Wavelength Shifters for Applications in Liquid Argon Detectors

Abstract

Wavelength shifters and their applications for liquid argon detectors have been a subject of extensive R&D procedures over the past decade. This work reviews the most recent results in this field. We compare the optical properties and usage details together with the associated challenges for various wavelength shifting solutions. We discuss the current status and potential future R&D directions for the main classes of wavelength shifters.

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

“…In one chamber, the ESR is coated with a 3 µm layer of vacuum-evaporated TPB. The other chamber instead contains a 25 µm PEN film (Teonex Q51) 1 attached in front of the ESR with up to 1 mm of LAr gap between both films, maintained by spacer rings at the endcaps. Both chambers are fully immersed in a LAr bath and are held with a frame attached to threaded rods suspended from the lid of the cryostat, as shown in Fig.…”

“…LAr acts as an efficient scintillator, however its emission peak is in the vacuum ultraviolet (VUV) at 128 nm. Due to lack of photo-sensors highly efficient at VUV wavelengths, the scintillation light is typically wavelength shifted (WLS) to the visible range by dedicated films or coatings applied on the detector walls [1].…”

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

“…In TPCs, the VUV component of S1 can be collected from the full solid angle by walls covered with an efficient reflector. For xenon scintillation, PTFE provides sufficient reflectivity, while in case of argon a layer of WLS on the reflectors is additionally required (typically TPB-coated ESR or PTFE are used [21]). S1 light can therefore reach the amplification structure directly as VUV, or after multiple reflections from the walls, either as VUV or visible photons, if the WLS coating on the walls is present.…”

“…Optical properties were taken from the literature, including: PEN and PMMA attenuation lengths of 1-5 cm [32] and ∼ 3 m [36], respectively, PEN emission spectrum [34], refractive indices of PEN and PMMA, and ESR reflectivity of 0.98. 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].…”

“…Another challenge in collection and detection of S1 and S2 light is its dominant vacuum ultraviolet (VUV) wavelength, 128 nm for LAr and 172 nm (178 nm) for GXe (LXe), strongly absorbed in most materials and in the region where the PDE of available sensors is reduced with respect to visible light. While direct VUV detection yields are satisfactory in the case of LXe, wavelength shifter (WLS) coatings on photosensors and detector walls are necessary to shift the VUV to visible light in case of LAr [21].…”