In this step, the PMTs are glued using an optical epoxy to the center of the scintillator panel. Because the optical epoxy has (nearly) the same index of refraction as the scintillator plastic and the glass of the PMT, it provides a very efficient optical coupling between the fluorescent light produced in the scintillator and the PMT.

PMTs need to be completely clean. Here, a razor blade is used to remove old epoxy from the glass surface of a PMT recycled from CASA.

The glass surface of the PMT is polished.

On the side opposite to the glueing side, the exact center of the panel is located using 2 diagonals from the corners. The center is marked using a water-based overhead pen, and concentric circles are drawn to aid in placement of the PMT. Carpenter's shims are used to ensure that each panel is level. The epoxy is rather slippery, and it's possible the PMT could slide downhill and become glued off-center, if the leveling were not done.

On each scintillator panel, cardboard wedges were lightly taped down with masking tape to hold each PMT in place overnight while the epoxy cures.

In SALTA, we used these cardboard pieces, masking taped to the panels, to provide additional stability and anti-sliding for the PMTs. Once glued, the PMTs were held in place by the cardboard templates

To keep the panels clean, the entire set up is done on a plastic sheet. Everyone picks a job, and an assembly line gets going...

The epoxy resin and hardener are measured by weight. To remove air bubbles (which would end up between the scintillator and PMT, disturbing light transmission), the well-mixed epoxy is placed in a high vacuum prior to application. The epoxy is applied with a syringe, about 5 cc per tube. The tube should be set down on the epoxy at an angle to avoid trapping air underneath. The most amount of sliding will occur at this moment, so care should be taken to check the centering of the PMT.