The Wavelength-shifting Optical Module

TL;DR

The Wavelength-shifting Optical Module (WOM) enhances single-photon detection in large detectors by decoupling the photosensitive area from the photomultiplier tube, significantly improving the signal-to-noise ratio especially in UV detection.

Contribution

This paper introduces the WOM concept, combining wavelength-shifting coatings with optimized geometry to improve sensitivity and noise performance in photon detection systems.

Findings

WOM achieves up to 8.9 times better signal-to-noise ratio in air.

Prototypes demonstrate effective wavelength-dependent photon collection.

The design is optimized for Cherenkov and scintillation detectors.

Abstract

The Wavelength-shifting Optical Module (WOM) is a novel photosensor concept for the instrumentation of large detector volumes with single-photon sensitivity. The key objective is to improve the signal-to-noise ratio which is achieved by decoupling the photosensitive area of a sensor from the cathode area of its photomultiplier tube (PMT). The WOM consists of a transparent tube with two PMTs attached to its ends. The tube is coated with wavelength-shifting paint absorbing ultra-violet photons with nearly $100\,\%$ efficiency. Depending on the environment, e.g. air (ice), up to $73\,\%$ $(41\,\%)$ of the subsequently emitted optical photons can be captured by total internal reflection and propagate towards the PMTs where they are recorded. The optical properties of the paint, the geometry of the tube and the coupling of the tube to the PMTs have been optimized for maximal sensitivity based on theoretical derivations and experimental evaluations. Prototypes were built to demonstrate the technique and to develop a reproducible construction process. Important measurable characteristics of the WOM are the wavelength dependent effective area, the transit time spread of detected photons and the signal-to-noise ratio. The WOM outperforms bare PMTs especially with respect to the low signal-to-noise ratio with an increase of a factor up to 8.9 in air (5.2 in ice). Since the gain in sensitivity is mostly in the UV-regime, the WOM is an ideal sensor for Cherenkov and scintillation detectors.