Optical performances of CKOV2

1. Gh. Grégoire Optical performances of CKOV2 1. Optical elements in relation with beam properties 2. Comparison of optical geometries 3. Optimization of light collection 4. Electron detection efficiency vs electronic threshold Frascati, MICE Collaboration meeting, 26 June 2005

2. Particle tracks Geant4 files generated by T.J. Roberts m+ande+tracks generated 1 mm ahead of CKOV2. TRD, Stage VI, Case 1 Configuration RF off Empty absorbers Particle samples 5527 muons from a simulation of the cooling channel 3312 electrons Muon decay in Tracker 2 2

3. Beam spots Muons at CKOV2 entrance Muons at CKOV2 exit Aerogel Exit window Projection on X-axis 3

4. Angular distributions • Muons are more focused than electrons • For electrons the divergence is larger if the parent m decays farther upstream 4

5. Momentum distributions T. Roberts T. Roberts Proposal (P. Janot) • Lower m momentum allows some freedom to choose the index of refraction of the radiator ! • The very low energy electrons have disappeared (all electrons are now fully relativistic). 5

6. Light yield for muons No light produced by muons for 1.02 < n < 1.04 Range chosen for this talk ! 6

7. Threshold curve for muons Upper limit of muon momenta Experimental fluctuations of index in a batch of a nominal value n Exp. distribution of index for aerogel n=1.03 FWHM  0.002 7

8. Pictorial view of optical elements Back mirror Particle entrance window Optical windows, Winston cones, PM’s Aerogel box Reflecting pyramid Front mirror + various small elements (clamping pieces for windows) Particle exit window 8

9. Design status - Choice of the shape of the reflecting pyramid with 12 cylindrical faces (this talk) - Internal walls of the aerogel box are covered by a diffuser - New shape of the reflecting pieces clamping the optical windows (to decrease the nr of trapped light rays) (instead of flat reflecting pieces) Some % more light collection 9

10. Aerogel and its container Aerogel tiles made by Matsushita Each 130 mm x 130 mm x 10 mm Hydrophobic aerogel (i.e. chemically modified) Number of tiles  320 ( Cost  260 € /tile ) for 100 mm thickness Aerogel container Polygonal honeycomb box Inside walls covered with a diffusing paint 10

11. Optical processes in aerogel H. Van Hecke (LANL), RHIC Detector upgrades workshop, BNL, 14 Nov 2001 P.W. Paul, PhD thesis (Part 2), The aerogel radiator of the Hermes RICH, CalTech, 12 May 1999 Photon production Since b = constant for small energy losses and n = constant at fixed l and for an homogeneous material, the photon source distribution is uniform along the track Transmission Experimentally « Clarity » C = 0.01 mm4 cm-1 ( = not scattered ! ) A ~ 0.96 (Hermes) ( in the UV and visible ranges ) Rayleigh-Debye scattering Here (MICE) Lscat = 25.6 mm at l = 400 nm Dipole-type angular distribution Absorption Experimentally very small (Hermes) Here 4% per cm 11

12. Simulation of aerogel All simulations performed at l = 400 nm for aerogels with n = 1.02 and n = 1.04 1. Uniform distribution of light ray sources (around a cone) along the thickness of the radiator NB. Relative phase between rays not taken into account since we neglect detailed polarization effects in reflections. 2. Scattering probability varies as where u is the distance along the ray path with Lscat = 25.6 mm 3. Isotropic angular distribution for scattering (approximation of the dipole distribution) 4. Transmission probability varies as where u is the distance along the ray path 5. Absorption Labsorption = 245 mm 12

13. Optical properties Also taken into account 1. Reflectivities of mirrors Front mirror 96 % at l = 400 nm Reflecting 12-sided pyramid Winston cones 2. Bulk transmittance Optical windows 90 % Labsorption = 94.9 mm 3. Reflectances and transmittances at interfaces Aerogel-air (angle dependent / unpolarized light) Opt. windows - air PMT - air 4. Diffuse scattering (Lambertian for 50% of the rays) Walls of the aerogel box i.e. 50 % undergo specular reflection 50 % are diffused around specular with a cos q distribution 13

