Sphenix-tpc-l mailing list

[Thomas K Hemmick <tkhemmick AT gmail.com>]

Hi John

Everything you say is why last fall we discounted bad gas as a theory from the outset.  This hesitancy cost of 100% of the AuAu data and we disproved quite convincingly after the AuAu beam disappeared that ionization theories were (in that case) wrong and also quite convincingly that it had been bad gas all along.  Under such circumstances, failure to pursue the gas seems unwise.  

Tom

On Mon, Mar 31, 2025 at 7:23 AM John Haggerty <haggerty AT bnl.gov> wrote:

I might not be able to make it to the TPC meeting this morning due to
the deluge of other meetings and activities, so I'll record some
comments on the gas here:

On 2025-03-30 13:00, "Hughes, Charles [PHYSA]" wrote:
> Hello All,
>
> Let's meet MONDAY 03.31 at 9:30 AM ET (3:30 PM CET) to discuss ongoing
> operations/analysis/planning for the TPC. In particular:
...
> Gas/Cooling: Our Online and Offline Monitoring are strongly hinting at
> a change in the gas that started absorbing drift electrons sometime
> between 8:00 AM 03/27 and 6:30 PM 03/28. We see sloping pulse heights
> in the OnlMon and a z dependence of dE/dx in the offline - exactly as
> what we saw last year at the end of the 2-week AuAu run (though less
> severe at time of writing). @Robert P. Pisani has already done some
> forensics with no clear indication that these symptoms can be
> correlated to a gas change. We should discuss.
...

- I generally don't like bad gas theories because they are almost
impossible to falsify, and we don't have the tools to do quantitative
analysis on the gas, so we have to rely on guesswork

- Rob is certainly right that we haven't needed extremely high purity
gas before, but the loophole in the argument is that we haven't drifted
charge a meter before, either.  The GEM's themselves are working fine,
apparently, but as the charge drifts, it either recombines or could it
be diffused so that we lose the charge to zero suppression?  I sometimes
have a pang about not trying to get the canary chamber in there--but it
probably wouldn't see this problem anyway, since it would only test the
GEM's.

- Ways of getting (damp) air into the TPC seem like the likeliest
explanation to me, and it seems to me there are lots of places and ways
that can happen.  The only remedy I can see is to increase the flow and
see what happens in a few days.  The other reason I don't like gas
explanations is that they take days to see a difference even if you have
nailed the cause.

- I also feel a little twitchy that both times we have had this problem,
we have dumped a lot of ionization in the chamber, first with Au+Au
collisions now with the line lasers.  I didn't take organic chemistry,
but it seems to me there's a lot of chemistry happending when you excite
gases with powerful lasers.  Could anything be seen on the baby TPC that
might... illuminate... this?

---
John Haggerty
haggerty AT bnl.gov
cell: 631 741 3358