  In the Deep Underground Neutrino Experiment (DUNE), the VUV LAr luminescence
is collected by light trap devices named X-Arapuca, sizing (480x93) mm2. Six
thousand of these units will be deployed in the first DUNE ten kiloton far
detector module. In this work we present the first characterization of the
photon detection efficiency of an X-Arapuca device sizing (200x75) mm2 via a
complete and accurate set of measurements along the cell longitudinal axis with
a movable 241-Am source. The MPPCs photosensors are readout by a cryogenic
transimpedance amplifier to enhance the single photoelectron sensitivity and
improve the signal-to-noise while ganging 8 MPPC for a total surface of 288
mm2. Moreover we developed a new photon downshifting polymeric material, by
which the X-Arapuca photon detection efficiency was enhanced of about +50% with
respect to the baseline off-shell product deployed in the standard device
configuration. The achieved results are compared to previous measurements on a
half size X-Arapuca device, with a fixed source facing the center, with no cold
amplification stage, and discussed in view of the DUNE full size optical cell
construction for both the horizontal and the vertical drift configurations of
the DUNE TPC design and in view of liquid Argon doping by ppms of Xe. Other
particle physics projects adopting Liquid Argon as target or active veto, as
Dark Side and LEGEND or the DUNE Near Detector will take advantage of this
novel wavelength shifting material.
