A novel wavelength shifter foil for IceCube

The IceCube optical modules are composed of a photomultiplier tube encapsulated in a glass sphere. The encapsulating glass is transparent for photons with a wavelength greater than 330 nm. Therefore, the part of the Cherenkov spectrum below this cut-off does not contribute to the signal production even if the medium in which these photons are generated is transparent down to 250 nm at least. Thus, the collection efficiency is not yet as high as desirable. We investigated the use of a wavelength shifter layer placed outside the lower hemisphere of the optical modules. The development of a novel wavelength shifter (WLS) has been motivated by the limitations of commercial available materials. In this paper, we report measurements of the improvement in light collection efficiency obtained with a wavelength shifter working in air and in water.