New optical filter for the ARTEMIS experiment

The properties of a new liquid filter for the ARTEMIS experiment are reported. ARTEMIS, which is run at the Whipple Gamma-Ray/Cosmic-Ray Telescope, aims to measure the negatively charged component of hadronic cosmic rays (presumably antiprotons) in the 1 TeV region. The Earth-Moon system is used as a spectrometer to perform a magnetic analysis of cosmic rays passing close to the Moon. The Moon acts as an aperture stop, resulting in a deficit of both protons and antiprotons. Air showers are detected using Cherenkov light emitted in the ultraviolet (UV) range in order to benefit from the large moonlight absorption through ozone in the upper atmosphere. The liquid UV filter needed to complement the ozone effect has been added to with a recently discovered dye. We report here on observations made using this dye. A background reduction to only 3% of its value without the dye has been achieved while maintaining the signal amplitude to at least 75% of its level. The reduced sensitivity to moonlight permits a lowering of the trigger threshold which, in turn, increases the air shower registration efficiency by a factor of three.