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

 Study of the unstable nuclei Incompressibility, Gamow-Teller strength, etc.  Forward scattering Need for identifying the  L of the reaction. ← For each  L, shape of d  /d  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  L of the reaction, Angular resolution : < 7.45mrad(RMS) Energy resolution : < 10%(RMS) Recoiled particle (  ) Beam ( 78 Ni :200MeV/u) Helium gas Motivation 3

Design of Active-Target GEM-TPC Beam Pad Recoil 25cm GEM (10cm×10cm) Beam Wire 4cm  Active-Target TPC Reaction occurs inside TPC. (Target is gas.) → Material budget can be smaller  Gas Depend on target → 4 He, 3 He, d 2 etc.  Mask the beam track area TPC can be operate in high rate beam condition (~ 10 6 Hz). (Rate of recoil nuclei has to be taken into account.)  Use of GEM GEM can multiply electron at higher rate than wire.  Pad shape : rectangular triangle Charge ratio of the neighboring pads (perpendicular to drift direction) Arrival time(drift direction)  Field cage Double layered, 2.5mm pitch mm

5  Following items were evaluated Distortion of electric field by ions created by beam Position resolution, angular resolution  Gas He(90%) + CO 2 (10%) was used for simulation. Electric field : 1.0 [kV/cm] Ion mobility : 2.5×10 3 [cm 2 ·Torr·V -1 ·s -1 ] Pressure : 760 [Torr] Temperature : 300 [K] Transverse diffusion coefficient : 250  m for 1cm  Electron velocity : 3 [cm/  s]  Ion velocity : 3.3×10 -3 [cm/  s] Simulation study

6 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. y=24cm Field cage Shielding wire mesh

7 : Without beam : Without shielding wire : With shielding wire (2.5mm pitch) 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) Active area of GEM Beam Beam rate : 10 7 cps Energy loss : 300 [keV/mm] ~ 10 4 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 Position difference Field cage x

8 Position resolution  Position derivation Position is derived by charge ratio of neighboring pads.  Pad size : ×16.45 mm 2  Recoil particle :   Energy loss 10 [electrons/mm] 50 [electrons/mm] 100 [electrons/mm] 190 [electrons/mm] ←  with 30MeV in He/CO 2 (5%) 300 [electrons/mm] → Position resolution : < 300  m (RMS) for energy loss > 100 [electrons/mm] Center : 10 [electrons/mm] : 50 [electrons/mm] : 100 [electrons/mm] : 190 [electrons/mm] : 300 [electrons/mm] Edge of pad Recoiled particle

9 Angular resolution Angular resolution : ~ 5 mrad < 7.45 mrad  = -30°  = 0°  = 30° z x Recoil particle

Date : Dec / 2009 Accelerator : 12UD Pelletron Scatterer Au (thickness : 2  m) Scattering angle ： 7° 4 He TPC Scintillator Beam Particle : 4 He 2+  Energy : 30MeV Beam rate : ~ 10 2 cps Q D Q 10 Collimator : 1mm  Performance Dipole magnet TPC Quadrupole magnet Au 10

Setup Gas : He(95%) / CO 2 (5%) (1 atm) E drift : 700 [V/cm] Drift velocity : 2 [cm/  s] Diffusion (transverse) : 250 [  m/1cm drift] Diffusion (longitudinal) : 180 [  m/1cm drift] Voltage applied to GEM : 450 V, 420 V, 390 V → Gas gain : Pad size : 16.45×16.45 mm 2 (Only 36 pads are used) Readout : FADC (SIS3301; 100MHz) Trigger system : TPC (self-trigger; signal sum for 4 pads) beam

Typical event Beam Inclined incidence

13 Position resolution 1 Perpendicular to drift direction Drift direction Position resolution is less than 700  m by charge division and about 50  m by arrival time 3D position derivation Charge ratio of the neighboring two pads.(2D) Arrival time.(drift direction)

14 Position resolution 2 Dependence of the drift length Drift direction Perpendicular to drift direction Charge ratio : no dependence of drift length. Arrival time : position resolution is improved as drift length become shorter.

15 Position resolution 3 Perpendicular to drift direction Drift direction Dependence of the gas gain Position resolution is improved as gas gain become larger.

16 Energy resolution  ~ 3.3% Energy resolution ~ 3.3 % < 10 % Particle :  with ~ 5.8 MeV/u → Energy deposit at field cage : ~ 700 keV

17 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 : < 700  m Energy resolution: < 3.3 % (  ) for  with 5.8MeV/u

End

19 Position resolution  Position derivation Position is derived by charge ratio of neighboring pads.  Recoil particle  (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 : ~ 300  m z x Center : 8.3mm(x)×25mm(z) : 16.6mm(x)×25mm(z) : 20mm(x)×20mm(z) : 16.6mm(x)×16.6mm(z) Edge of pad Recoil particle

20 Typical event 2 Use degrader to stop beam inside field cage Beam scatters inside field cage