  As a prototype detector for the SHiP Surrounding Background Tagger (SBT), we
constructed a cell (120 cm x 80 cm x 25 cm) made from corten steel that is
filled with liquid scintillator (LS) composed of linear alkylbenzene (LAB) and
2,5-diphenyloxazole (PPO). The detector is equipped with two
Wavelength-shifting Optical Modules (WOMs) for light collection of the primary
scintillation photons. Each WOM consists of an acrylic tube that is dip-coated
with a wavelength-shifting layer on its surface. Via internal total reflection,
the secondary photons emitted by the molecules of the wavelength shifter are
guided to a ring-shaped array of 40 silicon photomultipliers (SiPMs) coupled to
the WOM for light detection. The granularity of these SiPM arrays provides an
innovative method to gain spatial information on the particle crossing point.
Several improvements in the detector design significantly increased the light
yield with respect to earlier proof-of-principle detectors. We report on the
performance of this prototype detector during an exposure to high-energy
positrons at the DESY II test beam facility by measuring the collected
integrated yield and the signal time-of-arrival in each of the SiPM arrays. The
resulting detection efficiency and reconstructed energy deposition of the
incident positrons are presented, as well as the spatial and time resolution of
the detector. These results are then compared to Monte Carlo simulations.
