March 31, 2016 Drift Chamber meeting
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- Meeting ID: 290664653
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- NIM Papers
- Run Status
- Electronics (Fernando, Chris, Nick)
- fADC125 meeting (Naomi, Cody, Beni)
Participants: Curtis, Mike (CMU) Eugene, Beni, Luke, Cody, Simon, Nick, Naomi, and Lubomir (JLab).
- Simon showed results for two-track vertices from straight track data (see the logentry). We first discussed the two stripes on the 2d plot - r vs z of the vertex position. The upstream one is at the downstream CDC plate. The other stripe is in the middle of the first package (not clear why?). This may indicate background from photons(?) stopping in the CDC plate that has an inner radius of ~10cm; for r<10cm the photons are stopped in the first package. Eugene: e+/e- from the photon conversion can't be separated without magnetic field.
- Second part of Simon's results: projections on z. The positions of the target walls and the scattering chamber, as defined by the FDC with straight tracks, are off by almost 1cm as compared to the results when using the CDC with magnetic field. Later Simon included FDC-only track with magnetic field and confirmed an offset b/n FDC and CDC by about 8mm.
- Lubomir showed comparison b/n the FDC currents with full/empty target without magnetic field and 3.4mm collimator. The ratio is 2.4, to be compared to 1.4 we had before with magnetic field and 5mm collimator. The FDC currents when normalized to the active target with 3.4mm are about 20% higher than with 5mm collimator, indicating additional background not related to the collimator hole. Lubomir also showed how the FDC currents change with the magnetic field (see logentry): from 0 to 450A the currents decrease by a factor of ~4.5. There's additional factor of 5 from 450A to 1200A, so in total ~22. This is to be compared to a factor of ~40 from the spring of 2015, also indicating some extra background that we have now.
- Latest FDC wire resolution (see plot above) obtained with straight tracks is a little better than a year ago, when we had half the energy. Most likely this is due to the higher momentum of the tracks. The efficiency of the chambers (third package shown as an example) is ~90%, lower than the last year's 95%. Needs to be studied, could be a timing issue.
- The CDC & FDC crates are using the firmware version 2_00_F_3 but the DAQ still has the older driver library on it. The new firmware has some improvements made by Cody in the compilation. As previously, the pedestal output is the integrated pedestal (sum of 2**P2 samples), right-shifted by (P2+PBIT). The new firmware allows PBIT to have integer values from 3 down to -P2, so the possible output values for pedestal now include the mean scaled by 1/8 (PBIT=3), the mean (PBIT=0), and the integral (PBIT=-P2). Other scaling factors can be obtained using different PBIT values. Previous versions of the firmware and driver use PBIT >=0. This change was made because Lubomir had been expecting the firmware to output the integrated pedestal instead of the mean. Using PBIT=-P2 permits this while requiring no changes to the offline code. We need to remind Sergey to update the library.
- Cody suspected that some of the timing errors that we are seeing in the fa125 long mode data are due to triggers arriving too frequently. We thought that bufferlevel=1 would restrict the triggers so that only one trigger was sent to the fa125 at a time, but this turned out to be untrue, the value in the configuration file is only a place-holder and the true value is a constant 4. Beni increased the trigger holdoff from 15us to 30us and the number of timing errors halved. Naomi asked Sascha to recompile the CODA readout list with bufferlevel=1 and we took cosmic run 11075. After this, the readout list was recompiled with bufferlevel=4 as before. The timing errors in run 11075 are almost entirely from 6 channels, numbers 36,38,40 in two fadc modules (roc25 slot 9 and roc28 slot 16). We have not found any logic errors in the firmware.
- Sascha offered to make bufferlevel into a real parameter, we should take him up on this. We also mentioned the possibility of using the busy signal from the fa125 as a trigger inhibit, this should be more efficient in short mode, but this seems further back in Sascha's to-do list, as it is part of a larger makeover to do this for ALL the modules.
- Cody made the raw data output more robust in version 2_00_E, it turned out to be bufferlevel=4 causing the trouble with repeated raw data words, this was the clue that the triggers might be arriving too close together. We have not seen any more repeated raw data words since then, although there are very occasional errors (one per run) with channel number mismatch between pulse and window data.
- Lubomir estimated the timing algorithm resolution by comparing it with the TDC time from the wires. The TDC counts were taken w.r.t. to the trigger time as recorded by the f125 to eliminate the trigger jitter. For TH=100 and TL=25 we get 6ns resolutions for big signals (>600 fADC unist) and 8ns for amplitudes <600. From the plot one can see that a time-walk correction would improve the resolution there. For TH=50 and TL=12 the resolutions are much worse (>12ns). These are combined fADC/TDC resolutions, but the assumption is that the TDC contribution is smaller.