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METHOD USED FOR CALIBRATING PHOTOMULTIPLERS

We calculated the Photomultipler Gain _{HV} at the high voltage HV applied by measuring the response at different led amplitudes.

* The steps described below are automatized by our data acquisition setup and data analysis*

#### Runs at different HVs

- We took PM pulse height response at several led amplitudes.

**For each HV, the fit of the variance vs pulse height peak (The so called "Response line") gives us two parameters:**

- I -- interception with the Y axis (in ADC
^{2})
- S -- slope (in ADC)

**For each HV, the slope S is proportional to the Gain **_{HV}:

- S=Gain
_{HV}*(1+delta^{2})

* The factor depends on delta, where delta comes from fit of pm single photoelectron distribution. BUT:*

**In absence of a correct method for fine mesh single photoelectron spectra interpretation, we have decided to normalize our Gain**_{HV=2000} to the nominal gain given by Hamamatsu factory.

(If you want to know how data were analysed, please read PM data analysis )