The Inverse-Square Law
Gamma radiation from a source will spread out. The radiation from a small source can be considered the same in all directions (isotropic), imagine a sphere around the source. As we move further away from the source the bigger the sphere gets. The same amount of energy is shared over a greater surface area. The further we move from the source the less intensity of the gamma radiation.
Intensity is measured in Watts, W
The intensity, I, of the radiation at a distance x from the source is given as
Where I0 is the intensity at the source and k is a constant.
We do not always need to know the intensity at the source to find it at a given distance.
Consider two points, A and B, a certain distance away from a gamma source.
and
We can combine these to give