Liquid xenon ionization and scintillation studies for a totally active-vector electromagnetic calorimeter

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In these experiments MeV's to GeV's energies were deposited in a liquid xenon test cell by an electron accelerator with kinetic energy ≤100 keV, intensity ≤10 6 e and pulse width ≤30 ns FWHM. Purification of the liquid xenon is effected by continuous circulation of boil-off gas through Oxisorb. This technique will allow operation of a liquid xenon calorimeter in a high radiation ..environment because the liquid can be continuously purified and replenished. An ionization signal was detected by collecting drift electrons onto an anode mesh, a scintillation signal by collecting photoelectons in a silicon photodiode immersed in the liquid. The energy to create an ionization electron was measured to be W = 9.76±0.70 eV. The corresponding energy to produce a 175 nm scintillation photon was found to be W s= 14.2 eV. The scintillation signal is observed to be fast ( σ t≤14 ns). Anti-correlation of the ionization and scintillation signals was also seen. Intrinsic energy resolution σ E/ E ≈ 0.07%/√( E/GeV) was determined from ionization and σ E/ E ≈ 0.2%/√( E/GeV) from scintillation. Doping of xenon with up to 2% methane did not adversely affect the ionization/scintillation yields or resolutions but increased the drift velocity hence, the current signal by about 75%. The intrinsic energy resolution of a totally active xenon scintillation calorimeter will be limited by uniformity of light collection and the dead material of the calorimeter to ( σ E/ E ≤ 0.5%). Detection of scintillation with a silicon (or CsI gas) photodiode permits a fast energy measurement which can be used in a first level trigger. The ionization signal will be sampled to determine the vector direction of the photon (or electron) which initiates the shower. Directional resolution σ θ = 5 mrad/√( E/GeV) achievable with this method will allow association of a detected photon to its true vertex point. At LHC ( ≈ 20 interactions per beam crossing) this calorimeter can give a Higgs mass resolution σ M/ M H ≈ 0.6% via the 2γ decay mode compared to 3% for calorimeters with equal energy resolution ( σ E/ E = 0.5%) but without this vertex capability. Sampling of ionization in the early part of the shower will allow discrimination against π 0 production and detect individual photons from π 0 decays thus enhancing discrimination against these important sources of hadronic background. The essential problem in measuring the shower profiles is the large amount of readout electronics needed. Cheap, current sensitive, digital VLSI electronics has already been developed and produced for the FAST RICH detector project. Studies to adapt this electronics for use in liquid xenon are underway.