Liquid argon imaging: a novel detection technology

Ionisation electrons may drift over large distances (meters) in a volume of highly purified liquid argon (O₂ equivalent less than 0.1 ppb!) under the action of an electric field. With an appropriate readout system (i.e. a set of fine pitch wire grids) we have realised a massive, continuously sensitive 'bubble chamber' with multiple readouts of the same, small charge (a minimum ionising track segment, 2 mm long, yields &#149; 10000 electrons). We have developed this technology since 1987, initially with small laboratory devices and later with progressively larger and more sophisticated detectors, the latest being the T600 module (740 ton of liquid Argon), which has been operated in Pavia, as a step toward the ICARUS programme in the Gran Sasso Laboratory (LNGS). With cloning of T600 we aim at a 3000 ton detector by 2005. Argon is a medium with density 1.4 g/cm³, similar in characteristics to the heavy freon used in the famous Gargamelle. With wire pitches of 2-3 mm, it provides an extremely high spatial resolution, with a 'bubble' radius of a little over 1 mm. It is continuously sensitive, self-triggering by its own prompt scintillation light, and it provides an excellent measurement of the local energy deposition, relevant for electron/gamma separation and particle identification by means of dE/dx vs. range measurement. Energies of complex events can be reconstructed by charge integration. Unlike a bubble chamber, it is also an excellent calorimeter: <font face="Symbol">s</font>(E)/E = 3%/&#149;E(GeV) for em showers, <font face="Symbol">s</font>(E)/E = 17%/&#149;E(GeV) + 2% for hadronic cascades and <font face="Symbol">s</font>(E)/E = 7%/&#149;E(MeV) for electrons/<font face="Symbol">g</font>'s below transition energy. Multi-GeV muon momenta can be measured by multiple scattering. Possible applications of this technology beyond ICARUS are also part of our R&D programme. In particular we have demonstrated the possibility of extracting ionisation electrons from liquid to gas, where they can be multiplied, in view of a WIMP search in the LNGS, called WARP. Scintillation of slow Ar recoils (&#149; 50 keV) from WIMPs will be distinguished from backgrounds by the absence of ionisation (<font face="Symbol">b</font> < Ar atomic electrons).