A PMT (Photo-Multiplier Tube) is a device the measures light. Whenever a cosmic ray passes through the detector, it produces a small amount of light in the scintillator inside the detector. The light is detected by the PMT, which sends an electrical signal to the DAQ card or oscilloscope. PMTs must first be tested to ensure that they work. They are extremely delicate pieces of equipment and can easily be "burned out" if they are exposed to too much light (this includes ambient room light) while they are turned on, or if they are turned off quickly without lowering their power voltage. Therefore, it is important to keep the PMT covered while it is on and to always raise and lower voltages slowly.
CROP uses two different PMTs. Read more here: PMTs.pdf
Testing PMTs is an important part of the project. PMTs will vary on their efficiencies, noise bands, and more. CROP has two methods to test PMTs:
Alpha testing involves using an alpha radiation source (helium nuclei) to mimic the signals from cosmic rays. You will want to test the PMT's noise band, signal size, and threshold voltage.
Alpha testing tutorial: Alpha Testing.pdf
Monkey Box Testing
A "monkey test" is a generic term that basically means "keep doing what you're doing and eventually something will work." With that philosophy in mind, the CROP "Monkey Box Test" employs the use of a single detector with multiple PMTs attached. The idea is that you can test multiple PMTs at once and hope that at some point, you will find one that works. Since cosmic rays are hitting the detector at all times, we can gauge an "average" amount of signals and use this information to examine PMTs. In essence, measurements done with the monkey box should look about the same as the alpha test above.
Monkey box tutorial: Monkeybox Testing.pdf