A while back I complained about the fact that the chamber, after cleaning, becomes black-ish quickly to various people including Dennis and MikeZ, and was surprised to know that they didn't know.
I talked with MikeL, RobertS and Jodi since then. This alog just restates what was already known by some experts.
All chambers seem to be covered with black stuff. Attached is the picture of the glove with which I swiped the inside wall of BSC6 last week. Robert looked at it to confirm that it is quite similar to what he saw when he went into BSC8 some time after chamber cleaning to measure the particle count. Jodi also showed me her glove sample from BSC6, which looked quite similar. It's reasonable to say that we're seeing the same oxidization process. Jodi also say that she didn't see this right after the chamber cleaning, but it develops within a few days.
A few notes:
- You might think that the thing is grainy, but it's just that the high points on rubber surface are picking up the black thing.
- Unlike i/eLIGO days, these black stuff is very fine, and you cannot shake it off of the glove. In i/eLIGO days, when you swipe the inner wall of the chamber, you often had some bigger, scaly thing which you could shake off, on top of something you couldn't.
- Since Robert's study referenced above was done after the chamber wall already got covered with this black thing, and since it showed that the particle counts were a factor of 10 smaller than it used to be in i/eLIGO, there's no reason to believe that the particle count is back high again.
- I and Robert observed the glove under a microscope, and they're almost pitch black. As far as we can tell the size of the black particles is much smaller than the resolution of the instrument. There were several clumps of black stuff, though.
We don't know if this is going to become worse with time, though, and I think it's prudent to ask Jodi to make an assessment of the chamber surface once in a while, i.e. to go inside, swipe using a glove and a wipe and keep them as samples, and also measure the particle count. Of course she's done these things, but make sure that it's done repeatedly over some time. A perfect opportunity is before we pump down the Y arm, and after one arm test concludes.
There might be other things, e.g. Robert and I talked about wiping a part of the BSC thoroughly clean, have Jodi approve it, swipe using a glove right after that, and leave the glove for a few days to see if it becomes black.
One important thing to remember is this:
No worker should assume that the chamber wall is totally clean.
If your sleeve brushes the chamber wall, you need to at least be aware that your sleeve might become a carrier for the black thing to migrate to somewhere else in the chamber.
If possible at all, in-chamber workers should not touch the chamber.
I asked to be the first person into BSC6 so that I could check on this very thing! We noticed this phenomenon while cleaning BSC7, the first chamber cleaned with the production procedure. We had the whole chamber wiped down according to the procedure (last wipe for each section should have no visible color) and waited for FTIR results to come back (~7 to 10 days) before we continued to the 2nd vacuum and close out. After the FTIR results came back, we went into the chamber expecting to finish up in short order. But we found that we were seeing black "stuff" on our gloves when we touched the chamber, so the decision was made to wipe down the chamber a second time and then complete the chamber cleaning process. Later, we had to pull the door at BSC7 again (~10-14 days) to follow up on an FTIR result (false positive), and once again, we saw black "stuff" on gloves. FWIW, upon microscopic examination, Rai Weiss found no visible particulate on the "last wipes" that he examined. (If anyone needs me to firm up the number of days between "sampling events", let me know.) We'll have another good opportunity to assess re-oxidation (or whatever it is) when we do install at HAM 1-2-3. Chamber cleaning was completed at HAM1 on 20 April 2012, HAM2 on 13 April 2012, and HAM3 on 06 April 2012. I predict that enough time will have passed to observe re-oxidation and that initial assessment on re-entry will show black "stuff" on gloves. I have become interested in a vacuum-compatible fixative for chamber interiors: any one got a viable idea?