Initial Tests Of The Pion Beta Decay Detector

5.2.1 Pedestal subtraction

A normal effect of using electronics as described in section 3.4.1 is reading non-zero values from the digitizing electronics for channels where no signal is present. This is caused by a small DC offset between the PMT output and the ADC input and is called the pedestal. In this experiment, nominal pedestal values for each channel were measured online while more accurate pedestal values were determined offline.

The offline pedestal values were determined as follows: Differences between the raw ADC value of each CsI crystal and the raw ADC values of several select CsI crystals were used to generate histograms. These histograms are Gaussian shaped spectra centered at the difference between the pedestals of the two channels used to generate the histogram. The widths of these histograms were used to determine which channels had highly correlated noise (described below in section 5.2.2). Figure 5.1 shows an example of a raw pedestal histogram and an example of one of the difference histograms.

Figure 5.1: Pedestal histograms generated from raw ADC values. The plot on the left shows a pedestal spectrum for the ADC of a single photomultiplier tube (PMT). The plot on the right shows a histogram of the difference between ADCs for two PMTs. The narrow width of the plot on the right is an indication the noise is highly correlated between the two channels.