Wavelength-Shifting Performance of Polyethylene Naphthalate Films in a Liquid Argon Environment

Abstract

Liquid argon is commonly used as a detector medium for neutrino physics and dark matter experiments in part due to its copious scintillation light production in response to its excitation and ionization by charged particle interactions. As argon scintillation appears in the vacuum ultraviolet (VUV) regime and is difficult to detect, wavelength-shifting materials are typically used to convert VUV light to visible wavelengths more easily detectable by conventional means. In this work, we examine the wavelength-shifting and optical properties of poly(ethylene naphthalate) (PEN), a recently proposed alternative to tetraphenyl butadiene (TPB), the most widelyused wavelength-shifter in argon-based experiments. In a custom cryostat system with welldemonstrated geometric and response stability, we use 128 nm argon scintillation light to examine various PEN-including reflective samples' light-producing capabilities, and study the stability of PEN when immersed in liquid argon. The best-performing PEN-including test reflector was found to produce 34% as much visible light as a TPB-including reference sample, with widely varying levels of light production between different PEN-including test reflectors. Plausible origins for these variations, including differences in optical properties and molecular orientation, are then identified using additional measurements. Unlike TPB-coated samples, PEN-coated samples did not produce long-timescale light collection increases associated with solvation or suspension of wavelength-shifting material in bulk liquid argon.

Scintillation light detection in the 6-m drift length ProtoDUNE Dual Phase liquid argon TPC

“…TPB produces three times more visible photons than PEN, for the same amount of incident VUV photons. This agrees with the value of 0.34 ± 0.01 reported in [29] for the same PEN sample.…”

Direct comparison of PEN and TPB wavelength shifters in a liquid argon detector

“…The resulting value for WLSE of PEN relative to TPB for the preselected 25 µm thick Teonex Q51 foil is 47.2±5.7%, where the uncertainty is dominated by the systematic error. This is a significantly higher result than the previously projected/measured results for other PEN grades, particularly 34.0 ± 1.1 % for 125 µm thick Teonex Q53 [22] or 38.0 ± 7 % for 125 µm thick Teonex Q83 [3].…”

Development of very-thick transparent GEMs with wavelength-shifting capability for noble element TPCs

“…1. In particular, since injection moulded PEN [31] has at the moment a reduced WLS efficiency (WLSE) compared to PEN foils, an additional commercial layer was applied to the surface, aimed at higher S1 sensitivity. PMMA-based structures were prepared as well, for comparison.…”

“…The literature results for PEN WLSE, are typically relative to TPB, which itself has a debated absolute yield [21]. For the sake of this discussion we treat the published PEN WLS efficiencies as if they were absolute: 9.1% for the PEN tile and 34% for Teonex Q51 film, as recently measured for a similar grade in a LAr environment [31]. Assumed WLSE are constant in the entire VUV range, which again is debated [37,38] and makes the results nominally valid for both Ar and Xe and easy to scale to a more realistic WLSE in presence of new data.…”