Abstract
Liquid argon-based scintillation detectors are important for dark matter searches and neutrino physics. Argon scintillation light is in the vacuum ultraviolet region, making it hard to be detected by conventional means. Polyethylene naphthalate (PEN), an optically transparent thermoplastic polyester commercially available as large area sheets or rolls, is proposed as an alternative wavelength shifter to the commonly-used tetraphenyl butadiene (TPB). By combining the existing literature data and spectrometer measurements relative to TPB, we conclude that the fluorescence yield and timing of both materials may be very close. The evidence collected suggests that PEN is a suitable replacement for TPB in liquid argon neutrino detectors, and is also a promising candidate for dark matter detectors. Advantages of PEN are discussed in the context of scaling-up existing technologies to the next generation of very large ktonne-scale detectors. Its simplicity has a potential to facilitate such scale-ups, revolutionizing the field.
Highlights
Due to the lack of efficient photosensors capable of directly registering the vacuum ultraviolet (VUV) light from liquid argon (LAr) scintillation peaked at 128 nm [7], wavelength shifter (WLS) coatings, covering inner surfaces of the cryogenic volume, are used to convert VUV photons to visible. 1,1,4,4tetraphenyl-1,3-butadiene (TPB) has been successfully used for that purpose for the past few decades [8–10]
The R-mode uncertainty is enlarged by the reflectance of the smooth substrate at 8◦ to account for any reflected incident excitation beam contaminating the fluorescence spectrum, while the T-mode uncertainty is inflated by the substrate transmittance
For the case of standalone PEN (1) a 40% solid angle correction is applied to T-mode data to account for the absence of the substrate, and (2) the R-mode systematic uncertainty is increased by the 0◦ and 8◦ data difference which is substantial, indicating sensitivity to the specularly reflected incident light
Summary
Due to the lack of efficient photosensors capable of directly registering the vacuum ultraviolet (VUV) light from LAr scintillation peaked at 128 nm [7], wavelength shifter (WLS) coatings, covering inner surfaces of the cryogenic volume, are used to convert VUV photons to visible. 1,1,4,4tetraphenyl-1,3-butadiene (TPB) has been successfully used for that purpose for the past few decades [8–10]. TPB can be evaporatively coated, which requires a dedicated process control system and high vacuum conditions [11], or can be dissolved in a polymeric solution and applied as paint, which leads to a degradation in conversion efficiency by a factor >2–3 [12,13] due to VUV absorption in the polymer. These large-scale endeavors call for a simple, inexpensive, and robust WLS solution scalable to surfaces ranging in size between a few hundred and a few thousand m2. We show that poly(ethylene 2,6-naphthalate) (PEN) is well suited for this purpose
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