We tried to measure background radiation using a geiger counter for a experiment at school. The meter showed $0.12$-$0.21$ microSv/h during the day averaging at about $0.14$ mcSv/h.
As we tried to see ways how to shield incoming radiotion nothing seemed to work. Taking a cue from nuclear power plant we put the meter into a watertigt glasscontainer and sank it in the gym pool so that the container had a good three feet of water all around. The measurement did not budge at all. We tried all settings fast/slow/auto. The geiger meter should be okay as it had recently been calibrated.
Can someone explain.
Best Answer
If you are not living close to a uranium mine or other natural sources of radioactivity the main radioactivity that your geiger counter measures outside is cosmic radiation. At sea level this is composed mainly by muons , which are weakly interacting and will not be stopped by a meter of water. This is also true for cosmic origin neutrons . ( neutrons from reactors are of lower energy).
Electrons and gammas will be stopped but possibly your counter is not good for measuring electrons which can be stopped by a metal cover.
You could try measuring in a second basement, low energy muons will be absorbed by the ground above. On the other hand there is natural radioactivity in the ground and the cement of the basement. Still you might see a controllable difference. Have a look at this list of natural radiation.