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

[mpatsyuk at jlab.org]

Logentry Text:
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The modules for the first optical box were cabled and transported from the lab to the Hall D attached to the optical box frame, which had a plexiglas window. The edge brackets were not loaded and served as handles: [figure:6]

When we attached the first bracket as a test, the coupling was fine, but the edges of the cookie stuck out from the sides as shown here: [figure:3] 

Another example of how the cookie sticks out: [figure:7]

This is a potential problem, as the neighboring bracket is sitting close and the cookies might overlap.

So we decided to trim all the cookies to make them 2 mm narrower: initially the cookies in the second batch were 5.1 cm wide, so that we trimmed them to be 4.9 mm [figure:1]

All cookies from the second batch trimmed to 4.9 cm: [figure:5]

After we attached the first bracket as a test, we had to remove it as the window of the optical box needed cleaning on the inner side. When a bracket is detached, the cookie usually stays on the window. The following fig shows the cookies stuck to the window after the bracket is detached. [figure:2]

The coupling for the first two brackets: [figure:4]

One could see a small scratch on the cookie originating from the mold surface damage on the bottom row, second from the left PMT. We decided to leave this cookie in place but put aside all the other cookies which have damages (scratched from the same mold defect or tiny internal bubbles). Today we put aside about 5 cookies. We installed 14 brackets and it seems that we need to use a couple of cookies from the first batch, as we used up (applied or put aside) almost all the cookies from the second batch.

We were working on the cart, which stands right next to the optical box. We were greasing the PMTs and applying cookies, when the bracket was located on the cart. The other way of work - when the cookie is being applied when a bracket is standing on the ones installed, has one disadvantage. The nuts scratch the anodized surface of the bracket and create small particles, which then spread around and contaminate oily cookies and stick to the gloves. [figure:10]

When we installed a couple of modules, we saw that the coupling was not perfect, and that after a good coupling is created, something happens which destroys it. Greg tried tighten the screws holding the brackets, and it helped to improve the coupling. So we make sure all the screws holding brackets are tighten.

The following pic shows good coupling for the first two brackets, and the third bracket is attached, but the PMTs are not yet lowered to the window using the nuts: [figure:8]

After we installed each bracket, we checked with a ruler, that the two modules are aligned. But it seems that the modules are slowly rotated to some angle after that, and this angle needs to be calibrated out later. [figure:9]

With the first set of modules connected by the same bundle of optic fibers we noticed, that one cable was looped, so that it was impossible to install the brackets and get the four cables from one side, and another four - from the other side of the box. We had to disassemble the problematic bracket and reconnect the cable. All the other modules were checked in order to prevent this problem.

After about fifth bracket we started to notice that when a bracket is located on the frame, the pins do not fully go into the holes, so that there is a gap between the frame and the bracket. We were looking for the problem - maybe some cable was in the way, or the spacer hit the board of the neighboring module, but we did not see anything suspicious. So we were screwing the brackets slowly so that eventually they sit well on the frame without gaps.

I think I was putting slightly more oil than usually, because we need to grease the surfaces quickly and because more oil would come next Tuesday in time for the second box, so that we should not have shortage in oil. When the optical box will be vertical, the oil excess flows down. Maybe it makes sense to put some absorbing material to the bottom of the frame, where the PMTs are attached, so that oil does not get into the air cooling system and does not drip from the box to the floor of the hall.

After we installed five or six modules, it got hard to inspect the optical coupling for the first ones, but one could see that the coupling for the later ones was fine.  With some experience one could see that the coupling for all the modules got fine.


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