Efficiency of the PC Superlayers

There are four ITR superlayers in the PC region, they consist of 96 chambers organaized in 24 layers. The first and last superlayers consist of eight layers each. The two superlayers in between have only four layers each.

In order to avoid the bias which can appear if the pattern recognition program uses hits of the investigated chamber, a special version of the program was developed. Each event is reconstructed 24 times and every layer is excluded once from the reconstruction, in the excluded layer the efficiency estimation for all four chambers in the excluded layer is done. The parameters and covariance matrices of the nearest end-points of segments are extrapolated to the investigated $z$ plane.

For the analysis a sample of 30.000 - 50.000 events is used for each run. For each run a new mask is produced. The main contribution to the number of masked channels comes from superlayers MS10 and MS13 (up to 60%).

Figure 5.9: From left to right for the chamber MS10-+2: efficiency vs run number, signal over noise vs run number, fraction of masked channels vs run number. For the efficiency plot statistical errors are shown.

As an example, the efficiency, S/N and number of masked channels over time are shown in Fig. 5.9 for one of the relatively good chambers, MS10 +- 2. Some spikes are seen in the distribution of masked channels, they are caused by low voltage problems for half of the superlayer.

Drops of the high voltage in a group of chambers and problems caused by low voltage power supplies lead to the effect that even for properly working chambers it was not always possible to calculate the efficiency with sufficient statistic. In some areas the reconstruction program was not able to find enough hits in order to reconstruct segments.

Figure 5.10 shows pure efficiencies for all chambers of PC superlayers. In the range from 0 to 31, chambers of station MS10 are placed, the range from 32 to 63 corresponds to the superlayers MS11 and MS12 and the rest are chambers of MS13. One can see that the chambers in MS10 and MS13 show rather low efficiencies and large variations.

Figure 5.10: Mean efficiency distribution for the PC superlayers. Standard deviations are shown as error bars.

Superlayers MS10 and MS13 have approximately $40\%$ chambers with rather low efficiencies and large variations, this group mainly consist of chambers with only 10-20% of working channels. Another influence on the efficiency is due to non perfect masks. The non perfect masks can be produced for the chambers with rather low number of working channels ( only 10-20%) because it is difficult to distinguish between working and dead channels. In addition, some of these channels work only sporadically.

With the help of the obtained numbers, a GEM voltage tuning was performed in order to increase the efficiency of those chambers showing low efficiency. In some chambers the efficiency could be increased (in the order of 10%). Figure 5.11 shows the raise of the efficiency after GEM voltage increase for the chamber MS10 ++ 4.

Figure 5.11: From the left to the right for the chamber MS10 ++ 4: efficiency vs run, signal over noise vs run, average GEM voltage vs run.

Approximately 80% of the chambers did not show any change in the efficiency after a tuning of the GEM voltage, one of these chambers is shown in Fig. 5.12.

Figure 5.12: From the left to the right for the chamber MS12 - 1: efficiency vs run, signal over noise vs run, average GEM voltage vs run.

For approximately 5% of them the increased voltage affected the operation stability. For those chambers the voltage was set back to the original value.