D. Attié, P. Colas, Giganon, I. Giomataris, V. Lepeltier, M. Was

1. Gain measurement in various gas mixtures and optimization of the stability • D. Attié, P. Colas, • Giganon, I. Giomataris, • V. Lepeltier, M. Was TPC Jamboree, Aachen 14-16 March 2007 TPC Jamboree, Aachen – March 2007

2. Overview • Systematic measurements to provide gain curves • Simulations using GARFIELD/Magboltz • Comparisons between measurements and simulations for gain curves • How to optimize the detector stability for ILC-TPC ? TPC Jamboree, Aachen – March 2007

3. Micromegas: gas amplification system • Micromegas is a parallel plate gas avalanche detector with a small gap • Micromesh held by pillars 50 m above the anode plane Charge particle Drift electrode Edrift = 100-1kV/cm Conversion gap ~ 13 mm Amplification gap ~ 50 mm Micromesh ― Pillars ― Eamp = 50k-100 kV/cm Pads TPC Jamboree, Aachen – March 2007

4. Setup • Goals: • find gas mixture comfortable gain margin to use into a Micromegas TPC • know as much as possible the maximum gain (to avoid spark) • Description: • Transparent plastic box of • 23 cm x 23cm x 8 cm size • “Standard” 50 mm mesh of • 10 cm x 10 cm size • Sources: • - Fe 55 (5.9 keV) • - COOL-X (8.1 keV) • Monitoring of: • - pressure • - H2O • Gas mixing system with • triple mixture available TPC Jamboree, Aachen – March 2007

5. Gain measurements : simulation Drift Velocitiy Longitudinal diffusion Transversal diffusion Edrift ~ 220 V/cm Simulations from GARFIELD/MAGBOLTZ TPC Jamboree, Aachen – March 2007

6. Gain measurements : simulation Choice : Edrift = 150 V/cm Simulations from GARFIELD/MAGBOLTZ TPC Jamboree, Aachen – March 2007

7. Gain measurements : simulation Choice : Edrift = 300 V/cm Simulations from GARFIELD/MAGBOLTZ TPC Jamboree, Aachen – March 2007

8. Mixtures of gases containing argon: gain curves iC4H10 Cold gases: CO2, CH4 C2H6 Micromegas Mesh : 50 mm gap of 10x10 cm² size TPC Jamboree, Aachen – March 2007

9. Resolution vs. gain Argon/Isobutane • Best RMS for a gain • between 3.103 & 6.103 • Degradation increase in inverse • proportion to the quencher TPC Jamboree, Aachen – March 2007

10. Mixtures of gases containing argon: gain curves Micromegas Mesh : 50 mm gap of 10x10 cm² size TPC Jamboree, Aachen – March 2007

11. The Townsend coefficient a • We assumes homogeneous field gain vs. amplification gap d: • G(d) = eada = a (E) • a is calculated by Magboltz • Rose-Korff approximation: • a = P×A×e-B(P/E) • A, B: constants to be determined in measurements • But, in reality field could not be very homogeneous, not for very small gaps (i.e. high field) a = a(E(z)) • G(d) = ea[E(z)]dz Only applicable to fields E << 100 kV/cm. TPC Jamboree, Aachen – March 2007

12. Townsend coefficients measurements G = N/N0 = exp[a×d] With a = A×P×exp[-B×P/E] Argon-Isobutane TPC Jamboree, Aachen – March 2007

13. Ar/CH4 mixtures: measurements G < 104 TPC Jamboree, Aachen – March 2007

14. Ar/CH4: simulation vs. measurements Good agreement: measurements are Consistent with simulations, TPC Jamboree, Aachen – March 2007

15. Ar/CO2 mixtures: measurements G ~ 2.103 TPC Jamboree, Aachen – March 2007

16. Ar/CO2: simulation vs. measurements Agreement with the slope but not with the absolute gain TPC Jamboree, Aachen – March 2007

