Summary: Results from wiping experiments are consistent with the hypothesis that the black residue is brushing residue that has not been cleaned off by the wiping procedure because it has collected in scratches and crevices that are not reached by the stiff wipes. It comes off on latex gloves because they are less-stiff and can access those crevices. To avoid transferring the black residue from the walls to the optics, we could re-wipe the chambers with less stiff wipes, or we could instead use gloves made of a stiffer material than latex that doesn’t deform into the crevices that contain the residue and pick it up when a gloved hand touches the wall. Jodi will plan on trying nylon, nitrile, and co-polymer clean room gloves.

Jodi told me that that the black residue found after chamber cleaning (described here) does not require time to develop but instead blackens latex gloves immediately after chamber cleaning, even though it does not come off on the synthetic wipes that are used to evaluate chamber cleaning. This suggests that slow re-oxidation is not involved.

BSC3 was cleaned about a month ago, so I did a little study to see what wiped up the black residue and if the residue could be removed. Just as Jodi described, I found that a normal rub with the synthetic wipes we use to clean the chambers produced no black, but that the slightest rub with a latex glove immediately blackened the glove. I then found that I could get black on the wipes with a really forceful rub: as hard as I could press with an area of just one finger (see figure). I found that I could get black smudges with forceful rubbing of bunny suit material, in-chamber booties, and face masks, but nothing was as easy as getting smudges on the latex gloves.

I then checked to see if I could clean the black residue off of the wall so that there was no more that came off on latex gloves. To do this I rubbed a section of wall about 20 times with latex gloves. The figure shows that this worked well, resulting in no smudges on the test glove.

Thus our best hypothesis is that the black material is residue from brushing that collects in regions that are difficult to access with the normal wipes. A wipe that conforms more to the surface topography, such as latex, would remove more of the residue.

My biggest worry associated with the black residue (other than that the extra surface area may require longer pumping) is that we will transfer it from gloves to optics. We could mitigate this danger by re-wiping the walls with a less stiff wipe, or, less costly, by using gloves that were stiffer, e.g. made of the same materials as the booties, bunny suits, or wipes. For a start we could try knit filament nylon, nitrile or co-polymer clean room gloves.

Robert, Jodi