- Instructions for Bluejeans meeting connection
- Meeting ID: 290664653
- To join via a Web Browser, go to the page  https://bluejeans.com/290664653.
Thu 9/2/2021 2pm
- Run preparations
- FDC: FDC1 C1
The main discussion was about the problem with the first FDC chamber (FDC1 Cell1):
- In January 2020 (just when we got the first pulsed beam in the 2020 run) the pressure inside the chamber went down from 65 to 60Pa within couple of hours, while the pressure in all the other chambers slightly increased. From that time on, the pressure in that chamber remained always bellow the rest in the first package. 5Pa itself is not a big deal, but turned out to be indicative of a bigger problem.
- Since then the wire efficiency of the chamber started dropping slowly down to ~0.9 at the end of the 2020 run in September last year. The efficiency was dropping with the time even when the chamber was not in operation during the COVID downtime March-July 2020. The pseudohit efficiency went down to 0.7 (0.9*0.9*0.9) as it requires in addition hits from the two cathode planes. Alex prepared two animations from the FDC_Efficincy plugin, aboout the wire and pseudohit efficiency:
- Beni proposed the hypothesis that the gas input to that chamber got clogged and the pressure inside is coming from the output gas connection. As a result there's no flow through the chamber and it starts contaminating due to the outgassing inside the chamber.
- During the 2021 downtime we measured the oxygen contamination in the four packages 1-4 to be about 100ppm, 200ppm, 40ppm, 50ppm. Such levels would not produce the observed drop of efficiency. This also most likely excludes the other hypothesis that we have some leakage, otherwise it would result in higher oxygen levels.
- Naomi proposed to look at the charge vs drift time (or distance to the wire). After the meeting Alex re-run immediately the FDC_Efficiency plugin for four runs (with different color on the plots below) during 2020, where he calculates the chamber efficiencies vs distance. The results confirm that we have contamination in the first chamber that is worsening with the time: