Light yield measurements of GEM avalanches at cryogenic temperatures and high densities in neon based gas mixtures.

Gas Electron Multipliers (GEM) are being used in the baseline readout design of a cryogenic Time Projection Chamber (TPC). The GEMs provide amplification of an ionization charge signal but rather than reading out the charge signal, the proportional light produced in the GEM avalanche will be imaged using commercially available low-noise CCD cameras. Our measurements of neon-based gas mixtures at cryogenic temperatures indicate that visible light yields of one photon per avalanche electron are available. Light yields in neon based gas mixtures will be presented as well as spectral measurements of the light emission. We have been successful in imaging alpha tracks at 77 K and are actively working on imaging beta tracks at 45 K and below.