Before our eight bagels and two top hats could be flown on the sounding rocket, we had to calibrate them. This can be time consuming, so Professor Kletzing asked me to help Scott Bounds, another University of Iowa scientist, with the tests. I had never done anything like this before, so I had to learn how the detectors work and how to use the test equipment in our laboratory.

To calibrate the detectors, we put them into a big stainless steel vacuum chamber and pump out as much of the air as possible. This helps us simulate the conditions in space. Also, the microchannel plates in our detectors can be damaged if we power up the detectors at atmospheric pressure (760 Torr). To pump out our vacuum chamber we use two kinds of pumps. First, we use a mechanical pump to reduce the pressure in the chamber to about 1x10^-3 Torr. Then we use a cryopump to reduce the pressure in the chamber even further to about 5x10^-6 Torr (that's less than 1 millionth of atmospheric pressure). The difference between the air pressure inside and outside the vacuum chamber is so huge that it is impossible to open the vacuum chamber door when we are running the pumps. You would have to be as strong as Superman to open it! The pictures below shows me standing next to the vacuum chamber and the inside of the vacuum chamber with a detector that is ready to test.

Inside the vacuum chamber, an ultraviolet lamp provides a source of electrons. We need to know how many electrons our source produces, so we use another type of electron detector, called a Faraday cup, that has already been calibrated. To test the detectors for CHARM, we put one of the bagels or top hats inside the vacuum chamber. The detector sits on a special table that allows us to change the detector orientation relative to the electron source. The detector electronics sit on a shelf inside the vacuum chamber and are connected to a computer outside the chamber that controls the test and records the data.

It takes between 1 to 3 hours to pump the air out of the vacuum chamber. Once the air has been pumped out, it takes about 4 hours to test a bagel. The top hats take longer since we need to check many different detector orientations relative to the electron source. Fortunately, the tests are controlled by a computer, so we only need to check on the test once in a while to make sure everything is working. When a test is finished, we analyze the data files to make sure the calibration factors look okay. Sometimes the detectors didn't work properly, so we had to open up the vacuum chamber, fix any problems with the detector and electronics, and repeat the test. It was really frustrating when there were problems, but I sure learned a lot while helping Scott Bounds figure out what was wrong.