Performance of a Large GEM-TPC at FOPI

Transcription

1 Performance of a Large GEM-TPC at FOPI IPRD 2013, Siena, Italy Sverre Dørheim Physik Department E18 Technische Universität München Germany On behalf of the GEM-TPC collaboration Sverre Dørheim (sverre.dorheim@tum.de)

2 Outline The FOPI GEM-TPC Motivation FOPI Mechanics Electronics Performance Tracking Distortion correction Vertexing Gain calibration PID with de/dx Conclusion and Outlook Sverre Dørheim 2

3 The FOPI GEM-TPC Sverre Dørheim

4 Motivation TPC Positive Aspects: large volume tracking, low material budget, particle ID (de/dx). Limitations: ion backflow (MWPC), Gating mandatory, Low rate, Calibration challenging Sverre Dørheim 4

5 Motivation TPC Positive Aspects: large volume tracking, low material budget, particle ID (de/dx). Limitations: ion backflow (MWPC), Gating mandatory, Low rate, Calibration challenging. GEM-TPC Intrinsic suppression of Ion-Backflow, GEMs O(10-3 ), Gate O(10-5 ). Gating not needed, no dead time, continuous operation. Gain fluctuations? 140 µm 70 µm Sverre Dørheim 5

6 The GEM-TPC at (Darmstadt, Germany) CDC Barrel GEM-TPC RPC Superconducting solenoid, 0.6 T Spectrometer for charged particles produced in heavy ion collisions: 0.6 T magnet, ~7 % momentum resolution ps time-of-flight resolution (RPC, scintillators), ~5 mm secondary vertex resolution (xy- plane), z-resolution ~10 cm, FOPI with the GEM-TPC Prototype: significant improvement in vertexing, especially in z-direction, larger acceptance at low momenta, additional de/dx information, improvement of momentum res Sverre Dørheim (sverre.dorheim@tum.de) 6

7 The GEM-TPC at (Darmstadt, Germany) Spectrometer for charged particles produced in heavy ion collisions: 0.6 T magnet, ~7 % momentum resolution ps time-of-flight resolution (RPC, scintillators), ~5 mm secondary vertex resolution (xy- plane), z-resolution ~10 cm, FOPI with the GEM-TPC Prototype: significant improvement in vertexing, especially in z-direction, larger acceptance at low momenta, additional de/dx information, improvement of momentum res Sverre Dørheim (sverre.dorheim@tum.de) 7

8 Mechanics L. Fabbietti et al., Nuclear Instruments and Methods A 628, 2011 Construction completed fall 2010 Geometry: 72.8 cm drift length, 15 cm outer radius, 5 cm inner radius. Field cage: 792 strips, SMD-resistive divider, 4 mm thick. Modular design: media flange for supplies, GEM-flange, read out flange with water cooling hexagonal read out pads: 1.5 mm radius, Requires 3D clustering Sverre Dørheim (sverre.dorheim@tum.de) 8

9 Mechanics L. Fabbietti et al., Nuclear Instruments and Methods A 628, 2011 Construction completed fall 2010 Geometry: 72.8 cm drift length, 15 cm outer radius, 5 cm inner radius. Field cage: 792 strips, SMD-resistive divider, 4 mm thick. Modular design: media flange for supplies, GEM-flange, read out flange with water cooling hexagonal read out pads: 1.5 mm radius, Requires 3D clustering Sverre Dørheim (sverre.dorheim@tum.de) 9

10 Mechanics L. Fabbietti et al., Nuclear Instruments and Methods A 628, 2011 Construction completed fall 2010 Geometry: 72.8 cm drift length, 15 cm outer radius, 5 cm inner radius. Field cage: 792 strips, SMD-resistive divider, 4 mm thick. Modular design: media flange for supplies, GEM-flange, read out flange with water cooling hexagonal read out pads: 1.5 mm radius, Requires 3D clustering Sverre Dørheim (sverre.dorheim@tum.de) 10

11 The GEMs Triple GEM stack 1 side sectorized 8 Iris shaped sectors 10 MΩ loading resistors Double conical Made at CERN, double mask Sverre Dørheim (sverre.dorheim@tum.de) 11

12 The FOPI GEM-TPC L. Fabbietti et al., Nuclear Instruments and Methods A 628, 2011 Technische Universität München Sverre Dørheim (sverre.dorheim@tum.de) 12

