Laboratory Measurement of the X-ARAPUCA's Absolute Photon Detection Efficiency for the Deep Underground Neutrino Experiment's Vertical Drift Far Detector

TL;DR

This work reports the absolute PDE of the DUNE X-ARAPUCA photon detection device in cryogenic liquid argon, using two independent methods (reference and Geant4-based simulation) to cross-validate results across multiple XA configurations. The baseline single-/double-sided design achieves PDE values around $PDE obreak{=} obreak 3.1 ext{--}3.7 ext{ eplaced} ext{%}$ at $OV=4.5 ext{ V}$, with the highest measured value reaching $PDE = 4.5(4) ext{ eplaced} obreak{ ext{ ext{}}} imes 10^{-0} obreak{% obreak}$ at the same overvoltage; crucial corrections for bending and LAr purity are applied. A key result is that removing dichroic filters can improve PDE by up to about $18 ext{--}11 ext{ eplaced} ext{%}$ depending on the configuration, guiding design choices toward DF-free XA implementations. The study demonstrates robust, cross-validated PDE extraction strategies and establishes a practical path for optimizing the XA-based PDS in DUNE’s Vertical Drift module, with implications for light yield and event reconstruction performance.

Abstract

The Deep Underground Neutrino Experiment (DUNE) will probe fundamental questions in particle physics and cosmology. Its Far Detectors implement a Photon Detection System composed of light-sensitive devices called X-ARAPUCA (XA). These trap incoming VUV photons by total internal reflection in a Wavelength Shifter Light Guide to be collected onto wrapping Silicon Photomultiplier arrays, sensitive to visible light. In the baseline design, Dichroic Filters (DFs) are implemented to improve the collection efficiency of escaping photons. The configuration proposed for DUNE's Vertical Drift module has been characterised in Liquid Argon for the first time using dedicated cryogenic setups developed at CIEMAT and INFN Naples. Additionally, several alternative configurations, based on the design optimisation studies of an R&D campaign, have been evaluated. The results show an efficiency of up to 4.5 $\pm$ 0.4% at 4.5 V overvoltage, representing a high improvement with respect to previous XA implementations. Most noticeably, configurations without DFs show an improvement of up to 18%, which has been attributed to transmittance deficits.

Figures (33)

• Figure 1: Design for DUNE's VD FD module. Showing cathode XAs in grey and membrane XAs in green. The anode planes are shown in brown. The TPC is surrounded by a field cage, which is made semi--transparent for visual purposes.
• Figure 2: Schematic of the XA working principle TDR4_2002.031010v1. Not to scale.
• Figure 3: Scheme of the XA concept implementation for DUNE's VD FD module. Displayed are the substrate and WLS--LG layers, where the first can be seen surrounded by the XA mounting frame and the latter by the SiPM strips and spring--loaded mechanism.
• Figure 4: Schematic representation of the VD XA spring--loaded mechanism. SiPMs are mounted between the WLS--LG and an acrylic interface strip (blue arrow). Springs (red arrow) apply constant pressure orthogonal to this interface. Thermal shrinkage at cryogenic temperatures of the WLS--LG is 6mm, which causes the springs to partially decompress to 11.5mm. These springs are mechanically constrained to exert force and translate solely orthogonal to the WLS--LG face. This design enables SiPMs to translate in unison with WLS--LG shrinkage toward the centerline.
• Figure 5: DF transmission curves from ZAOT measured in demineralised water for different angles of incidence.