Production and installation of wavelength-shifting reflective light enhancers for the Short-Baseline Near Detector

Technical Summary: Production and Installation of Wavelength-Shifting Reflective Light Enhancers for the Short-Baseline Near Detector

Problem and Motivation
Liquid argon time projection chambers (LArTPCs) rely on scintillation light for fast timing, triggering, and calorimetry. However, liquid argon emits scintillation light in the vacuum ultraviolet (VUV) range at 128 nm, a wavelength to which conventional photosensors (such as photomultiplier tubes and silicon photomultipliers) are largely insensitive due to the opacity of standard materials like glass. While wavelength-shifting (WLS) coatings on sensors recover some of this light, the photon detection systems (PDS) in existing LArTPCs are limited by their physical coverage of the detector volume. This limitation restricts light collection efficiency, particularly for calorimetric applications and low-energy interaction studies. The Short-Baseline Near Detector (SBND), part of Fermilab’s Short-Baseline Neutrino Program, sought to enhance light collection and uniformity by implementing a passive, large-area wavelength-shifting reflective system on its central cathode.

Methodology
The SBND collaboration designed, produced, and installed 64 double-sided reflective plates to cover the cathode surface. The methodology involved three primary phases:

1. Plate Fabrication and Lamination:

   • Substrates consisted of 0.8 mm thick FR4 fiberglass plates, trimmed to 58.6 cm × 46.6 cm.
   • Plates were laminated on both sides with 3M DF2000MA, a non-conductive polymeric film exhibiting >99% specular reflection in the visible range. This process was performed at the Illinois Institute of Technology.

2. Wavelength Shifter Evaporation:

   • Tetraphenyl butadiene (TPB) was selected as the WLS material due to its high efficiency in converting VUV light to the visible blue spectrum (~425 nm), matching the peak quantum efficiency of SBND’s sensors.
   • TPB was deposited via low-temperature physical vapor deposition (PVD) in high-vacuum chambers at the University of Manchester and Universidade Estadual de Campinas (UNICAMP).
   • Plates were mounted on rotating discs to ensure uniform deposition. The target coating thickness was ~300 μg/cm² (approximately 3.0 μm).
   • Calibration was performed using small sample substrates weighed before and after evaporation, alongside profilometry measurements on glass substrates, confirming uniform coverage across >90% of the plate area.

3. Assembly and Installation:

   • To maintain electrical field uniformity and prevent high-voltage disruption, the non-conductive TPB-coated plates were sandwiched between two layers of thin stainless steel mesh.
   • These "mesh-plate-mesh" assemblies were tensioned onto sub-frames using a cryofitting technique involving liquid nitrogen cooling.
   • The assemblies were installed in situ onto the cathode frame within a controlled environment. Specialized stretching tools were used to align the assemblies with the cathode’s threaded holes.

4. Light Protection:

   • Recognizing that TPB degrades upon prolonged exposure to ambient blue/UV light, strict handling protocols were implemented. Plates were stored in nitrogen-filled, anti-static bags and kept in dark environments.
   • During installation, the detector area was enclosed in a clean tent featuring a dual-layer filter: an inner UV-filtering plastic and an outer blue-light-filtering plastic (50 μm thick) that blocked wavelengths below 450 nm while maintaining sufficient visibility for workers.

Key Contributions

• Scale: The project represents the largest area of TPB-coated reflective surface deployed in a neutrino detector to date.
• Integration: The successful integration of passive reflective plates into an existing LArTPC design without compromising the high-voltage cathode structure or electric field uniformity.
• Production Protocol: The establishment of a reproducible production pipeline for large-area WLS reflectors, including validated evaporation techniques, calibration methods, and protective handling procedures to mitigate TPB degradation.

Results

• Production Yield: A total of 81 plates were produced across three evaporation campaigns. 68 plates met quality standards for installation (covering the required 64 positions with spares), and 13 were designated as spares.
• Coating Quality: Calibration studies confirmed that the TPB coating density was uniform across >90% of the plate area, with an average surface density of ~325 μg/cm², consistent with the target thickness.
• Operational Performance: Early light-collection measurements indicate high uniformity and light response across the detector. Both uncoated PMTs and uncoated X-ARAPUCAs have successfully detected signals originating from the reflective cathode surfaces, confirming the system’s ability to recover VUV photons and re-emit them as visible light.

Significance
The paper claims that this system significantly increases the effective photon-sensitive area of the SBND PDS from ~15% to over 30% of the inner detector surface. This enhancement supports improved triggering, calorimetry, and position reconstruction in the drift direction. The work provides a validated framework for scaling evaporative TPB coatings to larger surface areas, offering critical lessons for future liquid noble element detectors, such as the far-detector modules of the Deep Underground Neutrino Experiment (DUNE).