Gain Changes for FMS high voltage changes


I have looked at Run 12025138. The ADC distributions are listed below.


The plan was to guess gain corrections that would correct the variations in energy response.

The guess is based on counting the number of of events with ADC>300 and estimating the gain change that would give us 150 counts in each channel for all channels of the FMS.

The guess is based on an assumed event energy distribution of

N(E)=N0 Exp(- E /(5 GeV) ).

Many of the channels have no events above the 300 threshold. For these channels I assume 1 event. The difference between the gain factor and unity  is then proportional to the log of the observed to required number of events above ADC = 300.

The files needed for further analysis are found at

The basic files are:

  • The gain corrections
  • The 3 parameters for the ADC vs HV for led data
  • PDF plots that summarize the distributions of "Number over 300" vs Pseudorapidity and the produced distributions of gain corrections as a function of Pseudorapidity are found here.
  • An Example of usage. A short root script that demonstrates the form of the above files and the definition of fits is found here. The output plots from this script are found here.  The script opens the FMSCor1-26-11.txt and plots the distribution of gain corrections for each of 4 FMS detectors.  It then open the fit parameter files and plots the gain corrections again but with a background color that shows the pulser gain at nominal voltages (1100 or 20 ) for corresponding cells. The script loads my distribution software (  which is provided and described in a earlier blog.  Finally there is a plot of the one ADC vs HV function for one example cell. This demonstrates the meaning of the parameters and how to reproduce functions of th required form. To remake this plot you need my FpdRoot source directory and you will need to set the environment variable pointing to that directory. You may need to rebuild.    (cd FpdRoot; rm; make). 
  • Overall Normalization. I have looked at the pi0 peak and I believe that these corrections will give a reasonable average pi0 mass (evaluated at 40 GeV) if the corrections are viewed as corrections to a nominal gain of about 50 to 55 MeV per channel.  
  • I think that these changes will bring us well toward all channel contributing to the trigger. When we have that, we will be able to iterate on pi0 mass for the next level of tuning.


PS: The Psuedorapidity variable is calculated as if the detector is closed. I don't know where it really is.