Radiation protection rests on three principles: justification (an exposure must do more good than harm), optimization (keep doses as low as reasonably achievable, ALARA) and dose limitation. For external exposure, the practical tools are time, distance and shielding; from a point source, doubling the distance reduces the dose rate to one quarter.
Reviewed 2026-09-24
Three principles
| Principle | Meaning |
|---|---|
| Justification | Any practice involving exposure must produce enough benefit to offset the harm. |
| Optimization (ALARA) | Keep doses as low as reasonably achievable, taking economic and social factors into account. |
| Dose limitation | Individual doses from planned exposure must not exceed the limits set by standards. |
Time, distance, shielding
The inverse-square law
- A point source emits radiation in all directions.
- At distance d, the dose rate is taken as 1.
- At 2d the same radiation covers four times the area: 1/4 of the dose rate.
- At 3d: 1/9. Halving the time also halves the dose; shielding removes a fraction per HVL.
- Time: dose is proportional to time spent in the field.
- Distance: for a point source, dose rate falls with the square of the distance (inverse-square law).
- Shielding: place attenuating material between the source and people, sized for the energy and geometry.
Internal exposure
Radioactive material taken into the body keeps irradiating it. Protection relies on containment, ventilation, contamination control and personal hygiene rather than on shielding.
References
- ICRP Publication 103 (2007): The 2007 Recommendations of the ICRP
- GB 18871-2002 电离辐射防护与辐射源安全基本标准
- IAEA — Radiation, People and the Environment (2004)
This encyclopedia is for general and engineering reference and is not medical advice. Consult radiation protection or medical professionals for individual exposure assessment.