Development of a Time Projection Chamber Using Gas Electron Multipliers (GEM-TPC) Susumu Oda, H. Hamagaki, K. Ozawa, M. Inuzuka, T. Sakaguchi, T. Isobe, T. Gunji, S. Saito, Y. Morino, Y.L. Yamaguchi 1, S. Sawada 2 and S. Yokkaichi 3 CNS, University of Tokyo, 1 Waseda University, 2 KEK, 3 RIKEN 1. Motivation of GEM-TPC Development 2. GEM-TPC Prototype 3. Performance Evaluation of GEM-TPC 4. Summary and Outlook 5. Development of GEM in Japan 1/ /10/25 Puerto Rico IEEE NSS 2005 N17-8

Motivation of GEM-TPC development Study hot and dense matter, Quark Gluon Plasma (QGP), by high energy heavy ion collisions. –e.g. PHENIX-RHIC-BNL, ALICE-LHC-CERN A high resolution (position, double track and energy) tracker with high collision rate and high particle multiplicity is needed. –Interested p T region is GeV/c  magnetic field should be kept low ~1T. –Required resolution  p T /p T 2 ~10 -3 (GeV/c) -1 : e.g. to resolve Upsilon states. –If 1-m radius solenoidal tracker, 200  m spatial resolution is required. –Since particle density is high, double track with >1cm interval should be separated. TPC using GEM (Gas Electron Multiplier) may satisfy the above requirements. –Fast signal. –No incident angle dependence by 2-dimension symmetry. –No E x B effect. –Ion feedback suppression. –Flexibility for magnification. 2/13

GEM-TPC prototype Field cage : 35cm(drift direction) x 17cm x 17cm. Triple-GEM : 10cm x 10cm (effective region), made at CERN. Pad : rectangle and zigzag (chevron), 1.09mm x 12mm. Charge sensitive preamplifier : gain is 3.2V/pC and time constant is 1  s. 24ch signals are read out using 100MHz FADC. Beam direction 3/13 10cm 12mm x 1.09mm

Gas Three kinds of gases with different characteristics were used for measurements. Measured energy resolution with 55 Fe X-ray source (5.9keV) was  =11%(Ar-CH 4 ), 10%(Ar-C 2 H 6 ) and13%(CF 4 ). Electric field Drift velocity Transverse Longitudinal Ar(90%)-CH 4 (10%)130 V/cm 5.48 cm/  s570  m378  m Ar(70%)-C 2 H 6 (30%)390 V/cm 5.01 cm/  s306  m195  m CF V/cm 8.90 cm/  s104  m82  m  =13% 55 Fe X-ray (5.9keV) spectrum with CF 4 4/13

Beam test A beam test was conducted at KEK PS to evaluate the performance of the GEM-TPC. Evaluated items –Detection efficiency (1GeV/c  ) –Spatial resolution (1GeV/c  ) –Beam rate dependence (2GeV/c e, ,p) –Double track resolution (2GeV/c e, ,p) –PID by dE/dx measurement (0.5-3GeV/c e, , ,p,d) Three kinds of gases were used. –Ar-CH 4 (P10), Ar-C 2 H 6 and CF 4 Without magnetic field. Setup schematic view 5/13

Typical GEM-TPC signal Time (6.4  s=640bin, 1bin=10ns) ADC Ar-C 2 H 6, drift length 85mm, rectangular pad 1GeV/c electron beam Track 6/13

Result 1: Detection efficiency 1.Events having hits in the 1st and 3rd pad rows were selected. 2. The fraction of existence of a hit in the 2nd pad row was used as detection efficiency. Result: The plateau is >99%. 7/13 1st 2nd 3rd Ar-CH % Ar-C 2 H % CF %

Result 2: Spatial resolution 1.Hit positions in pad-row direction (X) and drift direction (Z) are determined by simple weighted mean of charge. 2.Spatial resolution is estimated from a residual between the position of the middle pad row and the interpolated position. Result Best resolution was 80  m (X-direction) and 310  m(Z-direction) with Ar-C 2 H 6 gas, with rectangular pad, for drift length of 13mm. Zigzag pad and rectangular pad have similar spatial resolution. 8/13 1st 2nd 3rd Res X Pad-row direction Drift direction

Purpose To study the effect of ion feedback on the GEM-TPC performance, detection efficiency and spatial resolution were measured as functions of beam rates. Beam rate was changed by changing the width of beam slit. Beam rate was monitored by 2.5x2.5cm 2 plastic scintillator. Ar-CH 4 gas and 85mm drift length. Result With a large beam rate (<5000cps/cm 2, ~10 5 cps), high detection efficiency and spatial resolution were obtained. RHIC (Au+Au,  s NN =200GeV) |  =0 =170, Luminosity=1.4x10 27 /cm 2 /s,  inel =7barn ⇒ 300cps/cm 2 : 30cm away from vertex LHC (Pb+Pb,  s NN =5.5 TeV) |  =0 ~1000, Luminosity~1x10 27 /cm 2 /s,  inel ~8barn ⇒ 1400cps/cm 2 : 30cm away from vertex 9/13 Result 3: Beam rate dependence

Result 4: Double track resolution Double track resolution was evaluated by distribution of distance between two hits in drift direction. Detection efficiency of double tracks is defined as the ratio of measured distribution to expected one. Multiple hits were generated by high beam rate (~4000cps/cm 2 ) and a lead block (1X 0 ). Double track with >12mm distance can be distinguished. Since longitudinal diffusion is 1.0mm, electric noise limits the double track resolution. 10/13 12mm Secondary particle Accidental coincidence Ar-CH 4, 85mm drift Expected distribution Measured distribution

Result 5: Particle identification Energy loss was measured in momentum range of GeV/c with Ar-CH 4 gas. Performance of a larger TPC (50cm track, “real size”) was estimated from measured distribution (1.0GeV/c beam). –Energy resolution is expected to be 9.1% for pion and 8.0% for proton –Better resolution than STAR TPC (  E /E=8% with a 67cm track) –Pion rejection factor is 180 for proton efficiency of 99%. Measured energy loss 1.0GeV/c  + 11/13 Ar-CH 4, 85mm drift  p Expected energy loss spectra of 1.0GeV/c  + and p beams with 50cm track with truncated mean method.

Summary and Outlook A GEM-TPC prototype was constructed toward a tracker working with high rate and high multiplicity environment of high energy heavy ion collisions. Beam test for performance evaluation of the GEM-TPC was done. –Detection efficiency : >=99.3% –Spatial resolution : 80  m (pad-row direction), 310  m (drift direction) (Ar(70%)-C 2 H 6 (30%)) –Beam rate dependence: Good detection efficiency and spatial resolution were kept even with 5000cps/cm 2. –Double track resolution : 12mm (drift direction). –PID : Better performance than STAR TPC is expected. Our expectation was mostly satisfied! We are developing GEM foil itself (next page). 12/13

Development of GEM in Japan 13/13 We succeeded in fabricating a new type of GEM (CNS-GEM) using a dry etching method. CERN-GEM wet wet etching CNS-GEM dry dry etching Etching method The cross section of a hole cylindrical A hole with cylindrical shape double-conical A hole with double-conical shape Gain stability of CNS-GEM is better than that of CERN-GEM. –The shape of holes of GEM may significantly affect gain stability.