[Halld-pid-upgrade] [Revised Logentry] PMT installation and cookies

[mpatsyuk at jlab.org]

Logentry Text:
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We have completed today the first optical box - all 18 brackets are on the frame and a good optical coupling is established for 108 PMTs and dummies.

When we arrived in the morning and inspected the installed 14 brackets, we saw that the bracket 12 had effectively no optical contact for two PMTs closer to the middle of the window. The view of the PMT faces in the mirror looked like the two PMTs were still in the air, and drops of oil were visible on the cookie surface. It seemed that something interfered with the PMT or electronics boards and did not allow the PMTs to touch the window, so that they were hanging in the air.
It was hard to see the obstacle: brackets are packed together very dense and we did not see anything strange through the gaps between them. Neither we saw anything between the electronics boards.
We decided to take the bracket 12 out. For that we unscrewed 4 screws attaching the bracket to the frame and tried to pull it out. The bracket could be lifted a bit, but then the electronics boards interfered with the neighboring ones and the bracket got stuck. In our setup all the electronics modules are a little bit rotated relative to the bracket itself, and the spacers make sure the modules are parallel to each other. The spacers are attached to the top of each module. We unscrewed the screws (circled on the following photo) [figure:2]

and this enables us to disassemble the bracket and take out two modules (almost) independently. The two modules are connected by cables, but one can adjust each one to fit in the narrow gap between the neighboring installed brackets.
The structure of the bracket allows to detach one bracket from a stack.

The modules from bracket 12 looked like all the others, and we did not find any obvious reasons why the coupling was bad. We assembled the bracket 12 again, changed the cookies for the new ones and tried to put the bracket back in place. But we came across the same problem: the neighboring brackets had their modules slightly rotated, which did not allow to fit in the bracket 12. We loosened the screws circled in the following pic [figure:1]

and that allowed massaging each individual module, so that it fits into the slot.
After that both modules were lowered to the window using the nuts on the screws and a good optical coupling was established.

After that exercise we finished installation of 18 brackets. We have created one optical fiber looping, which seems to be not critical since the looped cable is long enough to reach the readout. We will keep that in mind for the second box.

We used up the second batch of cookies (using the latest mold) and also about 4-6 cookies from the old batch (thicker mold). We are left with about 40 cookies, which should be fine for the second optical box. I can bake another ~10 cookies at Bates, so that we do not feel shortage of them. Extra greasing oil should arrive tomorrow, but right now we are fine with the oil we have: we have 3 syringes 10 CC each, for the first box we almost used up one syringe, so the remaining two should be fine for the second box.

Conclusions:
1. It is possible and actually quite easy to extract and put back one bracket from the array.
2. We have enough greasing oil, and more cookies are coming.

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