Gas Pixel TRD/Tracker With the support of the TRT collaboration F. Hartjes, M.Fransen, W. Koppert, S.Konovalov, S.Morozov, N. Hessey, A.Romaniouk, M. Rogers, H. van der Graaf. With the support of the TRT collaboration

Motivation. A combination of a pixel chip and a gas chamber opens new opportunities for particle detectors. Pixelization of the information from the particle track offers new possibilities to combine in one detector many features: 1. Precise coordinate measurements (well below diffusion limit). 2. Vector track reconstruction. 3. Very good multi-track resolution. 4. Powerful pattern recognition features. 5. L1 trigger possibilities for stiff tracks. 6. Enhanced Transition Radiation separation 7. dE/dX measurements. TR clusters

What is the GasPixel detector ~ -500 V Pixel chip Mesh Gas volume Drift distance 16 mm 55 mm ~50 mm Track image on the pixel plane

Potential applications: everywhere. Issues important for TRD: But might be interesting for the Inner tracker in the collider experiments. Detector shell occupy the outer radii of the Inner Tracker and contain at least two layers which can be interleaved with TR radiator What is new? A vector tracking ! Precision space points: X ( f-direction) and Y (h – direction) 2. Vector direction f and h 3. Enhanced TR and dE/dx measurement function f Issues important for TRD: Limited space Number of layers 1 or 2 Effective TR radiator needed Minimum drift space (Rate constrains) May not be optimal for TRD Hope to work with each cluster

Particle ID: use TR and dE/dX information Total Cluster counting Relativistic rise vs d-electron energy B. Dolgoshein, NIM A326 (1993) 434 Number of primary Collisions vs gamma Number of primary collisions per cm of Xe Factor of 1.36 ln g MC Number of clusters above ~1 keV Factor of 2 Pions Plato Fremi : Mean 4 Min. ionising: Mean 2.7 Factor of 1.48 Detailed knowledge of the ionisation structure of the track gives more information for the particle ID. Although track the length of the drift volume is the main parameter! dE/dx, arb. units

X-ray absorption probabilities. Something more we may find useful. Particle ID: TR registration: The main issue is to separate dE/dX from TR Use the low: X-ray absorption probabilities. Something more we may find useful. 1+2 == 3 6% 3% 1% TR clusters Still there is dE/dX under the cluster B. Dolgoshein, NIM A326 (1993) 434

First test beam studies: Set-up Exposed to PS 5 GeV particle beam No detailed optimization done. InGrid TimePix detector, 14 x 14 mm2, 256 x 256 pixels (pixel size 55 mm) Two operation modes: time measurements and time-over-threshould measurements (amplitude) Exposed to PS 5 GeV particle beam Xe/CO2 (70/30) mixture. L = 16 mm 0.05 mm V0 V1 L for TR absorption ~ 16 mm TR-Radiator Beam p, el E 25o e- Length - 19 cm PP foils of 15 mm spaced by 200 mm h Pixel plane MC simulations studies were performed and compared with the test beam data using the same analysis tools.

Test beam studies. May 2008 test chamber: Operation parameters For the gas mixture 70%Xe+30%CO2 Total drift time ~ 300 ns Ion signal ~ 80 ns Transverse diffusion sT ~240 mm/cm1/2 Longitudinal diffusion sL ~120 mm/cm1/2 May 2008 test chamber: InGrid technology Drift distance= 16 mm Vdrift = 3800 V Edrift= 2000 V/cm Vamp ~470 V Gas gain ~ 800 – 3500 Protection layer 30 mm Amplification gap - 50 mm Orientation: 25 degree to the beam and horizontal. Gas Mixtures Ar/CO2 Xe/CO2 He/Isobutane DME/CO2 Mesh Si prot. layer Pixels Charge Electronics operating threshold: ~ 800 el. Gas gain range: 800 – 1800 Geometry + induced charge effect Effective threshold > 1600 el. OR > 1 primary electron. Estimated efficiency for 1 primary electron was ~30%

Test beam studies (5 GeV). Comparison EXP data with MC Particle tracking including all interactions + geometry - GEANT3 Ionisation losses - PAI model TR generation - ATLAS TRT code After all only 3 basic parameters were tuned to fit the MC model to the experimental data: Diffusion coefficient - was chosen to match a track widths. Threshold - tuned to satisfy number of fired pixels. Energy scale - scaled using calibration factor. Exp MC

Particle Identification. Time-Over-Threshold representing a signal amplitude was measured for each pixel Exp. data Amplitude (Arb. units Log scale) Pion Electron On the hip map color represents the amplitude or ToT information Amplitude (Arb. units Log scale) Pion Electron

Particle Identification Methods A total energy method The amplitudes of all the pixels on track are summed. 2. Cluster method Sum of the signals projected to the reconstructed track (binning is 55 mm). Exp. data rebin Cluster threshold Cluster energy

Particle Identification: Total energy. Test beam results Particle Identification: Total energy. Electrons Pions Distribution of a total ionization on the particle track for electrons and pions. Comparison MC and Data

Particle Identification: Total energy. Test beam results Particle Identification: Total energy. MC Exp Distribution of a total ionization on the particle track for electrons and pions Electron/pion separation.

