Radiation Protection and Shielding
Dose quantities and limits
Absorbed dose (Gy) becomes equivalent dose (Sv) after multiplying by radiation weighting factors, and effective dose after applying tissue weighting factors. Know the 10 CFR 20.1201 occupational limits: 5 rem (50 mSv) TEDE per year, 15 rem to the lens, 50 rem to skin and extremities; 0.5 rem to a declared pregnant worker over gestation; and about 100 mrem/y to a member of the public from licensed operations. ALARA (10 CFR 20.1101) is a requirement, not a suggestion, and dose constraints sit below the limits.
Shielding
Narrow-beam attenuation is I = I₀ e^(−μx); the half-value layer is HVL = ln 2/μ, and each HVL removes half the intensity. Real geometries require a buildup factor B for scattered photons, so thick shields attenuate less than the exponential predicts. Gammas are shielded with lead, iron, and high-density concrete; neutrons require hydrogenous material (water, polyethylene, borated polyethylene) to thermalize them, plus a capture gamma shield downstream. Inverse-square applies to point sources in open geometry.
Traps
- Mixing rem and Sv (1 Sv = 100 rem) or applying equivalent dose where effective dose is required.
- Treating HVLs as strictly multiplicative through thick shields while ignoring buildup.
- Confusing DACs, ALIs, and TEDE limits.
Worked example
A source reads 120 mR/h at 1 m. At 3 m the uncollided rate is 120/3² ≈ 13 mR/h. If 2 HVLs of concrete are added, multiply by ¼ - an easy cross-check for shielding questions.