Inverse square law for gamma

 

It was only in recent years that I learned the trick of putting a Geiger-Muller tube vertically for gamma, which can go through the metal sides. It eliminates alpha and beta. It is also supposed to make the distance measurement more accurate because of the circular symmetry. It is not possible to know exactly where in the tube the gamma is detected but they say this makes the distance measurement better because you measure to the middle of the tube. Sitting the gamma source firing downwards into lead also works because the reduced radiation leaking sideways out of the casing means more than one group can use the same source. The disadvantage is that the range is reduced. You'll see that I've had to set the distances at 5mm intervals which is much less than the diameter of the tubes. Nevertheless the results do work.