Radiation Protection and Health Physics
Domain 3 tests dose control, contamination control, and instrument use. The exam expects you to apply ALARA and the 10 CFR 20 limits, not just recite them.
Core rules
- Occupational limits: 5 rem/yr total effective dose equivalent; 15 rem/yr lens of eye; 50 rem/yr extremities and skin. Public limit: 100 mrem/yr TEDE. ALARA is a lower, as-low-as-reasonably-achievable operating goal.
- Control exposure by time, distance, and shielding. For a point source, dose rate falls with the square of distance; each half-value layer cuts gamma dose by half.
- Internal dose is controlled by DAC and contamination limits. Airborne radioactivity is sampled; protective clothing and respirators are chosen for the hazard, not habit.
- Survey meters must match the radiation type and energy: ion chambers for gamma dose rate, GM tubes for contamination, and alpha scintillation for alpha contamination.
Worked example
A point source gives 200 mrem/h at 2 m. At 4 m, inverse-square gives 200 × (2/4)^2 = 50 mrem/h. To keep a planned task below 2 mrem, allowed time = 2/50 h = 0.04 h = 2.4 min. If a 1 cm lead shield adds one half-value layer, the rate becomes 25 mrem/h and allowed time doubles to 4.8 min. Re-survey after shielding; do not assume the calculation survived the setup.
Traps
Adding doses in different units (mrem, mSv, µSv) without conversion; 1 mSv = 100 mrem. Using inverse square through walls or inside a shield. Ignoring internal dose when airborne contamination is present. Applying the public limit to a radiation worker.