[Halld-pid-upgrade] DIRC Optical Box mirror inspection: Oct 2020

[Justin Stevens]

Hi All,

Last week we drained both optical boxes and inspected the mirrors boxes, which have been in operation since December 2019.  Thanks to Bill for compiling the diagrams and pictures of the mirrors taken with both a camera and boroscope

South OB: https://logbooks.jlab.org/entry/3855960
North OB: https://logbooks.jlab.org/entry/3856148 <https://logbooks.jlab.org/entry/3856148>

Comments in those logbook entries also link to Google photos pages where you can see all the pictures we took last week.  The OBs are current empty of water and purging with dry nitrogen.

A few observations:
-No iron or residue was observed on the bar box flange or bottom of the OB (as we saw in summer 2019)
-The aluminum strong backs show signs of oxidation, but not as bad as our previous inspection in summer 2019
-Most mirrors show some evidence of water penetrating the seal between our borosilicate protectors and the front surface mirror.

From these observations its difficult to quantitatively say what impact this may have had on DIRC performance over the previous runs this Spring/Summer.  Some additional information we need to provide is a measure of the photon yield over time for the last ~10 months to quantify any degradation in reflectivity.

We’ll discuss this further at next week’s DIRC meeting on Nov 9, but if you have any thoughts or suggestions ahead of that meeting, please feel free to send it to this list to get us brainstorming about our possible next steps.

-Justin
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