14. Bulk transmission of optical windows Schott optical glasses For 10 mm thickness BK7 B270 choosen 90% transmission Cost ! 14

15. Mirrors • Substrate: polycarbonate (Lexan) 3 mm sheets supported by Honeycomb panels Very stiff at room T (but thermally deformable) Good surface properties of raw material and experience as a mirror support (HARP) • Reflecting layer: multilayer [ Aluminium + SiO2 + Hf O2 ]  Very good reflectivity (A. Braem/CERN) 15

16. Winston cones Raw material: transparent PMMA (lucite) milled on a CNC lathe polishing Reflecting surface: same as for mirrors Measurements from HARP at different points on the surface 16 ( Reflecting layer Al + SiO2 )

17. Photomultipliers EMI 9356 KA low background selected tubes (from Chooz and HARP) 8 " diam hemispherical borosilicate window / High QE 30% Bialkali photocathode / 14 stages / High gain 6.7 x 107 (at 2300 V) Positive HV supply ! (i.e. photocathode at ground and anode at HV !) Quantum efficiency (Electron Tubes Ltd) Transmission through window 17

18. Theoretical efficiency e Review of particle properties, July 2004 For a single particle loosing an energy DE in the radiator, with K= 370 cm-1 eV-1 L = thickness of radiator (cm) E = photon energy where e(E) = efficiency for collecting light and converting it in photoelectrons Threshold: or Since (sin qc) is slowly dependent on E (above threshold) with For a typical PMT working in the visible and near UV N0 = 90-100 cm-1 N0 already “contains” the Q.E. of a (typical) PMT and assumes all photons are collected ! 18

19. Realistic efficiency e Since the geometrical photon collection probability substantially varies for different tracks, we use where egeom is the geometric light collection probability ( probability that a given light ray reaches a photodetector detector) ephys is the physical attenuation of light in the device ( due to reflections, transmissions, absorptions) 19

20. Plan of the work 2 steps 1. Optimization of the geometrical configuration i.e. shooting Cherenkov photons from the aerogel for various shapes of reflectors Compare the probabilities of reaching the PMTs / minimizing the attenuation - Best light collection probability egeom among different geometries! - Minimal attenuation Minimize number of reflections/transmissions ( or ray path length!) i.e. maximize 2. Electron detection efficiency For the best geometry found in step 1, track the photons for each incident electron 20

21. Geometries # 1 # 2 # 3 12 flat faces at 45° 12 cylindrical faces 12 spherical faces (R = 843 mm) (R = 843 mm) 21

22. Increasing focusing power (still keeping mechanical simplicity …) Optical configurations Tested: three optical configurations externally identical (with the same external envelope! ) with the same optical elements except the reflecting pyramid # 1 12-sided pyramid with flat faces and back mirror # 2 12-sided pyramid with cylindrical faces (no back mirror needed) # 3 12-sided pyramid with spherical faces (no back mirror needed) 22

23. Sequence of operations Mechanical constraints Optical/material constraints Physical constraints This presentation Optical performances Analysis of ray data base Detection performances Mechanical design Zemax Engineering v. Feb 2005 Autocad 2000 Mathematica 3.0 iteration Generation of Cherenkov photons 23 MC files (T.J. Roberts)

24. Typical event (config. #1) No scattering Lots of rays 35 photons from a single electron - bouncing back and forth between front and back mirrors - trapped and/or absorbed inside aerogel box, … Losses ! Low light collection efficiency 24

25. Track #01 (config. #1) bouncing back and forth between front and back mirrors No scattering Incidence angle on the pyramid is too small and the initial ray gets reflected away from the PM 25

