The inverse square law describes the principle of dose reduction as distance from the source increases.
This assumes a point source. If radiation spreads over a spherical area, as the radius increases, the area over which the dose is distributed increases according to A=4πr^2 where A is the area, π is pi and r is the radius of the sphere.
Therefore, the dose is proportional to the inverse of the square of the radius. Thus if you double the distance you reduce the dose by a factor of four.
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