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 that absorbs ultraviolet 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.
Highlights
A viable method for reading out detectors with transparent target media is the detection of optical photons emitted by incident particles in the form of Cherenkov emission or scintillation light
In order to find the optimal refractive index of the filling material for the prototype geometry, the cylindrical geometry of the Wavelength-shifting Optical Module (WOM) is modeled in an Monte Carlo (MC) simulation, propagating incident photons with Fresnel equations and Snell’s law in three dimensions
A novel photosensor concept is described in which photomultiplier tube (PMT) are complemented with a tube with wavelength-shifting coating in order to enhance the signal-to-noise ratio
Summary
Many detectors for rare events in particle and astroparticle physics require large interaction volumes, ranging up to cubic kilometers, in order to achieve a reasonable detection rate. If the emission angle of the photons is larger than the critical angle, they are trapped by total internal reflection and are guided along the tube to small, low-noise PMTs. The performance of the WOM is determined by a number of efficiency factors, which are discussed throughout this work. UV photons incident on the WLS tube are absorbed in the paint layer and re-emitted isotropically as optical photons with the light yield LWYLS. This efficiency depends on the wavelength of the incident photons λinc, as well as the thickness of the paint layer d and the concentration of wavelength-shifting molecules cWLS. Performance Factors The efficiencies of the WOM components, which make up the overall efficiency of the WOM, as introduced in Equation (1), are derived in this chapter
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