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Design and performance of Active Target GEM-TPC R. Akimoto, S. Ota, S, Michimasa, T. Gunji, H. Yamaguchi, T. hashimoto, H. Tokieda, T. Tsuji, K. Kawase, H. Hamagaki, T. Uesaka, S. Kubono (Center for Nuclear Study, University of Tokyo) T. Kawabata (Kyoto), T. Isobe (Riken), A. Ozawa, H. Suzuki, D. Nagae, T. Morimoto, Y. Ito, Y. Ishibashi, H. Oishi (Tsukuba) 1 Contents • Motivation • Design of TPC • Simulation for the performance of TPC • Performance test 2 Recoiled particle (a) Motivation Beam (78Ni :200MeV/u) Study of the unstable nuclei Helium gas Incompressibility, Gamow-Teller strength, etc. Forward scattering • Need for identifying the DL of the reaction. ← For each DL, shape of ds/dW is very different. • Measurement of the recoiled light nuclei can lead to precise measurement. → Energy of the recoiled nuclei is very small. → Active-Target TPC Requirement Following spec are required to identify the DL of the reaction, • Angular resolution : < 7.45mrad(RMS) • Energy resolution : < 10%(RMS) 3 Design of Active-Target GEM-TPC Beam Active-Target TPC Reaction occurs inside TPC. (Target is gas.) → Material budget can be smaller Gas Recoil 25cm 4 3 Depend on target → He, He, d2 etc. Mask the beam track area TPC can be operate in high rate beam condition (~ 106Hz). Pad (Rate of recoil nuclei has to be taken into account.) GEM Use of GEM GEM can multiply electron at higher rate than wire. (10cm×10cm) Pad shape : rectangular triangle • Charge ratio of the neighboring pads (perpendicular to drift direction) Wire 16.45mm • Arrival time(drift direction) 4cm Beam Field cage Double layered, 2.5mm pitch. 4 Simulation study Following items were evaluated • Distortion of electric field by ions created by beam • Position resolution, angular resolution Gas He(90%) + CO2(10%) was used for simulation. • Electric field : 1.0 [kV/cm] • Ion mobility : 2.5×103[cm2·Torr·V-1·s-1] • Pressure : 760 [Torr] • Temperature : 300 [K] • Transverse diffusion coefficient : 250mm for 1cm Electron velocity : 3 [cm/ms] Ion velocity : 3.3×10-3 [cm/ms] 5 Distortion of electric field by ions High beam rate condition • When the beam rate is high, ions (electrons) created by beam are piled up, and distorts the electric field. • Shielding wire is used to suppress the effect from distortion. Effect of distortion of electric field • Drift electrons and evaluate the position difference. • The electric field was simulated using Garfield 9. 6 Field cage Field cage y=24cm Shielding wire mesh Field cage Field cage Position difference : Without beam : Without shielding wire : With shielding wire (2.5mm pitch) x Beam • Beam rate : 107 cps • Energy loss : 300 [keV/mm] ~ 104 ions/mm ← Ni with 50 [MeV/u] • Beam spread : 5cm (RMS) for drift direction 1cm (RMS) for other direction ← Dispersion matching mode beam in RIBF Active area of GEM • Without shield wire : Position difference is over 1mm • Shielding wire pitch : 2.5mm : Maximum position difference is 0.3mm → Change of track angle is less than 3mrad.(for flight length : 10cm) 7 Recoiled particle Position resolution Position derivation Position is derived by charge ratio of neighboring pads. Pad size : 16.45 ×16.45 mm2 : 10 [electrons/mm] Recoil particle : a : 50 [electrons/mm] Energy loss : 100 [electrons/mm] • 10 [electrons/mm] : 190 [electrons/mm] : 300 [electrons/mm] • 50 [electrons/mm] •100 [electrons/mm] • 190 [electrons/mm] ← a with 30MeV in He/CO2(5%) • 300 [electrons/mm] → Position resolution : < 300mm (RMS) for energy loss > 100 [electrons/mm] Edge of pad Center Edge of pad 8 Recoil particle Angular resolution x z q = -30° q = 0° q = 30° Angular resolution : ~ 5 mrad < 7.45 mrad 9 Performance test @Tsukuba 4He • Date : Dec. 1 - 3 / 2009 • Accelerator : 12UD Pelletron Beam • Particle : 4He2+ • Energy : 30MeV • Beam rate : ~ 102 cps Scatterer • Au (thickness : 2mm) • Scattering angle : 7° Q Quadrupole magnet Dipole magnet Au D Q TPC Collimator : 1mmf Scintillator TPC Quadrupole magnet 10 10 16.45 Setup beam 16.45 • Gas : He(95%) / CO2(5%) (1 atm) • Edrift : 700 [V/cm] Drift velocity : 2 [cm/ms] Diffusion (transverse) : 250 [mm/1cm drift] Diffusion (longitudinal) : 180 [mm/1cm drift] • Voltage applied to GEM : 450 V, 420 V, 390 V → Gas gain : 102 - 103 • Pad size : 16.45×16.45 mm2 (Only 36 pads are used) • Readout : FADC (SIS3301; 100MHz) • Trigger system : TPC (self-trigger; signal sum for 4 pads) 11 Typical event Beam Beam Inclined incidence Position resolution 1 3D position derivation • Charge ratio of the neighboring two pads.(2D) • Arrival time.(drift direction) Perpendicular to drift direction Drift direction Position resolution is less than 700mm by charge division and about 50mm by arrival time 13 Position resolution 2 Dependence of the drift length Perpendicular to drift direction Drift direction • Charge ratio : no dependence of drift length. • Arrival time : position resolution is improved as drift length become shorter. 14 Position resolution 3 Dependence of the gas gain Perpendicular to drift direction Drift direction Position resolution is improved as gas gain become larger. 15 Energy resolution Particle : a with ~ 5.8 MeV/u → Energy deposit at field cage : ~ 700 keV s ~ 3.3% Energy resolution ~ 3.3 % < 10 % 16 Summary • We are developing Active-Target TPC for study of nuclear property using unstable nuclei. Detect track and energy of recoiled particle with very low energy. (~ 1MeV/u) • Position difference in high beam rate condition : < 0.3mm → Can be used in high beam rate condition • Performance test has done. Position resolution : < 700mm Energy resolution: < 3.3 % (s) for a with 5.8MeV/u 17 End Recoil particle Position resolution x z Position derivation Position is derived by charge ratio of neighboring pads. : 8.3mm(x)×25mm(z) : 16.6mm(x)×25mm(z) : 20mm(x)×20mm(z) : 16.6mm(x)×16.6mm(z) Recoil particle a (energy : < 30 MeV/u) Four kinds of pad size were used • 8.3mm(x)×25mm(z) • 16.6mm(x)×25mm(z) • 20mm(x)×20mm(z) • 16.6mm(x)×16.6mm(z) → 16.6mm×16.6mm : ~ 300mm Edge of pad Center Edge of pad 19 Typical event 2 Beam scatters inside field cage Use degrader to stop beam inside field cage 20