Industrial Deposition of Wavelength-Shifting Films for Liquid Argon Photon Detection Systems

TL;DR

This work demonstrates an industrial physical vapor deposition process for p-terphenyl, a wavelength-shifting material, applied to large-area inorganic substrates to enable scalable photon detection in liquid-argon detectors. By adapting PVD techniques from OLED manufacturing and implementing substrate-cleaning, plasma activation, and in-situ ion bombardment, the authors achieve 1–2 μm thick coatings with sub-10% thickness nonuniformity across borosilicate glass, quartz, and sapphire substrates. Emission spectra align with canonical pTP behavior, showing a peak near 350 nm and strong reproducibility across substrates and production campaigns, while coatings survive cryogenic cycling without delamination. The process offers a clear, scalable path to mass-produce high-quality WLS coatings for the DUNE Phase-II Far Detector, with demonstrated throughput compatible with the order of ~2000 m$^2$ per year, and provides a foundation for broader industrial applications of organic scintillators in cryogenic detectors.

Abstract

The Deep Underground Neutrino Experiment (DUNE) Phase-II Far Detector is considering an unprecedentedly large-area ~2000 m$^2$ photon detection system to achieve a target mean light yield of 180 PE/MeV. Meeting this requirement demands scalable, cost-effective, and high-quality wavelength-shifter (WLS) coatings capable of converting 127 nm liquid-argon scintillation light into visible photons with high efficiency. We report on the successful realization of an industrial physical vapor deposition (PVD) process for \textit{p}-terphenyl (pTP) coatings, adapted from vacuum deposition techniques developed for OLED display manufacturing, to produce uniform WLS layers on large-area inorganic substrates, a task traditionally challenged by adhesion and uniformity issues at organic--inorganic interfaces. Surface characterization by profilometry and spectroscopic measurements confirms sub-10% thickness uniformity for 1-2 $μ$m films and emission spectra consistent with high-quality pTP reference samples. The industrial process demonstrates reproducibility, scalability, and significantly reduced production time compared to laboratory-based methods, while maintaining the optical quality required for large-scale liquid-argon time-projection chamber (LArTPC) photon detection systems. These results establish a viable pathway for mass production of high-performance pTP coatings for DUNE FD3 and future neutrino experiments.

Figures (8)

• Figure 1: (a) Working principle of an X-Arapuca cell, adapted from Ref. FALCONE2021164648. (b) Representative absorption and emission spectra of pTP. The blue curve shows the excitation light peaking at 266 nm, and the red curve shows the emission spectrum of pTP, reproduced from Ref. 4774796.
• Figure 2: Plasma-treatment system and substrate placement. Mesh shadowing during treatment produced observable thickness modulation in early trials.
• Figure 3: Uniform pTP film on a 143.75 mm $\times$ 143.75 mm B33 substrate.
• Figure 4: Throughput scaling with a rotating substrate dome.
• Figure 5: Monochromator setup used for emission-spectrum measurements.