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For the determination of the absolute scintillation yields -the number of scintillation photons per unit absorbed energy-for a variety of particles in liquid argon, a series of simultaneous ionization and scintillation measurements were performed. The results verified that scintillation yields for relativistic heavy particles from Ne to La are constant despite their extensive range of linear energy transfer. Such a constant level, called ''flat top response'' level, manifests the maximum absolute scintillation yield in liquid argon. The maximum absolute scintillation yield is defined by the average energy to produce a single photon, W ph ðmaxÞ ¼ 19:5 AE 1:0 eV. In liquid xenon, the existence of the same flat top response level was also found by conducting scintillation measurements on relativistic heavy particles. The W ph (max) in liquid xenon was evaluated to be 13:8 AE 0:9 eV using the W ph for 1 MeV electrons, obtained experimentally. The ratio between the two maximum scintillation yields at the flat top response level obtained in liquid argon and xenon is in good agreement with the estimation by way of the energy resolutions of scintillation due to alpha particles in both liquids.

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The scintillation yields and decay shapes for recoil Xe ions produced by WIMPs in liquid xenon have been examined. A quenching model based on a biexcitonic diffusion-reaction mechanism is proposed for electronic quenching. The total predicted quenching, nuclear and electronic, is compared with experimental results reported for nuclear recoils from neutrons. Model calculations give the average energy to produce a vuv photon, W ph , to be ~75 eV for 60 keV recoil Xe ions. Some aspects of ionization relating to liquid xenon WIMP detectors are also discussed.

Measurements were made of the average energy 8' per ion pair formed in liquid xenon by internal-conversion electrons from~Bi. We observed voltage pulses resulting from electron collection either in liquid xenon or in a gaseous mixture of argon (95%I and methane (5%). The relative pulse heights for the two material. s determine the ratio of the W values. Using the known W for the gaseous mixture, we obtained a liquid-xenon 8' of 15.6+ 0.3 eV. This value is considerably smaller than the gas-phase values, 21.5 or 21.9 eV. For interpretation, we adapted Platzman's energy-balance equation to liquids, assuming a conduction-band picture. Theoretical values thus calculated agree well with experiment.