In-situ measurement of the scintillation light attenuation in liquid argon in the Gerda experiment

Abstract

The Gerda experiment searches for the neutrinoless double beta (0νββ) decay in 76Ge 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, which peaks at 128 nm, 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 stainless steel housing with a photomultiplier tube (PMT) at one side and a moveable 90Sr 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. The analysis results in an absorption length of 15.8±0.7(stat)−3.2+1.5(syst)cm under the assumption of a scattering length of 70 cm at 128 nm. The obtained value of the absorption length is specific for the LAr in GERDA at the time of the measurement.