Procedure Set up the Geiger counter according to the unit's instructions, and calibrate it if possible. Keep the probe steady for 15 seconds, and find the average reading on the meter. The needle will shift around some, so choose a value in the middle of its oscillations. Record your findings in the data table. The audio units are not very useful for precise measurements.
Some counters have a meter with a needle moving on it like in a car's speedometer.
Others may have a digital meter that registers a count with every particle. Select the three source items with the highest readings. One at a time, place the source far enough away from the Geiger counter probe so that you will be able to fit the thickest piece of shielding in between the probe and the source. Test each of the shielding materials by placing them between the source and the counter. Follow the same procedure as in step 2 when taking the reading.