26. Performances of configuration #1 Pyramid with flat faces egeom = 0.61 26

27. Typical event (config. #2) 35 photons from a single electron ( same event as for config. #1 ) No scattering Only one ray is lost ! 27

28. Same event (config.#2) No scattering 3 detectors hit ! Some ring imaging clearly visible on the screen display . 28

29. Performances of configuration #2 Pyramid with curved cylindrical faces faces egeom = 0.67 29

30. Geometrical efficiency egeom Tracking 2000 rays best ! 30

31. Physical attenuation (no scattering) for l = 400 nm 31

32. ephys best ! 32

33. Conclusions of optimization process The configuration with cylindrical faces for the reflecting pyramid is better < egeom > = 0.67 < ephys > =0.75 (most probable values) < e> = < egeom > *< ephys > = 0.50 so that But, at this stage, there is yet no correlation between the photons and the corresponding electron The cylindrical geometry is choosen for the rest if this work ! 33

34. Two typical events Without bulk scattering in the aerogel With bulk scattering in the aerogel for the same two events There are no definite hit-PMT patterns for each particle entrance coordinates (position/direction) ! 34

35. i.e. generate 1 light ray of unit intensity (I0 = 1) per photoelectron and then track each ray. Structure of data 1. Tom’s electron files (cm, MeV/c) 2. Additional particle and Cherenkov effect data 3. Generation of Nc photons around a cone of angle qc (mm) ….. ….. ….. ….. 35 lines ….. ….. 35 photons with same x-y origin and unit intensity, starting from different z-positions, and having different directions along a cone of angle qc 35

36. Optical tracking database A typical good event Starting point of 7th photon (see previous transparency) Optical elements hit R = reflected T= transmitted S = scattered X= terminated B = bulk scattered Z = tracking error Physical path length between successive elements … ending on the PMT at 9 o’clock Physical attenuation 36

37. Analysis of database (1) For a given electron with (xe, ye, ze, px, py, pz) which generates Nc photelectrons of intensity I0=1, fill a row for each detected photon (i.e. one which reaches a PMT) Total number of photons detected Total detected intensity 37

38. Analysis of database (2) For this electron with (xe, ye, ze, px, py, pz) which generates Nc photelectrons of intensity I0, we detected Ndet photons and the total detected intensity is Itot - geometrical detection efficiency - global detection efficiency (Total detected intensity) - relative individual PMT signal - accepted individual PMT signal 38

39. Global efficiency About 50 % of the Cherenkov photons reach the PMTs n = 1.04 Global efficiency =  20 % of the Cherenkov light intensity (in relative photoelectron units) reach the PMTs n = 1.04 It does not change much for n = 1.02 (except for a small effect due to the different opening angle of the Cherenkov cone). 39

40. Efficiency versus momentum Momentum dependence n = 1.04 n = 1.02 - There is no obvious momentum dependence 40

41. Efficiency versus particle impact n = 1.04 Radial dependence - Trend of smaller efficiencies for larger impact radius n = 1.04 Azimutal dependence - Very sensitive to axial misalignments 41

42. PMT response table For example, for the first four particles (with n=1.02), we get not detected detected Assume that at least one PMT gets a signal equal to or greater than a given electronic threshold of, let’s say 2 photoelectrons 42

43. Preliminary optical assessment Assuming each PMT has a detection threshold of 2 photoelectrons Giving no signal in all 12 PMTs - It is obvious that n = 1.04 is the preferred index of refraction of the aerogel - What are the spatial and momentum distributions of undetected events ? 43

44. Distributions of undetected events x-y n = 1.02 x-y - no specific insensitive region n = 1.04 - no specific momentum range Most probably due to trapping inside a symmetric vessel and/or excessive path lengths. 44

45. Electron inefficiency versus threshold 45

46. What’s next ? 0. Comments, questions and criticisms 1. Update the CKOV2 part of the Technical Reference Document 2. Resume the final (?) mechanical drawings 46