One problem with using a gamma source and a Geiger-Muller tube to detect gamma rays is that we can't be certain whereabouts in the metal case the actual gamma source is and we cannot be sure whereabouts in the GM tube the detection occurs. This causes a systematic error in the results. An odd way round is to turn the source so that it fires into a solid plate and to turn the GM tube on its end. Enough gamma rays still escape the side of the source to be detected and they can get through the sides of the GM tube to be detected. The centre to centre distance is more certain.