17. TDR: Ar/CH4/CO2 (92:5:3) G < 3.103 TPC Jamboree, Aachen – March 2007

18. TDR: Ar/CH4/CO2 (92:5:3) TPC Jamboree, Aachen – March 2007

19. Ar/C2H6: simulation vs. measurements : G < 105 UV photons ? TPC Jamboree, Aachen – March 2007

20. Ar/C2H6: simulation vs. measurements : Agreement with the slope at low fields but not with the absolute gain and the photons effects UV photons ? Penning effect TPC Jamboree, Aachen – March 2007

21. Ar/iC4H10 mixtures: measurements G ~ 105 TPC Jamboree, Aachen – March 2007

22. Ar/iC4H10: simulation vs. measurements Agreement with the slope at low fields but not with the absolute gain and the photons effects TPC Jamboree, Aachen – March 2007

23. Ar/iC4H10+ 3% CF4: simulation vs. measurements + 3% CF4 TPC Jamboree, Aachen – March 2007

24. Penning effect ThePenning mixtureis a mixture of inert gas with another gas, that has lower ionization voltage than either of its constituents Penning effect : Is a energy transfer from molecule A to molecule B which have lower ionization potential than the first excited state of molecule A. TPC Jamboree, Aachen – March 2007

25. Penning effect ThePenning mixtureis a mixture of inert gas with another gas, that has lower ionization voltage than either of its constituents Penning effect : Is a energy transfer from molecule A to molecule B which have lower ionization potential than the first excited state of molecule A. TPC Jamboree, Aachen – March 2007

26. Penning effect TPC Jamboree, Aachen – March 2007

27. Penning effect TPC Jamboree, Aachen – March 2007

28. Penning effect • Possibilities of Penning mixtures: • Ne-Ar • Ar-Xe • Ar-Acetylene • Ne-Xe • Ar-impurities who have lower Ei ?! • … TPC Jamboree, Aachen – March 2007

29. Estimation of the gain Gain depends on: • Gas mixtures properties (Penning effect, ionization energies) • Detector geometry • Gas pressure • High voltage • Simulation: suppose that the detector is perfect, pressure and high voltage stable and constant. • Need to take care of Penning effect but the energy transfer from molecule A to molecule B in gases is not well simulated • It may be possible to estimate this effect using the ionization Pioni and excitation Pexci probabilities extracted from MAGBOLTZ • We suppose that a fraction of photons from molecule A deexcitation ionize molecule B with the measured probability of transfert Ptrans. • For the Penning mixture, the Townsend coefficient αcould be replaced by αeff: • Pexci,A excitation probability for molecule A • Pioni,B ionization probability for molecule B • Pioni,B measured probability of transfert TPC Jamboree, Aachen – March 2007

30. Cross section of gas Drift velocity and diffusion coefficients depend on the precision δof the gas cross sections. This precision decreases with the complexity of the rotational and vibrational cross sections. • He: δ < 0.5 % • Ar, Xe, Ne: δ < 1 % • Hydrocarbons with small nC (CH4, C2H6) : δ ~ 1 % • CO2, CF4: δ ~ 1-2 % • iC4H10: δ ~ 3 % • Heavier gases: δ > 3 % TPC Jamboree, Aachen – March 2007

31. Conclusions • Systematic measurements will help to choose and optimize the detector stability (best s, low voltage) for ILC-TPC • Simulations using Magboltz work well for drift velocities and diffusion • But, need to add correction factor for the Townsend coefficient used for gain calculation TPC Jamboree, Aachen – March 2007

32. TPC Jamboree, Aachen – March 2007

33. Pressure monitoring in Saclay TPC Jamboree, Aachen – March 2007

34. Gain vs. amplification gap TPC Jamboree, Aachen – March 2007

35. Gain vs. amplification gap TPC Jamboree, Aachen – March 2007

36. Gain vs. amplification gap TPC Jamboree, Aachen – March 2007

37. Gain vs. amplification gap TPC Jamboree, Aachen – March 2007

38. Gain vs. amplification gap TPC Jamboree, Aachen – March 2007

39. Micromegas gain • The resistive foil prevents charge accumulation, thus prevents sparks • Gains higher than obtained with standard anodes can be reached ― Without resistive foil ― With resistive foil (current) ArIso5% TPC Jamboree, Aachen – March 2007