In this experiment we will examine the Inverse Square Law. The law states that the intensity of radiation is inversely proportional to the square of the distance from the source.
I used a CS-137 10 uCi source for this experiment. I started measuring CPM counts at 16 centimeters from the source using the side of the GM tube. To obtain accuracy for this experiment the distance between the source and the detector (GM tube) needs to be several times greater than the diameter of the GM tube. The diameter of GM tube is approximately 1.5 cm (.59”).
Performing the experiment:Mark the distances away from the side of the Geiger counter wand. Place the gamma ray source at each distance mark and take five CPM readings. Average the results and fill in the table below. Once the table is filled, perform the calculations; d2, 1/d2. Use this information to graph your results. Once the results are graphed, it should resemble the graph provided below. Notice the red line doesn’t go through all the plotted points. Instead it is a straight line that best fits to the plotted points.
Record Your Results:
Below are the results I obtained. Notice that if you extend the red line it will not cross the y-axis at zero. Can you guess a reason why this is so?
If you took background CPM reading and subtracted them from every distance reading do you think the red line would cross closer to zero?
Extending this logic further, in the chart below, the point where the red line would cross the Y-axis represents what value?
