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

Particle tracks Geant4 files generated by T.J. Roberts  + and e + tracks generated 1 mm ahead of CKOV2. Configuration TRD, Stage VI, Case 1 RF off Empty absorbers 2 muons5527from a simulation of the cooling channel electronsMuon decay in Tracker Particle samples

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

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

Momentum distributions Proposal (P. Janot) Lower  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). T. Roberts 5

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

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

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

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

Inside walls covered with a diffusing paint Aerogel and its container Aerogel tiles made by MatsushitaEach 130 mm x 130 mm x 10 mm Hydrophobic aerogel (i.e. chemically modified) Number of tiles  320( Cost  260 € /tile ) Aerogel containerPolygonal honeycomb box 10 Aerogel typen=1.02n=1.04 Average nr of photoelectrons/electron3771 Density (g cm -3 ) Average Cherenkov angle (degrees) Nr of tracked light rays for 100 mm thickness

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 Transmission Rayleigh-Debye scattering Absorption « Clarity » C = 0.01  m 4 cm -1 Photon production Since  = constant for small energy losses and n = constant at fixed and for an homogeneous material, the photon source distribution is uniform along the track Experimentally A ~ 0.96 ( in the UV and visible ranges ) Here (MICE)L scat = 25.6 mm at = 400 nm (Hermes) ( = not scattered ! ) Experimentally very small (Hermes) Dipole-type angular distribution Here 4% per cm 11

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

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

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

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 + SiO 2 + Hf O 2 ] Very good reflectivity (A. Braem/CERN) 15

Winston cones Raw material: milled on a CNC lathe Reflecting surface: polishing transparent PMMA (lucite) same as for mirrors Measurements from HARP at different points on the surface ( Reflecting layer Al + SiO 2 ) 16

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 10 7 (at 2300 V) Positive HV supply ! (i.e. photocathode at ground and anode at HV !) Quantum efficiency (Electron Tubes Ltd) 17 Transmission through window

Theoretical efficiency  For a single particle loosing an energy  E in the radiator, Review of particle properties, July 2004 K= 370 cm - 1 eV -1 L = thickness of radiator (cm) E = photon energy with Since (sin  c ) is slowly dependent on E (above threshold) where  (E) = efficiency for collecting light and converting it in photoelectrons with For a typical PMT working in the visible and near UVN 0 = cm -1 Threshold:or N 0 already “contains” the Q.E. of a (typical) PMT and assumes all photons are collected ! 18

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

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

Geometries # 1 # 2# 3 12 flat faces at 45° 12 cylindrical faces12 spherical faces (R = 843 mm) 21

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

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

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

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

Performances of configuration #1 Rays emitted2000 Rays detected1210 Average nr of reflections 4.68 Most probable nr of reflections 2 Average path length 2065 mm Most probable path length 1100 mm  geom = 0.61 Pyramid with flat faces 26

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

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

Performances of configuration #2 Rays emitted2000 Rays detected1347 Average nr of reflections3.31 Most probable nr of reflections2 Average path length1647 mm Most probable path length1100 mm  geom = 0.67 Pyramid with curved cylindrical faces faces 29

Geometrical efficiency  geom Tracking 2000 rays Type of reflecting pyramidFlat faces Cylindrical faces Spherical faces Average nr of reflections Most probable nr of reflections222 Average path length2065 mm1647 mm1774 mm Most probable path length1100 mm Geometrical light collection efficiency  geom best ! 30

Physical attenuation (no scattering) for = 400 nm 31

 phys Type of reflecting pyramidFlat faces Cylindrical faces Spherical faces Average attenuation factor Most probable attenuation factor0.750 best ! 32

Conclusions of optimization process The configuration with cylindrical faces for the reflecting pyramid is better   geom > = 0.67   phys > = 0.75 so that   > =   geom > *   phys > = 0.50 (most probable values) 33 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 !

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 photons with same x-y origin and unit intensity, starting from different z- positions, and having different directions along a cone of angle  c Structure of data 1. Tom’s electron files 2. Additional particle and Cherenkov effect data 3. Generation of N c photons around a cone of angle  c ….. 35 lines (cm, MeV/c) (mm) i.e. generate 1 light ray of unit intensity (I 0 = 1) per photoelectron and then track each ray. 35 …..

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

Analysis of database (1) For a given electron with (x e, y e, z e, p x, p y, p z ) which generates N c photelectrons of intensity I 0 =1, fill a row for each detected photon (i.e. one which reaches a PMT) Light ray detected Index of PMT hit Detected intensity 1h1h1 I1I1 2h2h2 I2I2 3h3h3 I3I3 4h4h4 I4I4 ……… ihihi IiIi ……… nhnhn InIn N det = n Total number of photons detected Total detected intensity 37

Analysis of database (2) - geometrical detection efficiency - global detection efficiency For this electron with (x e, y e, z e, p x, p y, p z ) which generates N c photelectrons of intensity I 0, we detected N det photons and the total detected intensity is I tot - relative individual PMT signal (Total detected intensity) - accepted individual PMT signal 38

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

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

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

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

Preliminary optical assessment Index Photon sample Electrons not detected Particle Sample Detection inefficiency n = % n = % - 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 Assuming each PMT has a detection threshold of 2 photoelectrons Giving no signal in all 12 PMTs

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

Electron inefficiency versus threshold Electronic Threshold (p.e.) n = 1.02n =

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