  Large-volume water-Cherenkov neutrino detectors are a light-starved
environment, as each interaction produces only $\sim 50-100$ photons per MeV.
As such, maximizing the light collection efficiency of the detector is vital to
performance. Since Cherenkov emission is heavily weighted towards the near UV,
one method to maximize overall detector light collection without increasing the
number of photomultiplier tubes is to couple each tube to a wavelength-shifting
plastic plate, thus shifting photon wavelengths to a regime better suited to
maximize photomultiplier efficiency and potentially detecting photons that miss
the photocathode. To better understand the behavior of such plates, a scan of a
rectangular wavelength-shifting plate was performed, and the results were used
to calculate the overall percentage improvement in light collection that could
be expected for individual PMTs in a large water-Cherenkov detector.
Measurements of a 15.1 in. by 11.5 in. wavelength-shifting plate using a 365 nm
LED were found to increase overall light collection at the photomultiplier tube
by $7.4\pm0.7\%$. A simulation tuned to reproduce these results was used to
predict the behavior of a wavelength shifting plate exposed to Cherenkov
spectrum light and found increases in light collection that were linear with
edge length, assuming square geometries. These results demonstrate the
potential of wavelength-shifting plates to increase the overall light
collection efficiency in a large detector.
