Doping Liquid Argon (LAr) with Xenon is a well known technique to shift the light emitted by Argon (128 nm) to a longer wavelength to ease its detection. The largest Xenon doping test ever performed in a LArTPC was carried out in ProtoDUNE Single Phase (ProtoDUNE-SP) at the CERN Neutrino Platform. From February to May 2020, a gradually increasing amount of Xenon was injected to compensate for the light loss due to air contamination. The response of such a large TPC (770 t of Liquid Argon and 440 t of fiducial mass) has been studied using the ProtoDUNE Photon Detection System (PDS) and a dedicated setup installed before the run.
With the first, it was possible to study the total light detected in the system as a function of the xenon concentration and to characterise the light collection efficiency with respect to the track position. With the second system it was possible to disentagle the LAr (128 nm) light from the Xenon (178 nm) light using two dedicated X-Arapuca modules. The run was fully satisfactory, it was possible to measure directly the increase of the Xenon light component during doping; furthermore most of the LAr light quenched by impurities was fully recovered even at small Xenon concentration (< 20 ppm in mass), which implies an efficient energy transfer between LAr and Xe.
Xenon distribution was uniform in space and stable in time, not affecting the charge collection by the TPC. A study of the collected scintillation light as a function of the track position, performed on a sample of horizontal muons, led to the estimation of an increased Rayleigh scattering length, that improves the detector response uniformity.