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The CLAS Cerenkov Detector
Mirror Alignment
After building the sectors, the PMTs and mirrors must be installed.
The mirrors must be aligned precisely, so that as much Cerenkov light
as possible is focussed onto the PMTs. To do this, we used Alex
Vlassov's simulation. As part of his work for the CLAS collaboration,
he simulated the response of the detector to electrons at varying
energy and angle, and (in the simulation) adjusted the positions of
the mirrors to obtain the highest light collection efficiency. He
then ran the program again, turning off the magnetic field, and
setting the index of refraction so that the Cerenkov light is emitted
precisely along the direction of the incident electron. We duplicated
this in the laboratory by placing a laser at the target position,
relative to the sector, and aiming it at different points along the
mirrors. By comparing the reflections of the laser beam with those in
the simulation, we can obtain the optimum mirror alignment.
After adjusting all of the mirrors, they are attached to a spine
running down the center of the detector. This spine provides extra
rigidity to the mirror array, and prevents droop in the mirrors over time.