Radiation Physics and Dosimetry
What to know
Photon interactions (photoelectric, Compton, pair production), inverse square law, attenuation, absorbed dose units, and dosimeter types. Also brachytherapy source physics (half-life, inverse square) and radioisotope properties.
Concepts to master
- Photoelectric dominates at low energy/high Z; Compton dominates at megavoltage energies in tissue; pair production requires >1.022 MeV.
- Inverse square law: dose ∝ 1/d². Doubling distance quarters dose.
- Half-value layer (HVL) and attenuation: I = I₀e^(−μx).
- Units: 1 Gy = 1 J/kg; 1 Sv for equivalent dose; 1 Ci = 3.7×10¹⁰ Bq.
- Dosimeters: ionization chambers (reference), TLD (integrating), diodes (in vivo), film (2D).
Traps
- Using inverse square across scatter-contaminated fields (fails near low-energy/large fields).
- Confusing exposure (C/kg) with absorbed dose (Gy).
- Mixing Bq (activity) with Gy (dose).
Rule of thumb
At megavoltage, Compton dominates in tissue - so dose per unit exposure is relatively Z-independent. At kV energies, photoelectric dominance makes bone absorb disproportionately.
Worked example
A source delivers 400 cGy at 50 cm. What at 100 cm (ignoring attenuation)? Dose ∝ 1/d². Distance doubles → dose quarters → 100 cGy at 100 cm.