ATLAS straw-tube Transition Radiation Tracker test beam results

1. ATLAS straw-tube Transition Radiation Tracker test beam results Vladimir Tikhomirov P.N.Lebedev Physics Institute, Moscow Presented on the CBM meeting, 9-11 March 2005, GSI, Darmstadt, Germany

2. Motivation • The Transition radiation tracker (TRT) is one of the component of the ATLAS Inner Detector. It combines electron identification capability with charged-particle track reconstruction. This is achieved by interleaving layers of xenon-filled drift tubes of small diameter (straws) with radiator. • In order to test the physics performance of the proposed detector, several small-scale TRT prototypes and also one TRT production module have were tested in the H8 beam line at the CERN SPS accelerator over the past 14 years. The TRT performance was evaluated using electron, pion and muon beams with energies varying from 1 to 300 GeV. • Results obtained during last years with four different detectors are presented here: • - sector prototype • - radiator prototype • - high-rate prototype • - TRT barrel production module V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

3. Straw-tube chambers • The total number of straw-tubes chambers in the ATLAS TRT - 372K (52.5K in the barrel and 319.5K in the end-cap). Total number of electronics channels - 425K (some of barrel tubes are divided into two independent readout channels) • Straws, the detector elements of the TRT, are tubes of 4 mm diameter made from a conductive polyimide (Kapton) film. The anode wires, made from gold-plated tungsten, have a diameter of 30 μm. For all measurements made before 2002, the straws were operated with a gas mixture consisting of 70% Xe + 20% CF4+10%CO2. More recently, the TRT prototypes were operated with a new gas mixture, consisting of 70% Xe + 27% CO2+3%O2, which was chosen to solve etching problems observed with the glass wire-joint in the barrel straws with the original gas mixture. The nominal gas gain was 2.5·104. In order to make straw rigid, 4 C-fibres are attached along the straw Straw wall Reinforced straw V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

4. Electronics Front End Back End • Designed to work at 40 MHz • Front-end electronics should work during 10 years with a total ionising dose of up to 5 Mrad TRT-TTC 8 ch 16 ch ASDBLR DTMROC ROD DMILL Amplifier Shaper Tail cancellation Discriminator Baseline restorer IBM DSM Time encoding Track&TR Thresholds Test pulses Pipeline Sparsification Data Format Error checking Event transfer (ROB) V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

5. ASDBLR boards DTMROC board Electronics (2) End-cap 3 ASDBLR boards (64 channels) connected to one DTMROC flex board (192 channels) Barrel 64-channels board V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

6. Typical experimental setup • H8 beam line at CERN SPS • π, e andμ with wide energy range (1-300 GeV) • Two beam chambers (BC) and three silicon (Si) detectors for precise external tracking. Intrinsic Si resolution is around 10 μm • Two Cherenkov counters (Ch), preshower and lead glass calorimeter for determination of beam particle sort • Several different TRT prototypes to study V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

7. Sector prototype • 1/32 of one ATLAS TRT end-cap 16-plane wheel • 384 radially oriented straws with space between layers filled with radiator • 4 mm straw diameter • Regular radiator: 15 μm polypropylene foils with 200 μm spacing • 70% Xe + 20% CF4 + 10% CO2 gas mixture (70% Xe + 27% CO2 + 3%O2 since 2002) • 2.5 ·104 nominal gas gain • LHC type electronics V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

8. Sector prototype: event display V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

9. Barrel module setup V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

10. 3.35 1.01 2.05 1.28 2.13 3.34 Barrel module • 2003, 2004 years • 1/16 of ATLAS barrel TRT production module • 6 submodules, 73 layers on radius (along the beam), 3284 straws • Fiber radiator • New gas mixture • Final version of electronics V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

11. Barrel module: event display V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

12. Drift time and threshold measurement • Two adjustable thresholds: low threshold for drift time measurements and high one – for registration of TR hits • Three time slices during 75 ns, each time slice – 8 bit, 3.125 ns bin width Noise Trailing edge Leading edge 75 ns Two threshold analysis TR:6 keV MIP: 0.2 keV V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

13. Particle sort identification • Shower detector, Cherenkov counters and lead glass calorimeter were used for identification of beam particle sort Pions Electrons V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

