Calibration of Photomultiplier Tubes and wavelength-shifting fibers using a low-intensity light source

Precise calibration of the response to single photon detection is an important step in the construction of the trigger scintillation detectors for the MOLLER experiment. We calibrate Photomultiplier tubes (PMTs) using a low-intensity light source placed inside a light-tight box. The PMT and the low-intensity LED light source are connected by a clear fiber, which will transmit the incident photons to the PMT. We collect the PMT response to a pulse-width modulated (PWM) signal of the LED when it is on. A histogram analysis with Gaussian fitting to obtain the single photoelectron peak of the PMT was performed using the ROOT analysis software.

Measuring the light yield of wavelength shifting (WLS) fibers is crucial to ensure that the scintillation light of the detector can be efficiently and uniformly detected by the fibers. WLS fiber connected to a PMT was tested using a fixed LED light source placed inside a light-tight box. The signal distribution of different fibers and the photoelectron pulse count of each fiber were observed. The outcome of the PMT calibration method and the progress of light yield measurements of the WLS fibers will be presented.