FCALPhotos
The cell wall must hold and shield the 2800 PMTs that are used to read out the lead glass blocks. The current GlueX design is based of a design used for both the E852 and RadPhi cell walls. It consists of two thin plates with holes drilled at the PMT locations. The plates are then connected with soft iron sleeves that help magnetically shield the enclosed tubes. The void in between the plates and outside of the tubes is filled with epoxy to make the entire structure mechanically rigid.
We expect the a stray field from the GlueX solenoid to be a couple hundred Gauss in the vicinity of the FCAL. To shield from this, the PMTs will need to be recessed about a photocathode diameter ( ~2.5 cm) back into the cell wall tubes. To handle achieve this while improving light collection efficiency we plan to use aluminized cylindrical light pipes or light guides to connect the PMTs to the lead glass. The light pipes will be glued to the end of the PMT and will be coupled to the lead glass using a Sylgard cookie.
To accommodate this design the cell wall and base must satisfy a variety of mechanical constraints include:
- provide soft iron enclosure for PMTs
- allow for +- 1/16" variance in length of assembled PMT and base
- permit variance on concentricity of PMT pin pattern and glass envelope
- apply pressure on Sylgard optical joint between light guide and lead glass bar
- minimize production costs by utilizing stock tubing materials (tubing) wherever possible
