Physical Sciences Flashcards

Which interaction is primarily responsible for the attenuation of low-energy X-rays (below ~30 keV) in soft tissue?

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.

A capacitor stores 0.5 J of energy with a capacitance of 100 μF. What is the voltage across it?

Answer: 100 V

Using E = ½CV², V = √(2E/C) = √(2×0.5 / 100×10⁻⁶) = √10000 = 100 V.

What does the term 'acoustic impedance' represent in ultrasound physics?

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.

In nuclear medicine, what does the effective half-life of a radiopharmaceutical account for?

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.

Which phenomenon explains why electrons do not spiral into the nucleus despite electrostatic attraction?

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.

Which detector type is used in gamma cameras for nuclear medicine imaging?

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.

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)

Answer: 0.123 nm

λ = h/√(2meV) = 6.63×10⁻³⁴ / √(2×9.11×10⁻³¹×1.6×10⁻¹⁹×100) ≈ 0.123 nm.