Cosmic Calibration

How to adjust the voltage in a cosmic ray detector photomultiplier

Materials

• Very small slotted tip screwdriver (one can be made with a small metal rod)
  

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Procedures

1. For detectors with end mount photomultipliers, two holes must be made in the case that holds the detector at the end where the phototube is to be able to reach the small screw that adjusts the voltage. These holes should be large enough to look through and adjust the knob that controls the voltage. Side mount photomultipliers should have easily accessible screws right through the Lucite panel.
   
2. Before changing anything, record a preliminary set of data.
   1. Take a timed coincidence count with the detector on its side (the circuit board should be facing up and the paddles in a vertical position).
   2. Turn the detector right side up with the circuit board facing you and the paddles in a horizontal position. Take a singles count for the upper paddle and another coincidence count.
      
3.  Adjust the photomultiplier
   1. If the initial coincidence count is at or below 10, use the slotted-tip screwdriver and turn the screw on the upper paddle half a turn clockwise to increase the voltage slightly. Repeat step 2 and record the data. Continue turning the screw half a turn clockwise and recording the counts until the coincidence ceases to increase. There should be at least 10 data sets.
   2. If the initial coincidence count is higher than ten, turn the screw on the upper paddle counterclockwise two turns then repeat step 3 and record. Continue this pattern until the coincidence rate is lower than ten. Then, as in part a, begin turning the screw clockwise half a turn at a time and record the rates each time until the coincidence rate becomes constant. There should be at least 10 data sets.
4. Plot the points on a graph with the single count as the independent variable and the coincidence count as the dependent variable. Determine the optimal singles rate value
5. Adjust the photomultiplier until the singles count is close to that optimal point
6. Take a final data set.
7. Repeat steps 2-6 for the lower paddle. 

NOTE:

• Pay attention when the counter passes 1000 because the numbers turn back to 000 after 999 and continue counting. Be sure to keep track of the thousand digits.
• Data taken of natural phenomena is believed to have a margin of error of the square root of the selected measurement.