2.3 Hit Finding

  The CST Hit Finding is called in Hits&Cluster as well as in Monitor mode. It is performed only if the FADC is selected and if the Sigmas have been initialized correctly (see Sec. 2.4). It consists of a two-pass algorithm processing the Raw Data of every strip. The main loop runs over one halfladder, i.e. 640 strips.

In a first pass the Common Mode for every APC (i.e. 128 strips) is calculated. This is done by just taking the mean raw data value from all strips (We call it CM128). To minimize large contributions from ``hit strips'', the pulse height is then evaluated as

and a 3$\sigma$-cut is applied. The Common Mode is evaluated again from the remaining ``non-hit-strips''. The first pass hence actually contains two loops over the data. In case the number of ``non-hit strips'' is below 64, the CM128 is used.

In a second pass the pulse height is evaluated again (eventually with a correct Common Mode):

Two criteria have to be met for a $PulseHeight$ to be marked as a Hit:

1. (Hits are negative!)
2. $PH2>\; Variance\; CSTStripCut$

Bit 14 of the raw data word of a hit strip is used as a hit flag to be checked by the updating routine, see Sec.2.4. As long as the above conditions are true for neighbouring strips, the $PulseHeights$ as well as the $Variances$ are summed up and stored. If the first non-hit strip is reached, the $Cluster\; Criterion$ for the candidate strips is checked:

$(\sum PulseHeight)2ClusterWidth\; >\; \sum Variance\; CSTClusterCut$

where the $ClusterWidth$ is the number of strips belonging to the cluster candidate. For $CSTStripCut$ and $CSTClusterCut$ values of 1 and 25 resp. are used at present.

If the above statement is true, a hit word is written to the Hit Buffer for each of the strips.

• 2.3.1 Handling Special Voltages