[Sbs_gems] [EXTERNAL] BB Results with Mixed Gas System

[Sean Jeffas]

Hi All,

I finished analyzing the new cosmic run with the mixed gas system last
night. It is run 11916, and the full results can be seen at the HALOG.
Because it was our first test with the new mix we operated at a slightly
lower HV to be safe. For comparison we can look at an old run, 10408, taken
back in September at the exact same HV settings and with premixed gas. So
the only difference between run 10408 and 11916 is switching from premixed
gas to the new mixed gas.

*Full Results:*
Run 10408 (premixed gas): https://logbooks.jlab.org/entry/3940003
Run 11916 (mixed gas): https://logbooks.jlab.org/entry/3940008

The notable differences are that it looks like run 11916 has a slight
pedestal shift in J0, so that will have a tiny effect on the results. Also
run 11916 has two APVs turned off on J2, which artificially lowers the
efficiency that we see on module 5. Also run 11916 has not been aligned by
the software yet, so that will also have a slight effect on the tracking,
but it is only a mm or two difference.

We originally found the mixing system was not working by noting a drop in
efficiency back a couple months ago. Below I have just attached the
efficiency results, for comparison, but I highly recommend that everyone
looks through the HALOG that I attached to see all the results.

Conclusion: Mixed gas system is accurately at 75/25 Ar/CO2

Every module except for 1, 5, and 6 have less than 2% change in efficiency.
Module 1 is on J0, which has a slight pedestal shift, so the apparent 7%
increase in efficiency may be due to that. And modules 5 and 6, are on
layer J2, which has a whole host of problems, so I wouldn't worry about
them too much.

Best,
Sean
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