C. Eckardt

The baseline technology for the Surrounding Background Tagger of the recently approved SHiP experiment relies on liquid scintillator composed of linear alkylbenzene and 2,5-diphenyloxazole as active detector material. The primary scintillation photons are collected by Wavelength-shifting Optical Modules, and the secondary photons are guided by total reflection to an array of 40 silicon photomultipliers. Here we present a direct comparison of the performance of three detector prototype cells constructed from different alternative materials known to provide good chemical compatibility with the liquid scintillator: aluminium and stainless steel. To increase the internal reflectivity, one of the two aluminium prototypes was polished on the inside, while the inner walls of the stainless steel cell were clad with sheets of polytetrafluoroethylene. Using 5 GeV muons from the CERN PS T10 beamline, we studied detected light yield and time resolution attained by the three prototypes. For both the polished AlMg4.5 cell and the PTFE-clad stainless steel prototype, the achieved detected light yield and time resolution meet the requirements of the SHiP Surrounding Background Tagger. Concerning another crucial parameter, the uniformity of the detector response across the detector cell, the polished AlMg4.5 cell shows the best performance among the tested prototypes. These results will significantly affect the final design of the SHiP Surrounding Background Tagger.

As a prototype detector for the SHiP Surrounding Background Tagger (SBT), we constructed a cell (120 cm × 80 cm × 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.