CDC Pedestal Studies

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Pedestal Subtraction

In this study the file hd_rawdata_003221_000.evio from run 3221 is used. It is a BCAL trigger run with the collimator in blocking position so no beam but only cosmics. Events are selected with the following signature in the CDC looking for hits with no signal in the first 125 samples.

  • Wave form of CDC signal

The first test was done by using the region P_{{i}} (blue) to determine a pedestal and the region I (red) to determine an Integral and then subtract the proper pedestal from the Intergral as follows: D=I-P_{{i}}/N_{{i}}*80. where N_{{i}} is the number of samples used to calculate the pedestal and 80 is the number of samples in the Integral I. In the case of no signal in the region I the distribution D is expected to be around zero with a sigma that reflects the resolution and depends on the number samples N_{{i}} used in determining the pedestal. As an example the distribution, with 4 samples and 32 samples in the pedestal calculation, looks like this:

  • Pedestal subtraction distribution Pedestal subtraction distribution
N_{{i}} mean F sigma F mean I sigma I
4 102.2 1293 96.5 1276
16 119.0 1121 163.4 1120
20 121.1 1080 145.1 1080
32 102.4 984 159.0 978
64 40.9 680 84.6 685

Note this is not what one would expect. The mean should be around zero!


Looking just at the pedestals you get the expected results like here, a pedestal determined over the first 16 samples:

  • Pedestal over 0-16 samples Pedestal over 96-112 samples

However if you look at the distribution of the above two pedestal calculations P6-P0 you get the following:

  • Pedestal difference P6-P0

The following table shows the mean and sigma of the distributions when P0 is subtracted from all the other pedestal distributions, once in full floating point and once with integers only.

Diff mean F sigma F mean I sigma I
P1-P0 2.5 201.5 2.8 199.7
P2-P0 26.6 254.9 27.7 257.9
P3-P0 23.1 272.3 24.9 274.1
P4-P0 25.2 280.0 25.9 275.0
P5-P0 30.3 277.5 28.8 276.8
P6-P0 21.3 280.1 19.7 282.1