B. Effect of absorber thickness

1.  Correct all of the actual counting data that you have taken by subtracting the background rate to fill in the corrected Counts/Minute columns in both Tables.

2.  Let I_o be the intensity (counts/minute) with zero thickness of absorber. Let I represent the intensity for other thicknesses. Make a plot of the natural logarithm of I, ln(I), vs. d, where d is the thickness of the aluminum absorbers in millimeters. You can use the Excel Chart Maker if you wish to.

3.  This plot should be a straight line. Find the slope of this line and enter it in cell D36 on the Worksheet.

4.  Find the thickness that will reduce the intensity to one-half of its initial value and record the value in cell D38.

5.  The straight line on the graph of ln(I) vs. d indicates that intensity vs. thickness follows the mathematical relationship:

where α is the absorption constant,, a measure of how well the material absorbs radiation. With this information, answer Question 1 on the Worksheet.

C. Effect of source distance

1.  Draw a graph of N_D (the number of counts in 5 minutes) vs. 1/d.

2.  Draw a graph of N_D vs. 1/d².

3.  Answer Question 2 on the Worksheet.

4.  Make a modified graph of N_D vs. . Find the slope and enter it in cell D70. We are assuming that a = 12 mm following eqn (5).

5.  Answer Question 3 on the Worksheet.