13 Front End Electronics Based on the AFTER (Asic For TPC Electronic Readout) chip for the T2K experiment. Both signal polarities, 72(64) channels, sampling frequency MHz (Adjustable), shaping time: 116 ns (Adjustable), analog circular buffer of 511 samples, ~0.8 W/chip. Digitization by custom made ADC. Noise: ~680 e - ENC, connected to prototype (12-15 pf) Sverre Dørheim (sverre.dorheim@tum.de) 13

14 Noise uniformity 1 ADC ch=~400 e - Based on the AFTER (Asic For TPC Electronic Readout) chip for the T2K experiment. Both signal polarities, 72(64) channels, sampling frequency MHz (Adjustable), shaping time: 116 ns (Adjustable), analog circular buffer of 511 samples, ~0.8 W/chip. Digitization by custom made ADC. Noise: ~680 e - ENC, connected to prototype (12-15 pf) Sverre Dørheim (sverre.dorheim@tum.de) 14

15 Event display π - +C Sverre Dørheim (sverre.dorheim@tum.de) 15

16 Gain Calibration with 83m Kr Results from Roman Schmitz, HISKP Bonn, 83m Kr mixed in with the TPC gas Decays with a set of discrete lines in the kev region, the most prominent beeing 41.6 kev Each pad is assigned a gain calibration factor in an iterative procedure. 25 % improvement on energy resolution. An event with 83m Kr decays in the TPC Sverre Dørheim (sverre.dorheim@tum.de) 16

17 Gain Calibration with 83m Kr Results from Roman Schmitz, HISKP Bonn, 83m Kr mixed in with the TPC gas Decays with a set of discrete lines in the kev region, the most prominent beeing 41.6 kev Each pad is assigned a gain calibration factor in an iterative procedure. 25 % improvement on energy resolution. 4.4 % Energy resolution for the main Peak (41.6 kev) after correction Sverre Dørheim (sverre.dorheim@tum.de) 17

18 Performance Sverre Dørheim

19 Tracking GENFIT C. Höppner et al., Nuclear Instruments and Methods A 620 (2010),518 Modular software framework Kalman filter for fitting Full field and material treatment Used in PANDA, Belle II, GEM-TPC, (ILC) True space point treatment Concept of virtual detector planes Proper residual minimization Sverre Dørheim (sverre.dorheim@tum.de) 19

20 Residuals from Cosmics L. Fabbietti et al., Nuclear Instruments and Methods A 628, 2011 Ar/CO 2 (90/10) Drift field: 360 V/cm Gain 3800 No magnetic field 230 µm Biased residuals Two-Gaussian fit Red: Central Gauss Black Weighted mean of both widths Dashed line: Single e - transverse diffusion Sverre Dørheim (sverre.dorheim@tum.de) 20

21 Residuals from Cosmics in 2D X-residual. Magnetic field: 0.6 T. Clear structures: E x B effects. Need to make a detailed model of electric field. Short strips at the cathode. Gap between last strip and GEMs Sverre Dørheim (sverre.dorheim@tum.de) 21

22 Distortion Correction Detailed finite elements model of our field cage, including the mentioned problems. Microscopic model to calculate the drift distortions in inh. fields ( E x B ), F.V. Böhmer et al., NIM A 719, 2013: allows calculation of drift distortions, directly from fields or space charge, allows correction of distortions based on simulations or measurments with reference tracks. Radial component of distortion field from FE Field Cage simulation Sverre Dørheim (sverre.dorheim@tum.de) 22

23 Distortion Correction Detailed finite elements model of our field cage, including the mentioned problems. Microscopic model to calculate the drift distortions in inh. fields ( E x B ), F.V. Böhmer et al., NIM A 719, 2013: allows calculation of drift distortions, directly from fields or space charge, allows correction of distortions based on simulations or measurments with reference tracks. x-distortions real data x-distortions simulations Sverre Dørheim (sverre.dorheim@tum.de) 23

24 Vertexing Realized with RAVE [1] interfaced to GENFIT [2]. Preliminary Substantial improvements, esp. z. xy distribution dominated by beam width. z-distribution dominated by target thickness. [1] W. Waltenberger, IEEE Trans. Nucl. Sci. 58 (2011) 434 [2] C. Höppner et al., NIM A 620 (2010), Sverre Dørheim (sverre.dorheim@tum.de) 24

