Dr. Jeffrey Wilkes of the University of Washington
discusses the project with a SALTA workshop participant. Note
the blueish color of the piece of scintillator plastic he's holding.
Ultraviolet light from the sun "excites" a fluorescent
molecule in the plastic. When that molecule loses energy a short
time later, it does so by giving off light, but at a blue, rather
than a UV frequency. For more on fluorescence, click here.
The first step is to unwrap the panels, which are in
plastic housings sealed with black tape to make them light - tight.
Dr. Greg Snow of the University of Nebraska points
out that panels were originally cut with a band saw and have rough
edges. To save on costs in the CASA array, the scintillator panels
were left in this state. This allows light to escape, ultimately
causing loss of signal, or a less efficient detector. By polishing
to bring the edges to a smooth finish, more light of the light
created within the plastic will reach the PMT, resulting in a
more efficient detector. To learn more details about this, go
here.
Scrape each edge with a razor. Hold the blade at a
45 degree angle and draw it toward you. This step should remove
nearly all imperfections : the more time spent here, the quicker
all of the later polishing tasks will go.
Occasionally wiping the surface with a wet sponge will
allow you to see your progress.
After the razor work is complete, time for sanding.
Use 3 rounds of wet/dry snadpaper : 600 grit, then 1500 grit,
then 2500 grit. Wetting the paper will keep the paper from getting
clogged with plastic. Using a block will keep the sandpaper on
the edge and prevent the flat sides of the plastic from getting
scratched by the paper. A woodblock made from a 2 x4 will do,
but a commercial sanding block from a hardware store is a lot
easier to use.
After sanding, the panels are polished on all sides
with plastic polish, then cleaned with alcohol or windex.
