1MediumMedium (few mm aluminium)
γEM photonNo change in A, ZLowestHighest (thick lead/concrete)

Half-life

  • T½: time for half of the radioactive nuclei (or activity) to decay. Characteristic of each isotope.
  • λ = 0.693 / T½. N = N0(½)n where n = t/T½; N = N0e−λt.
  • After 1, 2, 3, 4 half-lives remaining = 1/2, 1/4, 1/8, 1/16.

Biological effects

  • Ionizing radiation damages cells and DNA. Somatic effects (in the exposed person): burns, radiation sickness, hair loss, cataract, leukaemia, cancer. Genetic effects: mutations passed to offspring.
  • Absorbed dose: gray (Gy) = 1 J/kg. Equivalent dose: sievert (Sv) = Gy × RBE/quality factor (α has high RBE ≈ 20). Safety: time, distance, shielding; film badges for workers.

Medical uses

  • Radiotherapy: cobalt-60 γ-rays to destroy cancer cells; iodine-131 for thyroid cancer/hyperthyroidism; gold-198, radium needles.
  • Diagnosis / tracers: technetium-99m (most used imaging tracer, T½ ≈ 6 h), iodine-131 (thyroid scan), phosphorus-32 (bone, blood), sodium-24 (blood circulation). PET scans use positron emitters (fluorine-18).
  • Sterilization of surgical instruments, dressings and syringes with γ-rays. X-rays and CT for imaging.
PMDC learning outcomes for this unit
  • Nucleus, decay, radiation uses: 16.1 Describe a simple model for the atom to include protons, neutrons and electrons
  • Nucleus, decay, radiation uses: 16.2 Identify the spontaneous and random nature of nuclear decay.
  • Nucleus, decay, radiation uses: 16.3 Describe the term half-life and solve problems using λ = 0.693/T½.
  • Nucleus, decay, radiation uses: 16.4 Describe biological effects of radiation and explain the different medical uses of radiation.

Nuclear Physics MCQs with answers

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Q1
The number of neutrons in ²³⁸₉₂U is:
A238
B330
C146
D92
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Answer: C (146). N = A − Z = 238 − 92 = 146.
Q2
Isotopes have the same:
ANumber of neutrons
BAtomic number
CNuclear mass
DMass number
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Answer: B (Atomic number). Isotopes have the same number of protons but different neutrons.
Q3
Radioactive decay is:
ASpontaneous and random
BControlled by pressure
CPredictable for each nucleus
DAffected by temperature
Show answer
Answer: A (Spontaneous and random). Decay cannot be influenced by physical/chemical conditions and is random.
Q4
The relation between decay constant and half-life is:
Aλ = T½/0.693
Bλ = 1/T½
Cλ = 0.693 × T½
Dλ = 0.693/T½
Show answer
Answer: D (λ = 0.693/T½). λ = ln 2 / T½ = 0.693/T½.
Q5
A radioactive sample has half-life 10 days. The fraction left after 30 days is:
A1/4
B1/8
C1/2
D1/16
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Answer: B (1/8). 30 days = 3 half-lives → (½)³ = 1/8.
Q6
Emission of an alpha particle decreases the mass number by:
A0
B4
C1
D2
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Answer: B (4). α = ⁴₂He: A decreases by 4, Z by 2.
Q7
After β⁻ emission, the atomic number:
AIncreases by 1
BRemains same
CDecreases by 1
DDecreases by 2
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Answer: A (Increases by 1). A neutron turns into a proton; Z increases by 1.
Q8
Which radiation has the highest penetrating power?
AAll equal
BGamma
CAlpha
DBeta
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Answer: B (Gamma). Gamma rays are uncharged high-energy photons.
Q9
The radioisotope commonly used to treat thyroid disorders is:
ACarbon-14
BUranium-235
CCobalt-60
DIodine-131
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Answer: D (Iodine-131). The thyroid absorbs iodine, so I-131 targets it directly.
Q10
The SI unit of equivalent (biological) dose is:
AGray
BCurie
CBecquerel
DSievert
Show answer
Answer: D (Sievert). Sievert accounts for the biological effectiveness of radiation.
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