Particle Identification: Cluster counting Test beam results. Particle Identification: Cluster counting Electrons Pions No clusters 29/368 – 7.9% No clusters 711/925 - 78% Cluster energy distribution on the particle track for electrons and pions

Particle Identification: Cluster counting Test beam results Particle Identification: Cluster counting Cluster energy distribution on the particle track for electrons and pions Electrons 1 cluster 175 Cluster energy (arb units) Events For PID additional CUT on the cluster energy is applied,. Electrons and pions 1 cluster 177 Electrons 2 clusters Events Only 1% of pions have 2 clusters Maximum cluster energy (arb units)

Particle Identification Rejection factor Pion registration efficiency as a function of electron efficiency. For 90% electron efficiency pion rejection factor is ~7 for the total energy method. Total energy Pion registration efficiency as a function of electron efficiency for the total energy method and Cluster counting method. Cluster counting method has a bit larger rejection power.

Particle Identification Rejection factor with cluster counting method Pion registration efficiency as a function of electron efficiency for 1 and 2 layers of the detector. Cluster counting method. TRD with two detector layers (total thickness ~ 40 cm) allows to achieve rejection factor of ~ 50 for 90% electron efficiency. The radiator is one of the biggest issues for the compact detectors.

TR generation and absorption (test beam set-up). Particle ID. TR generation and absorption (test beam set-up). MC MC Average: 2.5 TR photons. No TR photons: 12.5% Number of photons at the radiator output Number of detected photons in one layer per event (test beam chamber)

TR generation and absorption. Particle ID. TR generation and absorption. MC MC 20 keV Spectrum TR absorbed in 1.6 cm thick chamber Spectrum TR absorbed in 4 cm thick chamber Number of detected photons in inserts. For 4 cm chamber 6% electrons have no TR cluster

TR generation and absorption Particle ID. TR generation and absorption For 1-2 layer detector the biggest problem is the compact radiator which should provide a soft photon spectrum. Be - Li materials? MC Polypropylene radiator Beryllium radiator Number of absorbed photons as a function of the pass in the gas in 1.7 cm chamber (70%Xe + 30% CO2).

TR generation and absorption Particle ID. TR generation and absorption Radiators: 17mm/100mm/9 cm MC Polypropylene Beryllium Spectrum of absorbed TR for different types of radiator. There is a way to get a compact radiator with good TR yield But much more optimisation and test work is required.

Technique used for the tacking property studies. How well we understand the results? The answer is in MC simulations which use the same technique. Reminder: Diffusion coefficient and threshold in MC were chosen to satisfy a track widths and number of fired pixels observed in test beam. Only tack projection to the pixel plane is used (no time information taken into account). Y X « f » MC What we can measure with the beam without reference detector? Represent the chamber as two independent interleaved units and measure parameters of two “tracks” (pseudo-tracks) reconstructed independently! Track 1 Track 2 Chamber 1 Chamber 2 A real resolution is about ½ of the measured one with this method.

Results strongly depend on the gas properties. Tacking. Results strongly depend on the gas properties. DME/CO2 (50/50), 19o incident angle Xe/CO2 (70/30), 19o incident angle Low diffusion gas Electron NO TR Electron with TR Apart of detector geometry a track reconstruction accuracy depends on diffusion, single electron efficiency and also on a presence of the TR clusters.

Test beam results: Tacking. Low diffusion gas. Particle Chip DME/CO2 (50/50), 20 mm drift, Incident angle of 10o. s = 0.6o s = 11.5 mm At 10o incident angle an angle measurement accuracy of 0.6o for the track projection would mean: At B=2 T this corresponds to 15% momentum measurement accuracy for Pt of 40 GeV with one layer of the GasPixel tracker

Test beam results: Tacking. Xe - mixture Xe mixture: Pions, 5 GeV,, 55 mm pixel, 240 mm diffusion. 25o of incident angle Intrinsic special accuracy of the gas pixel detector (MC and EXP). Spatial accuracy MC intrinsic MC based on pseudo tracks EXP Effective electronics threshold (primary electrons) Spatial accuracy (mm) Angular resolution Angular resolution (o) MC based on pseudo tracks MC intrinsic MC: for the diffusion coefficient factor of 2 less EXP Effective electronics threshold (primary electrons) Intrinsic angular resolution of the gas pixel detector. (MC and EXP). 30 mm space accuracy and 0.6 o angular resolution obtained in the experiment for Xe based mixture (without radiator) . It is a bit worse (~55 mm, 0.9o) for electrons with TR clusters but effect still to be studied in details

Conclusions. Gas pixel technology offers new possibilities for the tracking and particle ID detectors Combining these detectors in one will certainly reduce tracking properties for the particles which produce TR but still still very much adequate to the electron tracking. For the High Energy Physics Experiments data flow limitations require some data processing at the Front-End level. This also makes natural the use of these devices as L1 track trigger. Definitely more test–beam and MC studies are needed to understand all the details and accumulate knowledge which would allow to produce a real detector.