Measure the radius, R, of the disk magnets, and use the gaussmeter to determine the average magnetic induction, B, between them. Note also the direction of vec{B}. Place the Strontium (^{90}Sr) source under the spring clip in the source arm. Make sure that the center of the source is centered on the hole in the aluminum. Set the source arm at one of the 90^o positions.
Measure an entire angular distribution in steps of 10^o. Count at each point at least for 5 min. Make a background measurement for at least 10 min. If the time permits it, measure additional points to have an angular distribution with a step size of 5^o. Estimate the error of your background subtraced counts. Are 5 min enough ?