# Inverse Square Law of Gamma Radiation Experiment

When performing this experiment with a cup source and GM Tube it says that you should start at a distance several times the diameter of the GM tube in order to obtain accurate results (https://www.imagesco.com/nuclear-science/geiger-counter/experiment-4.html). Which is why this link starts taking readings at 16cm as its many more times the diameter of the GM Tube he used. Please someone explain why starting, at say 3cm, would not give accurate results?

Thanks

• The source is not a point source. You want it looking more like all the gammas come from the exact same place, not over an area. Feb 8 at 14:29

The further the detector is from the sealed source the better the approximation is that it is a point source although the type of source you are likely to be using will be very small in dimension, $$\approx$$ 1 mm.