Improvement in light collection of a photomultiplier tube using a wavelength-shifting plate

TL;DR

This study demonstrates that attaching wavelength-shifting plates to photomultiplier tubes can significantly enhance light collection efficiency in water-Cherenkov neutrino detectors, with measured improvements of about 7.4%.

Contribution

The paper provides experimental measurements and simulations showing the effectiveness of wavelength-shifting plates in increasing light collection in large neutrino detectors.

Findings

Wavelength-shifting plates increased light collection by 7.4%.

Simulation predicts linear increase in light collection with plate size.

Potential to improve detector sensitivity without adding more photomultiplier tubes.

Abstract

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.