The ATLAS Muon Spectrometer 1 has ~355,000 drift tubes installed into 1200 precision Monitored Drift Tube (MDT) tracking chambers arrayed over a 22 m high, 45 m long barrel-shaped detector. The 2%-3% (100 GeV) momentum resolution and high efficiency of the spectrometer rests on maintaining 80 um single drift tube resolution. Significant performance degradation occurs for miscalibration of the maximum drift time of a few ns. The drift tube is the fundamental sensitive element in the spectrometer. It consists of a 3 cm diameter extruded aluminum tube, pressurized to 3 bar with 93%Ar, 7% CO 2 gas mixture. A 50  m diameter gold-plated tungsten wire stretched to 350 grams, operated at 3KV is coaxial to the tube. The operating gain is The spectrometer’s precision coordinate is the radial distance of closest approach of an ionizing particle passing through a drift tube. The spatial resolution depends critically on RT transfer function relating drift time to track impact parameter. We have constructed mini-MDT chamber employing 96 closed- packed drift tubes arrayed in two multi-layers of 3 closed-packed layers each. This chamber dedicated to online gas monitoring and calibrations, is staged at the ATLAS gas-mixing facility at CERN. It is configured with two independent gas distribution manifolds and enables simultaneous direct comparison from different input sources. It routinely samples the gas supply and return lines servicing the underground spectrometer. The chamber uses standard ATLAS muon spectrometer readout electronics and has an automated data acquisition and analysis program. Plastic scintillators trigger on cosmic ray muons at 20 Hz, two-fold coincidence. Drift times are measured and collected into drift spectra. Which are analyzed to extract the maximum drift time. The maximum drift time is sensitive to the gas composition as well as gas temperature and pressure. The gas temperature and pressure are measured by embedded sensors and collected drift times are corrected to standard conditions of 3000 mb and 20 C. After these corrections any changes to the drift spectra are attributable to changes in the gas composition. expressed as drift radii. Also from these drift times track are reconstructed and used to iteratively in autocalibration 1 process to extract an RT function. The primary task of the Monitor Chamber is to provide continuous recording of gas properties and to flag deviations sufficient to signal a re-determination of the RT calibration constants. A sensitive parameter to gas properties is the maximum drift time, T max. A 5 ns change in the maximum drift corresponds to ~75  m error in track drift radius- about equal to a drift tube’s intrinsic resolution. Left un-calibrated this change in T max degrades efficiency and momentum resolution by several percent 2. Drift times are affected not only by the gas composition, but also by the temperature and pressure in the monitor chamber. Corrections are computed & validated with data. ns mb C % ppm T max vs Pressure T max vs Temperature T max vs Water content T max vs % CO 2 ns CONFIGURATION Drift Time Calibration & Gas Monitoring of the ATLAS Muon Spectrometer Precision Chambers Daniel S. Levin, 1 Nir Amram 2, Meny ben Moshe 2, Erez Etzion 2, Tiesheng Dai 1, Edward Diehl 1, Claudio Ferretti 1, Jeffery Gregory 1, Mike Kiesel 1, Rudi Thun 1,Curtis Weaverdyck 1, Alan Wilson 1, Bing Zhou 1 for the ATLAS Collaboration 1 University of Michigan, Department of Physics 2 Tel Aviv University, School of Physics and Astronomy  drift spectrum from one group of 24 tubes. The rising edge and trailing edge are fit with Fermi-Dirac functions : The maximum drift time is defined : p 3 (trailing)-p 2 (leading) In normal operation:  the MDT Supply/Return gas lines are connected to the Right /Left partitions.  the gas flows at ~40 l/hr through both chamber partitions Monitor Chamber Electrons drift to anode muon Drift tube with High Voltage anode wire at center Above: drawing of chamber showing the two multi-layers. Readout electronics are hidden from view in faraday cages on the near side. The results of one week’s output is shown above. In this plot: 1)Each point represents one hour of data. Measurement error is < 1 ns. 2)The T max is computed as the difference in the tail and leading edge fit parameters 3) The difference in the gas drift time is consistent with ~50 ppm H 2 O. (Diffusion of small amounts of water through the ceramic endplugs of the drift tubes is possible) REFERENCES PERFORMANCE 1. ATLAS MUON TDR ATLAS TDR 10, CERN/LHCC/ R. Veenhof, “GARFIELD”, CERN Program Library W A. Wilson et al, “Z   ”  ATLAS Physics Workshop, Rome, (2005) After corrections to temperature and pressure the T max from a reference gas source is very stable. In plot below the dispersion over 115 hours is only 0.6 ns CONCEPT Below: Chamber in the MDT gas system. Left/Right sides receive inputs from: Fresh mix MDT gas MDT Supply gas from main trunk line MDT Return gas from trunk line Reference calibration gas Above: The variation of T max to pressure, temperature, CO 2 fraction and water are computed from the Garfield program 3 Top: Temperature and pressure diurnal variations in the monitor chamber for 1115 data run with a reference gas source. Bottom: the T max is extracted and plotted vs elapsed hours. All deviations can be attributed to the small diurnal pressure/temperature fluctuation as shown in the top plot. The bottom plot shows T max before and after the corrections are applied. MONITORING RESULTS Day/Date monitor chamber Left: Muon track reconstructed by pattern recognition algorithm which finds the line tangent to the drift radii. Drift radii are obtained from drift times with an RT function from an autocalibration 2 method. Right: Hit distribution for each tube layer of the chamber: This shows: all channels functioning high efficiency no spurious noise hits tube hits by chamber layer The results of one month (Sept 07) output In this plot: 1)Each point represents one hour of data. Measurement error is < 1 ns. 2)The T max is computed as the difference in the tail and leading edge fit parameters 3)T max from both gas lines is stable to < 1 n 4)There is a small, but easily measurable difference in the input/output T max 5)Aug 22: Argon supply flow interruption  immediate effect on drift time Day/Date