25 de/dx-extraction F.V Böhmer et al., First Measurement of de/dx with a GEM-based TPC, submitted to NIM A Fully 3D treatment. Base is a combined TPC+CDC track. Step along each track with a fixed steplength, Δx = 5 mm. Planes perpendicular to track created. Collect all pad hits Ω i between two planes. Sum up amplitudes k Spectrum: A i / x A i a Truncated mean applied to get a stable estimator. The lowest 5 % and the highest 25 % are cut. i k Sverre Dørheim (sverre.dorheim@tum.de) 25

26 de/dx-spectrum F.V Böhmer et al., First Measurement of de/dx with a GEM-based TPC, submitted to NIM A Particle bands clearly visible Only matched tracks, momentum from combined TPC+CDC fit Amplitude information taken only from TPC Cuts Outer and inner edge of active area. Circle in gain calibration pad map (backup) #de/dx samples >13 Preliminary Preliminary Sverre Dørheim (sverre.dorheim@tum.de) 26

27 Slicing for Feature Extraction F.V Böhmer et al., First Measurement of de/dx with a GEM-based TPC, submitted to NIM A Technische Universität München 25 MeV/c momentum slices. Gaussian shape, tail towards higher energy-loss values. Fit function: Convolution of an exponential decay and a Gaussian: P eg 2 ( x;,, ) exp[ (2 2 2 Maximum likelihood: Start values from Bethe parametrization. π Preliminary 2 x 2x)] erfc( ) 2 π Preliminary π π Red line: Fit Sverre Dørheim (sverre.dorheim@tum.de) 27

28 de/dx Resolution F.V Böhmer et al., First Measurement of de/dx with a GEM-based TPC, submitted to NIM A Technische Universität München Resolution (Gaussian σ) in the range % for Protons Comparison with expectation: W.W.M Allison and J.H. Cobb, Ann. Rev. Of Nuclear and Particles Science 30 (1980) 253 Approximation (based on PAI-model) of experimental data from MWPCs Expectation (FWHM) : R( N, x, P) 0.96 N N: number of samples Δx: sampling length (cm) P: gas pressure Obtained in regime of much larger sample size FWHM/μ larger than prediction Asymmetries well understood and consistent σ/μ in perfect agreement 0.46 ( x P) 0.32 π Preliminary Preliminary Uncertainties are shown in shaded areas π Sverre Dørheim (sverre.dorheim@tum.de) 28

29 Summary The GEM-TPC with the largest active volume to date Spatial resolution of 230 μm achieved at small drift lengths Drift field distorions: consistent results from Finite Elements simulations and data Vertexing performance within expectation Gain calibration working well de/dx: First measurement of this kind for a GEM-based TPC de/dx resolution(σ) in the range %, in agreement with A.C. model Gain fluctuation in the same order of magnitude as MWPCs Outlook: Upgrade the ALICE TPC with GEM read out chambers Sverre Dørheim (sverre.dorheim@tum.de) 29

30 The GEM-TPC collaboration TU München, E18 M. Ball, F. Böhmer, S. Dørheim, K. Eckstein, A. Hönle C. Höppner, B. Ketzer, I. Konorov, S. Neubert, S. Paul, J. Rauch, S. Uhl, M. Vandenbroucke 1 1 Now at Temple University, Philadelphia TUM, Exc. Cluster Universe M. Berger, J. Chen, F. Cusanno, L. Fabbietti, P. Gasik, R. Münzer GSI, Darmstadt R. Arora, J. Frühauf, T. Hackler, J. Hehner, M. Kis 2, V. Kleipa, J. Kunkel, N. Kurz, Y. Leifels, K. Peters, H. Risch, C. Schmidt, L. Schmitt, S. Schwab, D. Soyk, B. Voss, J. Weinert 2 also at RBI Zagreb Stefan-Meyer-Institut, Wien P. Bühler, P. Müllner, J. Zmeskal HISKP Bonn R. Beck, D. Kaiser, M. Lang, R. Schmitz D. Walther Universität Heidelberg N. Herrmann Sverre Dørheim (sverre.dorheim@tum.de) 30

