Pushing the WOM technology for liquid scintillator detectors to the next level

About a decade ago, wavelength-shifting optical modules (WOMs) have been proposed as a new type of photon detectors for a large-volume extension of the IceCube detector. The idea was picked up right after for the Surrounding Background Tagger (SBT) of the recently approved Search for Hidden Particles (SHiP) experiment. The SBT is a large-area liquid-scintillator detector, for which for the first time wavelength-shifting optical modules (WOMs) are used as photon detectors. A WOM is made of a long tube coated with a wavelength-shifter (WLS) that shifts the absorbed UV
photons into the visible range. The secondary photons are transported by total reflection to the end of the WOM tube, where they are detected by a photon sensor. In the IceCube case, the photon sensor coupled to the WOM tube was supposed to be a photomultipler (PMT), while in the SHiP-SBT case it is an array consisting of 40 silicon photomultipliers (SiPMs). The use of WOMs with large liquid-scintillator detectors has been successfully demonstrated by our groups over the last years.
Based on this work, we plan to push the WOM-for-liquid-scintillator technology to the next level by targeting two directions:
1. We will explore a ”granular” WOM, by replacing the solid WOM tube by a circular array of smalldiameter WOM cylinders. This increased granularity has the potential to significantly increase the spatial resolution of any future large-area liquid scintillator detector using the WOM technology. However, since the area covered by the array of WOM cylinders will be in general smaller than the one of a WOM tube, an optimized reflector behind the WOM cylinders has to be developed to guarantee sufficient light-collection efficiency. A granular WOM might also help further improving the homogeneity of such a detector concerning the collected light yield response and particle arrival time across the detector.
2. By equipping the inner walls of a large-volume liquid-scintillator detector with small-diameter WOM cylinders equipped with large-area reflectors placed behind, we will explore a novel technology of separating Cherenkov light from scintillation light by wavelength filtering. Such separation is of interest for neutrino detection in liquid scintillator detectors. This will require using two different sets of WOM cylinders: one set of WOM cylinders coated with a WLS that is mainly sensitive to the scintillation light spectrum and one set of WOM cylinders that is mainly sensitive to the part of the Cherenkov spectrum not overlapping with the scintillator emission spectrum.