Validation of the VUV-reflective coating for next-generation liquid xenon detectors

• Published in: Journal of instrumentation 2024-06, Vol.19 (6), p.P06008
• Main Authors: Bajpai, D., Best, A., Ostrovskiy, I., Poitras, D., Wang, W.
• Format: Article
• Language: English
• Subjects: Dark Matter detectors (WIMPs, axions, etc.); Detectors; Double-beta decay detectors; Incidence angle; Magnesium fluorides; Noble liquid detectors (scintillation, ionization, double-phase); Polishes; Polishing; Reflectance; Substrates; Thin films; Xenon
• ISSN: 1748-0221
• DOI: 10.1088/1748-0221/19/06/P06008

Coating detector materials with films highly reflective inthe vacuum ultraviolet region improves sensitivity of thenext-generation rare-event detectors that use liquid xenon. In thiswork, we investigate the MgF2-Al-MgF2 coating designed toachieve high reflectance at 175 nm, the mean wavelength of liquidxenon (LXe) scintillation. The coating was applied to an unpolished,passivated copper substrate mimicking a realistic detector componentof the proposed nEXO experiment, as well as to two unpassivatedsubstrates with “high” and “average” levels of polishing. Afterconfirming the composition and morphology of the thin-film coatingusing TEM and EDS, the samples underwent reflectance measurements inLXe and gaseous nitrogen (GN2). Measurements in LXe exposed thecoated samples to -100°C for several hours. No peeling ofthe coatings was observed after several thermal cycles. Polishing isfound to strongly correlate with the measured specular reflectance(Rspec). In particular, 5.8(5)% specular spikereflectance in LXe was measured for the realistic sample at20° of incidence, while the values for similar angles ofincidence on the high and average polish samples are 62.3(1.3)% and27.4(7)%, respectively. At large angles (66°–75°),the Rspec in LXe for the three samples increases to23(5)%, 80(8)%, and 84(18)%, respectively. The Rspec at around 45° was measured in both GN2and LXe for average polish sample and shows a reasonable agreement.Importantly, the total reflectance of the samples is comparable andestimated to be 92(8)%, 85(8)%, and 83(8)% in GN2 for therealistic, average, and high polish samples, respectively. This isconsidered satisfactory for the next-generation LXe experiments thatcould benefit from using reflective films, such as nEXO and DARWIN,thus validating the design of the coating.