Performance of BIL tracking chambers for the ATLAS muon spectrometer

1. Performance of BIL tracking chambers for the ATLAS muon spectrometer A.Baroncelli, P.Branchini, F.Ceradini, E.Graziani, M.Iodice, D.Orestano, A.Passeri, F.Pastore, F.Petrucci, E.Spiriti, S.Tagliaventi and A.Tonazzo Università Roma Tre and INFN Sez. Roma IIII A.Tonazzo –Performance of ATLAS MDT chambers /

2. Outline • Description of ATLAS MDT chambers • Equipment and tests in Roma Tre • Calibration and resolution • Effects of varying operational conditions • Temperature • Gas composition • Use of charge information to improve the spatial resolution A.Tonazzo –Performance of ATLAS MDT chambers /

3. The ATLAS muon system Air-core toroidal spectrometer (4 T/m) with 3 measurement stations  good momentum resolution 6GeV/c-1TeV/c + Dedicated trigger detectors MDT chambers BIL chambers: 2 MultiLayers x 4 Layers x 36 (or 30 or 24) MDTs Length=2.6m A.Tonazzo –Performance of ATLAS MDT chambers /

4. MDT (Monitored Drift Tube) Al tube R=15mm 400mm thick Anode wire 50 mm W start TDC ADC electron drift time signal amplitude • Gas mixture: Ar-CO2 93-7% • Absolute pressure 3 bar • Gain 2x104 (HV=3080 V) • Discriminator threshold: 20 primary e stop Max drift time 700 ns Single point resolution ~80mm A.Tonazzo –Performance of ATLAS MDT chambers /

5. MDT chamber equipment and tests RomaTre total 62 chambers , 32 done A.Tonazzo –Performance of ATLAS MDT chambers /

6. Cosmic ray hodoscope Simultaneous test of up to 3 chambers Trigger = 3 planes of RPCs 6 segments along tube CHAMBER 1 CHAMBER 2 CHAMBER 3 A.Tonazzo –Performance of ATLAS MDT chambers /

7. Data acquisition system • VME architecture • 3 Chamber Service Modules (CSM0) read the events from front-end level-1 buffer of chamber TDCs and perform single chamber event building • Digitization performed on the chamber by mezzanine boards hosting 3 ASD circuits and an Atlas Muon TDC (AMT) integrated circuit, that digitize the drift time and sampled charge • The CSM0 also distribute the trigger and the main clock (40 MHz) and initializes the ASD and AMT parameter via a JTAG interface A.Tonazzo –Performance of ATLAS MDT chambers /

8. Cosmics data • 1.3 M events in 24 h • Occupancy distributions: spot dead/hot channels • Check response of each tube Chamber RM012: central position A.Tonazzo –Performance of ATLAS MDT chambers /

9. Single tube response Drift time spectrum Distribution of drift time spectrum fit parameters for all tubes in one chamber noise TDC counts T0 Tmax A.Tonazzo –Performance of ATLAS MDT chambers /

10. Autocalibration: space-time relation • Iterative procedure: • Straight line tangent to drift circles • Evaluate residuals • Compute mean value of residuals in each time slice • Use it as correction to r-t relation A.Tonazzo –Performance of ATLAS MDT chambers /

11. Resolution • Select “good” events (single track, n≥8 hits) • Cut on c2 (a tight cut selects hard ms, reducing contributions from MS) • Compute residuals and extrapolation error for each point with track constructed with n-1 points • Width of residuals distribution is s(r)=  [Resolution(r)]2+[<extrapolation error>(r)]2 s(r) A.Tonazzo –Performance of ATLAS MDT chambers /

12. Temperature effects • Effect of temperature variations: • Increase/decrease drift spectrum length • Modify r-t relations • R-t variations on cosmic samples at different temperatures: DT=1.1°C DT=3.1°C Systematic error not included Variations of r-t relations scale linearly with T DT=2.1°C DT=5.1°C In ATLAS: measure T and apply corrections to r-t relations A.Tonazzo –Performance of ATLAS MDT chambers /

13. Effects of varying gas composition • Effect of gas mixture variations: • Increase/decrease drift spectrum length • Modify r-t relations • r-t variations on cosmic samples with precisely known Ar-CO2 mixtures: • 93-7% and 93.3-6.7% DT=7.1°C …. DT=0.2°C In ATLAS: Control gas composition stability at % level DCO2=-0.3% A.Tonazzo –Performance of ATLAS MDT chambers /

14. Charge information 8-bit Wilkinson ADC to measure the charge collected in a given time gate after threshold crossing Charge→ADC conversion is non-linear Spread ~10% Equalize channels Width vs peak A.Tonazzo –Performance of ATLAS MDT chambers /

15. Charge information: noise rejection TDC counts cut cut ADC vs TDC counts ADC counts A.Tonazzo –Performance of ATLAS MDT chambers /

16. Charge information Average signal amplitude vs position along the wire Average signal amplitude vs drift time <ADC>(t) in the 6 trigger zones along the wire Signal attenuation length ~30 m consistent with estimate from impedance value A.Tonazzo –Performance of ATLAS MDT chambers /

17. Charge information: slewing corrections Charge information can be used to improve the spatial resolution • Estimate Dt from track residuals • Use local linear correction for • Dt vs ADC-<ADC> in each r interval A.Tonazzo –Performance of ATLAS MDT chambers /

18. Slewing corrections Improvement on resolution most relevant at small radii Average resolution 90 mm 80 mm = ATLAS design ~50 mm The method is based solely on the data from the chamber itself A.Tonazzo –Performance of ATLAS MDT chambers /

19. Summary • 32/62 MDT chambers for the ATLAS muon system have been equipped and tested at the Roma Tre site • Cosmic ray data is used for calibration and resolution measurement • Effects of varying operational conditions have been studied • Temperature • Gas composition • A method to improve the spatial resolution using charge information has been developed A.Tonazzo –Performance of ATLAS MDT chambers /