Figure 1 shows the raw ADC spectrum for 29, 29 in a 50K event replay for run 68144. Note that there is a "hit" for almost every event, at a constant position of about 1015 channels
Figure 2 shows the pedestal output file for run 68144, with channel 29,29 highlighted. Note that the pedestal mean is around 747 channels. This indicates that the engine did not calculate a new pedestal mean and used the default value instead.
Figure 3 shows the pedestal histograms page of the online GUI for run 68144. Notice that channel 29, 29 is missing.
I suspected that the problem is in the pedestal event analysis. And I was correct. I found that signals above 1000 channels are excluded (Nominally a Lecroy 1881M is supposed to have a pedestal between 200 and 900 channels) in order to prevent large signals from entering into the calculation of the pedestal mean and width. However, the pedestal shift in this channel was so large and positive that it now has a pedestal of more than 1000 channels, hence, it's pedestal is not calculated correctly in run 68144. I will raise the limit in the analysis code so that the pedestal is calculated correctly until we get a chance to go down there and fix whatever problem there is with the ADC or otherwise.
A copy of this log entry has been emailed to: pentchev@jlab.org, weiluo@jlab.org
Figure 1
Figure 2
Figure 3
