First Year Performance Paper - Section 3.2
3.2 Calibration of the PMT gain
Both DOM acceptance testing and detector operation require the DOMs to operate at a specified gain. Each PMT has a unique gain response to anodephotocathode voltage; therefore, it is necessary to characterize this response and calculate the voltage yielding a specified gain for each DOM.
The uncorrelated noise in the optical module results from thermal background of the photocathode, which is significantly reduced at the cold temperatures, and by radioactive decay of isotopes contaminating the glass pressure sphere. The electrons from the beta decays produce photons by Cherenkov radiation and by scintillation. This results in a significant rate of SPE signals. The spectrum of PMT signals is shown in Fig. 7.
At each high voltage setting, several thousand PMT waveforms are acquired with a special low discriminator threshold. Each waveform is integrated around the peak. The total charge is obtained by dividing the integrated value by the load impedance and the ATWD sampling frequency. The charge spectrum is fitted to a Gaussian + exponential function. The mean SPE charge is proportional to the mean of the Gaussian component. The gain is the SPE charge divided by the electron charge. This procedure is repeated for high voltage settings between 1200V and 1900V in 100V increments. A linear fit of log10(gain) to log10(V ) is performed. This fit is later used to compute the PMT voltage yielding a desired gain. The gains of the PMTs on String-21 were set at 107.