31 Backup slides Sverre Dørheim

32 ADC ch. Technische Universität München Chip Noise 1 ADC ch = ~400 e Sverre Dørheim (sverre.dorheim@tum.de) 32

33 Gain Calibration Gain Calibration Map used for de/dx studies RMS ~ % improvement in de/dx resolution Radial cuts to exclude fringe effects Sverre Dørheim (sverre.dorheim@tum.de) 33

34 Vertexing [1] W. Waltenberger, IEEE Trans. Nucl. Sci. 58 (2011) 434 [2] C. Höppner et al., NIM A 620 (2010), Sverre Dørheim (sverre.dorheim@tum.de) 34

35 Momentum Resolution [F. Cusanno, TUM] Sverre Dørheim 35

36 Separation Power F.V Böhmer et al., First Measurement of de/dx with a GEM-based TPC, submitted to NIM A Technische Universität München de/dx spectrum fitted in 25 MeV/c momentum slices Fit function convolution of a Gaussian and an exponential decay function μ x, μ x : Gaussian mean of de/dx distribution; σ x,y : average of Gaussian σ s Direct measure of PID capabilities p - π separation important for Λ- reconstruction S XY Preliminary x x, y y Sverre Dørheim (sverre.dorheim@tum.de) 36

37 Straggling #Samples A i / Δx too small to reconstruct straggling function In addition, long tail is always cut off Simple mean is not a good estimator Solution: Truncation Discard certain fraction of smallest and largest A i / Δx Move mean towards MPV A more stable estimator Truncated mean is a reasonable estimator Truncation parameter needs to be optimized Sverre Dørheim (sverre.dorheim@tum.de) 37

38 Data samples for Allison & Cobb model W.W.M Allison and J.H. Cobb, Ann. Rev. Of Nuclear and Particles Science 30 (1980) 253 GEM-TPC data within red square Sverre Dørheim 38

39 Monte carlo study: Fit Function Obtain raw, un-truncated proton sample with help of FOPI RPC Select small angle and momentum window for a clean de /dx spectrum MC: Sample N times from spectrum, perform id. analysis! simulate longer tracks Asymmetry vanishes towards longer tracks: Effect of limited sample size N FWHM resolution systematically above prediction (obtained in the regime of larger N!) Gaussian resolution component in perfect agreement (Additional) asymmetry is fully absorbed in exponential component, single parameter Preliminary Sverre Dørheim (sverre.dorheim@tum.de) 39

40 Monte carlo study: Fit Function Obtain raw, un-truncated proton sample with help of FOPI RPC Select small angle and momentum window for a clean de /dx spectrum MC: Sample N times from spectrum, perform id. analysis! simulate longer tracks Asymmetry vanishes towards longer tracks: Effect of limited sample size N FWHM resolution systematically above prediction (obtained in the regime of larger N!) Preliminary μ μ Gaussian resolution component in perfect agreement (Additional) asymmetry is fully absorbed in exponential component, single parameter Sverre Dørheim (sverre.dorheim@tum.de) 40

41 Gain measurements in different Gases Effective gain: Includes losses inside the GEMs Red: Ne/CO2 (90/10) Blue: Ar/CO2 (90/10) Green: Ar/CO2 (70/30) Measured with a small planar GEM powered by a resistor chain Photons from Cu X-Ray tube Sverre Dørheim (sverre.dorheim@tum.de) 41

42 Voltage configuration Sverre Dørheim 42

43 Radiation Length Sverre Dørheim 43

44 Pulse Shape analysis Simple PSA algorithm Find local minima Pulse amplitude: Integral of pulse Pulse time: Time of maximal sample minus the rise time of the shaper Sverre Dørheim 44

45 Clustering Simple clustering: Create cluster around local maxima in pad hit amplitudes Attach pad hits to clusters, if They are adjacent (Pad-wise) They have a z-value within a certain time slice around the cluster center of gravity Pad hit splitting between clusters possible Sverre Dørheim (sverre.dorheim@tum.de) 45

46 Pattern recognition J. Rauch et al., Journal of Physics: Conference Series 396 (2012) Performed standalone in TPC Track-following algorithm Conformal mapping onto Riemann Sphere Fast helix fits Track piece merging Serial performance ~ 50 μs/track on regular office machine Single track efficiency >95 % Very robust against track distortions Sverre Dørheim (sverre.dorheim@tum.de) 46

47 Principle of Ion-Backflow Supression Sverre Dørheim 47