What it shows:
Gamma rays are electromagnetic radiations which we detect as quanta of energy or photons. When the radioactive source is confined so that it acts as a point source, the diminution in the number of photons incident on a given area is such that the intensity is inversely proportional to the square of its distance from the source.
How it works:
A Co-60 source (1.173 and 1.332 MeV gammas) radiates isotropically. A Geiger-Müller counter is used to detect the radiation intensity at distances of 2, 3, and 4 meters. The intensity at 3 and 4 meters should be (1.5)-2=0.44 and 1/4 the value at 2 meters, respectively.
Setting it up:
The Co-60 sources are stored in the "radiation closet" of the Physics 191 lab. Sources #3 and #4 (square aluminum holders) are reasonably strong sources (0.52mCi and 1.21mCi as of 1/1/95; half-life is 5.26yr) and should be kept in the cylindrical lead shield when not in use. For the demonstration, it is held by a lab clamp/stand on the lecture bench. Set out 2-meter stick and bright yellow tape (to mark the measurement positions).
The GM counter's analog meter needs to be video projected for visibility. Therefore it's best to keep the GM counter in a fixed position (with video camera on it) and move the radioactive source. Set the counter to "slow response" so that it time averages and reduces the statistical fluctuations. If the source is moved too close to the counter (1 meter or less), the high count rates will produce an appreciable "dead time" effect and give an apparent low count rate. Remember to put the source back in its lead shield immediately after the measurements to minimize radiation exposure to the lecturer. Sample data is plotted below. Note that the data extrapolates to about 100 CPM background.
The plot speaks for itself.