GlueX TOF Meeting, March 28, 2018
GlueX Time-of-Flight Meeting
Wednesday, March 28, 2018
10:00 am EDT
JLab: CEBAF Center, Room F226
BlueJeans Meeting number is 350 531 998
- Review of minutes from the March 14 meeting
- Calibration Status
- ADC thresholds (Beni)
- Amplified Base Prototype (Beni)
- PMT testing
- NIM Paper (Paul)
- Action Item Recap
Talks can be deposited in the directory
/group/halld/www/halldweb/html/talks/2018 on the JLab CUE. This directory is accessible from the web at https://halldweb.jlab.org/talks/2018/ .
- FSU: Sasha Ostrovidov
- JLab: Thomas Britton, Eugene Chudakov, Mark Ito (chair), Simon Taylor, Beni Zihlmann
There is a recording of this meeting on the BlueJeans site. Use your JLab credentials.
Changing the Hit Factory
Beni has changed the TOF hit reconstruction code to use the FADC integral for the time-walk correction in the case that the amplitude is set to zero. Recall that this can happen when the voltage is above threshold at the beginning of the FADC read-out window. The ratio used is a hard-wired 0.2 (amplitude/integral). He analyzed a file both before and after this change and sees an increase in TOF efficiency. See logbook entry 3547610 for plots that demonstrate the improvement. It can be greater than 10% in certain regions of the detector.
The timing resolution appears to be a bit worse with the change, but a quantitative assessment is still in progress. Part of this may come from an apparent shift in the timing calibration constants. This is not expected and not understood.
Review of minutes from the March 14 meeting
We went over the minutes.
High Rates and Missed Pulses
There was further discussion of the presentation Beni made at the last meeting on losses at high rates.
- There were "missing channels" where FADC read-out was completely apparently absent from the event for channels were a pulse was expected. This was a bug. Those pulses are indeed there in the data.
- The FADC read-out thresholds have been raised for the TOF from 110 counts to 160. This should improve efficiency. The level was chosen to be a bit below that of the discriminatory threshold (thus on the TDC side, note that the signal split is 50-50).
- The feature of the firmware where "peak=0" is reported is allegedly fundamental in the algorithm and not easy to change.
- We need to explore schemes where full waveform read-out is done, either selectively, or for every event, because of lost information at high rates.
- Beni found out that there is an intrinsic dead-time in the discriminator of 20 to 25 ns, independent of input pulse width. This was chosen to match the dead-time of the F1-TDC. Since we are using CAEN TDCs in the TOF, this limits our double pulse separation capability. As a result, a discriminator width of 20 ns is used universally in GlueX.
We need to study how to deal with the significant baseline shift incurred by the AC coupling of the amplifier-in-the-base scheme.
In order to look the apparently poor quantum efficiency he was seeing for some PMTs with an LED light source, Sasha set up a test with an Americium source. With this set all PMTs give roughly similar and acceptable efficiency. The cause of the problem with the LED set-up is still under investigation.
Beni asked about gains. The new tubes have gains in the range of 4 to 6×106.
Action Item Recap
- Add items to the list of tests to perform with the remaining run time during Spring 2018:
- Test the effect of the raised FADC read-out threshold.
- Look with a scope at the discriminator input for the amplifier-in-the-base channel.
- Initiate process of shipping tested PMTs back to JLab from FSU.