Three levers reduce an external radiation dose: spend less time near the source, keep more distance from it, and put mass between you and it. Halving the time halves the dose, doubling the distance from a small source in open air cuts the dose rate to one quarter, and each half-value layer of material halves a gamma beam, with the mass per square metre, not the material name, deciding how much each layer does.
Part of Radiation Safety
Radiation protection against an external source rests on time, distance and shielding. Each works alone, and together they multiply.
Time: Dose Is Rate Multiplied By Time
Dose = dose rate x time (see Units of Measurement). The parent article, Nuclear Safety, says half the time means half the dose, and that planning a route and getting out quickly is the method. Time also works through decay: waiting lets the field fall; see Fallout Patterns.
Distance: The Inverse Square Law
For a small source in open air, the dose rate falls with the square of the distance. Double the distance and the rate is one quarter; at 10 m it is 1/100 of the rate at 1 m (parent article). The arithmetic is (1 / 10)^2 = 1/100. The law holds for a small source far smaller than the distance. It does not describe a wide contaminated field, a long pipe or a pool, where the rate falls much more slowly with distance. Distance is the parent’s most powerful tool and costs nothing; see Safe Distances.
Shielding: Mass Between You And The Source
Each half-value layer (HVL) of a material halves the gamma beam. The parent gives 7 half-value layers as the thickness that brings gamma to about 1 percent: 42 cm of concrete, 63 cm of packed earth or 126 cm of water. Those are different thicknesses holding a similar mass per square metre, which is why material choice matters less than quantity of mass. Full treatment: Shielding Materials and Half-Value Layers. For alpha and beta particles, shielding is simple (paper, thin metal), but the danger there is inside the body; see Penetration & Shielding.
The Three Together
Because the levers multiply, small gains in each add up. Hypothetical: halve the time (x 0.5), double the distance (x 0.25) and add one half-value layer (x 0.5) and the dose is 0.5 x 0.25 x 0.5 = 0.0625, or 1/16 of the original. The factors are illustrative; the multiplication is the point.
Protection Against Contamination Is A Different Job
None of the three levers keeps radioactive dust off skin, out of lungs or out of food. That is the work of cover, hygiene and decontamination; see Personal Protection and Contamination vs Irradiation.
Common Mistakes
- Using the inverse square law near a wide source.
- Counting only walls. Radiation reaches you through the roof too.
- Thinking shielding makes a source safe to approach. It lowers a rate you cannot measure.
Safety
Without an instrument you cannot know how much shielding is enough
Use the thickest shield you can build, the greatest distance you can keep and the shortest time you can manage, and treat the result as unmeasured. See Shelter Design.
What’s Next
- Learn next: Shielding Materials — what to build with
- Combine with: Shelter Design — putting the mass in the right places