STP Physical Sciences 3 — Questions and Answers
Question 1: Which interaction is primarily responsible for the attenuation of low-energy X-rays (below ~30 keV) in soft tissue?
- Compton scattering
- Pair production
- Photoelectric effect (Correct answer)
- Rayleigh scattering
Correct answer: Photoelectric effect
At low photon energies, the photoelectric effect dominates because its cross-section is strongly dependent on atomic number and inversely on photon energy cubed.
Question 2: A capacitor stores 0.5 J of energy with a capacitance of 100 μF. What is the voltage across it?
- 50 V
- 100 V (Correct answer)
- 3162 V
- 10 V
Correct answer: 100 V
Using E = ½CV², V = √(2E/C) = √(2×0.5 / 100×10⁻⁶) = √10000 = 100 V.
Question 3: What does the term 'acoustic impedance' represent in ultrasound physics?
- The frequency of sound waves in tissue
- The product of tissue density and sound speed (Correct answer)
- The attenuation coefficient of sound in tissue
- The wavelength of ultrasound in a medium
Correct answer: The product of tissue density and sound speed
Acoustic impedance Z = ρc, where ρ is the density of the medium and c is the speed of sound in that medium.
Question 4: In nuclear medicine, what does the effective half-life of a radiopharmaceutical account for?
- Only the physical decay of the isotope
- Only the biological excretion of the compound
- Both physical decay and biological elimination (Correct answer)
- The energy of emitted radiation only
Correct answer: Both physical decay and biological elimination
Effective half-life combines both physical half-life (radioactive decay) and biological half-life (clearance from the body): 1/T_eff = 1/T_phys + 1/T_biol.
Question 5: Which phenomenon explains why electrons do not spiral into the nucleus despite electrostatic attraction?
- Special relativity
- The Pauli exclusion principle
- Wave-particle duality and quantised energy levels (Correct answer)
- The Heisenberg uncertainty principle only
Correct answer: Wave-particle duality and quantised energy levels
Electrons occupy discrete quantised energy levels described by quantum mechanics; they cannot lose energy continuously and therefore cannot spiral inward.
Question 6: Which detector type is used in gamma cameras for nuclear medicine imaging?
- Geiger-Müller tube
- Proportional counter
- Sodium iodide scintillation crystal with photomultiplier tubes (Correct answer)
- Silicon drift detector
Correct answer: Sodium iodide scintillation crystal with photomultiplier tubes
Gamma cameras use large sodium iodide (NaI:Tl) crystals that convert gamma photons to light, detected by an array of photomultiplier tubes.
Question 7: What is the de Broglie wavelength of an electron accelerated through a potential difference of 100 V? (m=9.11×10⁻³¹ kg, h=6.63×10⁻³⁴ Js, e=1.6×10⁻¹⁹ C)
- 0.123 nm (Correct answer)
- 1.23 nm
- 0.0123 nm
- 12.3 nm
Correct answer: 0.123 nm
λ = h/√(2meV) = 6.63×10⁻³⁴ / √(2×9.11×10⁻³¹×1.6×10⁻¹⁹×100) ≈ 0.123 nm.
STP Physical Sciences 3
Which interaction is primarily responsible for the attenuation of low-energy X-rays (below ~30 keV) in soft tissue?