1. Remove the Strontium-90 sample from the chamber and drop it back on its pad. We'll assume that it and the Barium-137 sources are both far enough away from the Geiger tube so as to provide no measureable radiation. To measure the background radiation, set your voltage at your selected operating voltage and take a four-minute reading. Calculate and record your background radiation below in counts per minute. Background Radiation 1907 counts/min