14. Tracking and efficiency • Si track is used for precise prediction of beam particle position inside the straw • Full collection time is around 44 ns (with new gas mixture) • Third-degree polynomial fit of RT dependence is used to reconstruct hit coordinate via measured drift time • Two kinds of efficiency: total (hit registration efficiency) and 2.5 (drift-time efficiency). Later takes into account the position of reconstructed hit V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

15. Coordinate accuracy and efficiency • Coordinate accuracy is around 130 μm at 200-300 eV threshold • Smooth dependence of coordinate accuracy and efficiency on (low) threshold • ~95% of total and ~85% of 2.5 efficiency at 200-300 eV threshold • The straw noise probability increases rapidly from 1.1% at 400 eV thr. to 11% at 200 eV. Taking this into account, a reasonable choice of low-level threshold seems to be 250-300 eV. V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

16. High threshold hit efficiency • Relatively big variation of high threshold probability from channel to channel • TR accumulation effect is clearly seen at the barrel front and after module’s walls Walls between modules High threshold hits V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

17. Rejection by number of high threshold hits • Traditional method for pion/electron separation: calculate the number of high threshold hits in straws, crossed by the beam particle V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

18. Rejection: threshold scan • Optimal threshold is around 6-7 keV • Rejection power better than 2% can be achieved • ATLAS Inner Detector TDR gives 5% at central rapidity, but: 1) with smaller number of active straws due to double wire join 2) with magnetic field 3) with full material in front of TRT taken into account V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

19. High rate prototype • Goal: estimate straw performance in multiparticle environment. • Single straw in the beam line • Straw matrix, irradiated by Fe55 source • Mixed output signal V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

20. High rate prototype (2) • Deterioration in both tracking accuracy and efficiency with rate increasing. V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

21. High rate prototype (3) • Still good performance is demonstrated up to 20 MHz counting rate V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

22. High rate prototype (4) • High particle density, up to 500 kHz/cm expected in the most exposed parts of the TRT straws. It may induce space charge effects, which can affects the TRT performance. These effects have been studied in details for the original TRT gas mixture. No change was observed for the signal amplitude up to 500 kHz/cm and for the drift time accuracy up to 750 kHz/cm. • These studies were repeated for the new gas mixture. A straw exposed to test-beam particles was irradiated at the same time by 6 keV X-ray source with a maximum particle rate of 350 kHz/cm of wire (equivalent to 900 kHz/cm from minimum ionising particles). The straw drift-time accuracy degraded fro 110 (0 rate to 130 mkm (maximum rate) without significant change of the drift-time efficiency. • The degradation observed in this measurements is assumed to arise from the ozone concentration in the avalanche region. V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

23. Radiator prototype Goals: precise measurement of dE/dx and TR spectra; different radiators performance study; comparison with MC predictions. V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

24. Pion and electron spectra in a straw 20 GeV pions 20 GeV electrons Straw number along a beam V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

25. Pion and electron spectra in a straw 20 GeV pions 20 GeV electrons Straw number along a beam TR accumulation effect V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

26. Comparison of different working conditions V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

27. Radiator prototype (2) • TR radiation from straw walls has to be taken into account to describe spectrum above 8 keV V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

28. Radiator prototype (3) • Rich collection of data with different energies and sort of particles • Very good agreement between data (histograms) and Monte Carlo (dots), except, may be, small region around 4 keV V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.

29. Conclusions • Such a big and complex detector, as the ATLAS TRT, requires intensive R&D work do be done • Many different TRT prototypes have been studied in the test beam to understand/improve detector performance • Different radiators, gas composition, set of electronics and operational conditions have been investigated • Tracking accuracy around 130 µm in one straw is achieved with nominal working conditions • ~95% of total and ~85% of drift-time efficiency for single particle at nominal conditions • Pion/electron rejection of 1.5% is achieved for barrel module • Good performance is demonstrated up to 20 MHz counting rate • At 900 kHz/cm a small degradation in drift-time accuracy is observed without efficiency degradation. • TR performance has been studied with different beam energy and particle sort. Good agreement with Monte Carlo is observed. • General TRT performance is found to satisfy ATLAS requirements. V.Tikhomirov. ATLAS straw-tube TRT test-beam results. 9-11 March 2005, CBM meeting at GSI, Darmstadt, Germany.