French Cerenkov PMTs High Voltage Calibration

By plotting the Cerenkov PMTs normalized rates as a function of the threshold, we obtain the integral of the gaussian photoelectron amplitude distribution, which is a cumulative distribution function. The amplitude corresponding to half the maximum rate is the mean value of this distribution, so we can watchful eye extract it and then adjust the high voltage to apply to the Cerenkov PMTs in order to normalize this amplitude.

Here are the plots obtained for octant 2, octant 4, octant 6 and octant 8, and the following table gives these measured amplitudes, in mV (note that to find the photoelectron amplitude, we don't take into account the first bin, corresponding to a 10mV threshold) :

	Octant 2	Octant 4	Octant 6	Octant 8
PM1	95	80	100	75
PM2	80	85	90	90
PM3	80	55	70	70
PM4	75	60	60	105

As one can see on the plot, there was something weird with the PMTs 2 and 4 of the octant 2 (we were however able to find their photoelectron amplitude by switching the PMT 1 with the PMT 2, and the PMT 3 with the PMT 4, runs #29694 and #29695). In fact this problem comes from crosstalks between odd and even scaler channels, and affects all even channels, even if it is not obvious by just looking at these plots. It seems to be due to the VME crates power supply (ROC8), and has mysteriously disappeared since the electronic room air conditioning has been fixed...

We can fit this distribution by the following function :

where A, B, C, D, sigma and mu are the fit parameters. The first term corresponds to the integral of a decreasing exponential with low amplitude photoelectrons (see Photonis documentation). The fit seem pretty good, as one can see on the figure below, which also plots the photoelectron amplitude distribution resulting from this fit. We are waiting for non-crosstalked data with beam to check the values listed in the above table and extract a sigma for the gaussian distribution. We will later be able to check them again with the ARS...

We can thus calculate the high voltage to apply in order to normalize these amplitude to 100mV, using the formula :

where 0.153162 is the slope of the straight line obtained by plotting in a log/log scale HV = f(gain), and 157 corresponds to the ordinate at the origin.

Here are the new high voltage values (in V) :

	Octant 2	Octant 4	Octant 6	Octant 8
PM1	1984	1795	1985	2109
PM2	1826	1759	2233	2177
PM3	1893	1765	1681	2055
PM4	2109	1976	2193	2070

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(Contact : guillard [at] jlab.org)