  Doping of liquid argon TPCs (LArTPCs) with a small concentration of xenon is
a technique for light-shifting and facilitates the detection of the liquid
argon scintillation light. In this paper, we present the results of the first
doping test ever performed in a kiloton-scale LArTPC. From February to May
2020, we carried out this special run in the single-phase DUNE Far Detector
prototype (ProtoDUNE-SP) at CERN, featuring 720 t of total liquid argon mass
with 410 t of fiducial mass. A 5.4 ppm nitrogen contamination was present
during the xenon doping campaign. The goal of the run was to measure the light
and charge response of the detector to the addition of xenon, up to a
concentration of 18.8 ppm. The main purpose was to test the possibility for
reduction of non-uniformities in light collection, caused by deployment of
photon detectors only within the anode planes. Light collection was analysed as
a function of the xenon concentration, by using the pre-existing photon
detection system (PDS) of ProtoDUNE-SP and an additional smaller set-up
installed specifically for this run. In this paper we first summarize our
current understanding of the argon-xenon energy transfer process and the impact
of the presence of nitrogen in argon with and without xenon dopant. We then
describe the key elements of ProtoDUNE-SP and the injection method deployed.
Two dedicated photon detectors were able to collect the light produced by xenon
and the total light. The ratio of these components was measured to be about
0.65 as 18.8 ppm of xenon were injected. We performed studies of the collection
efficiency as a function of the distance between tracks and light detectors,
demonstrating enhanced uniformity of response for the anode-mounted PDS. We
also show that xenon doping can substantially recover light losses due to
contamination of the liquid argon by nitrogen.
