In-situ measurement of the scintillation light attenuation in liquid argon in the GERDA Experiment

The GERDA experiment searches for the neutrinoless double beta ($0\nu\beta\beta$) decay in $^{76}$Ge in order to probe whether the neutrino is a Majorana particle and to shed light on the neutrino mass ordering. For investigating such a rare decay it is necessary to minimize the background of the experiment. In Phase II of the GERDA experiment the scintillation light of liquid argon (LAr) is used as an additional background veto. In order to estimate the efficiency of such a LAr veto it has to be known how far the scintillation light can travel within the LAr. A dedicated setup was built to measure the attenuation length of the scintillation light in the LAr in-situ within the cryostat of GERDA. The setup is composed of a steel tube with a photomultiplier tube (PMT) at one side and a moveable $^{90}$Sr source at the other side to measure the light intensity at different distances between source and PMT. Furthermore, a sophisticated simulation was developed in order to determine the solid angle correction as well as the background for this measurement, both are needed for the analysis. Additionally, a set of simulation data was generated to confirm the analysis which is performed for the measured data afterwards and to find a combination of simulation parameters that matches the measured data. The analysis results in an absorption length of $15.79 \pm 0.70$ (stat) $^{+1.42}_{-3.14}$ (syst) cm under the assumption of a scattering length of 70 cm at 128 nm. The simulation matching best was produced with an absorption length of 18 cm and a light yield of 2500 $\gamma$/MeV.