The Wavelength-Shifting Optical Module

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.

Wavelength-shifter coated polystyrene as an easy-to-build and low-cost plastic scintillator detector

“…To produce the modified dye, 22% of the toluene in the standard dye was evaporated. This leads to an increase in the viscosity of the dye, which in turn increases the thickness of a dip-coated dye layer [1].…”

“…The dip-coating procedure was optimized in dedicated studies to maximize the absorption probability of PMMA WOM tubes developed for the SHiP Surround Background Tagger [12]. With respect to the standard WLS dye, 22% of the toluene from the WLS-2 dye was partially evaporated before dip-coating to enhance the dye viscosity, since this helps in producing a larger WLS layer thickness [1]. Coating with the standard paint results in WLS layers of about only 5 µm thickness, as measured on dip-coated glass slides using the stylus profiler DektakXT from Bruker Corporation.…”

“…For several years, wavelength-shifting optical modules (WOMs) are being considered as possible photon detectors with large area and low noise and costs [1,2]. They have been originally proposed as water-Cherenkov photon detectors in the context of the Upgrades of the IceCube neutrino telescope [2].…”

“…A standard WOM is a large-area transparent tube, typically made of PMMA or quartz glass, coated from outside with a wavelength-shifting (WLS) dye, e.g. using a dip-coating technique [1,2]. Typically, wavelength-shifting in this context refers to absorption of ultraviolet (UV) photons by the WLS dye followed by re-emission of photons in the blue range, although the principle can of course be applied as well to absorption and re-emission in other wavelength ranges.…”

First measurement of the surface tension of a liquid scintillator based on Linear Alkylbenzene (HYBLENE 113)

“…In the proposed SHiP experiment [1,2] at the CERN SPS, the vacuum decay vessel has to be surrounded by a detector to detect muons entering the decay vessel from the beam-dump or from inelastic scattering events induced by neutrinos or muons in the decay vessel walls or its vicinity. The baseline detector concept for this surround background tagger (SBT) is based on cells filled with liquid scintillator, hermetically enclosing the lateral sides of the decay vessel which are readout by wavelength shifting optical modules (WOMs) [4] equipped with silicon photo multipliers (SiPMs).…”