PIXEL NOISE AT THE BOTTOM OF THE TPC

The Problem

In E895 and in the EOS experiment at the Bevalac, there are often the first few timebuckets for all pads above thresold. It is unclear whether this is a residual effect of the gated grid, or whether it is particles passing through the wire plane. In principle, since we fired the grid while taking pedestal data, the first-order effect of the grid is taken away. On the other hand, to second order, these pixels may be simply "noisy".

In any event, this happens on some large fraction of events (I would say most) but not all events. This leads to a lot of "bad" hits showing up at the bottom of the TPC, and therefore a lowered "fraction of hits used by tracker parameter" (but still better than the 1d code, which has its own explicit removal of these hits at the bottom).

The Fix

The clean-up proceedure that has been adopted is a very simple one:

For a given padrow, if a certain fraction (set by the new switch h2d_sw.cleanup_maxocc = 0.25 by default) of pixels corresponding to a given timebucket are above thresold, then kill (zero) all pixels with that time bucket, and those below. The search for these "bad buckets" starts at some bucket set by a switch (h2d_sw.nbuck_cleanup = 3 by default). This cleanup and switches were implemented in v1.14 of h2d. Not suprisingly, such a simple mechanism works fine. Here are ADC vs time vs pad for padrows 50 and 52 in a 2 GeV event that showed the problem. The z axis (ADC) is logarithmic.

Here is padrow 50 before and after the cleanup was implemented.

Here is padrow 52 before and after the cleanup was implemented.

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