The SoLid antineutrino detector: construction and commissioning with cosmic ray muons

The SoLid experiment, located at the BR2 reactor at SCK.CEN, is designed to resolve the reactor antineutrino anomaly. This anomaly refers to a set of short-baseline reactor experiments, where, after a re-evaluation of the neutrino flux, the ratio of measured over predicted rate of antineutrinos deviates from unity at the 98.6 % C.L.. A result clearly in tension with the existing framework of three families of neutrinos oscillating into each other. Chapter 1 will provide more details on the existing neutrino framework and the anomalies. The approach used by SoLid consists of a novel technique to detect reactor anti-neutrinos via the inverse beta decay interactions (IBD), combining 555 cm3 PVT scintillator cubes equipped with 6LiF:ZnS(Ag) neutron sensitive screens. This technology is explained in dept in chapter 2. Chapter 3 discusses the SM1 prototype detector built at Ghent University in the summer of 2014. As a part of this thesis I was heavily involved in the construction and quality assurance of this detector. Despite a limited reactoron data set collected in February 2015, a sub-optimal electronics system and a relatively low light-yield, the prototype was able to show that neutrons can be distinguished from other particle interactions. The results of the SM1 prototype lead to several design changes for the full-scale Phase 1 detector. Since the SoLid experiment is located above ground, it will detect high rates of cosmic particles. These cosmic neutrons and muons can generate several backgrounds to the IBD signal and it is therefore very important to investigate these interactions. Chapter 4 provides information on a dedicated cosmic simulation package “CosmicGen”, developed as part of this thesis. A good agreement with SM1 data shows that the simulation chain is trustworthy and can thus provide valuable information on cosmic backgrounds. The full-scale Phase 1 detector was built from December 2016 until November 2017 at Ghent University. The construction protocols and quality assurance are discussed in chapter 5. The procurement of materials, training of iii iv manpower and the construction and quality control of the detector took up a large part of this thesis. In October 2017, 4 out of 5 Phase 1 modules were transported to the BR2 reactor. After a relatively short installation and commissioning period, the detector was able to collect reactor-on and -off data in December. This dataset was used as a burn sample for the proto-IBD analysis group. The goal of this group was to understand the basic properties of the detector by performing initial studies of the data quality, object reconstruction, backgrounds and to perform an initial IBD search. Chapter 6 discusses the reconstruction of the muon objects, performed within the scope of this thesis. These reconstructed muons were used as the basis of a muon veto in the IBD search of the proto-IBD group. Analysis of the muons could also indicate a stable behaviour of the detector. Coincidence studies of muons with Michel electrons and neutrons resulted, respectively, in a value of the muon decay time and of the neutron capture time on the Li screens. The results of the proto-IBD group were able to show a good understanding of the Phase 1 detector and provide a decent basis for further studies that will be performed within the SoLid Collaboration. It is expected that SoLid will be able to provide an explanation for the reactor antineutrino anomaly within a few years of data taking. Thereby either rejecting or confirming the existence of the sterile neutrino, one of the most enigmatic particles in physics.