Optical performances of CKOV2

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Optical performances of CKOV2
Gh. Grégoire
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
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Particle tracks
Geant4 files generated by T.J. Roberts
m and e tracks generated 1 mm ahead of CKOV2.
TRD, Stage VI, Case 1
Configuration
RF off
Empty absorbers
Particle samples
muons
5527
from a simulation of the cooling channel
electrons
Muon decay in Tracker 2
3312
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Beam spots
Muons at CKOV2 entrance
Muons at CKOV2 exit
Aerogel
Exit window
Projection on X-axis
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Angular distributions

• Muons are more focused than electrons

• For electrons the divergence is larger if the
  parent m decays farther upstream

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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).

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Light yield for muons
No light produced by muons for 1.02 lt n lt 1.04
Range chosen for this talk !
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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 n1.03
FWHM ? 0.002
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Pictorial view of optical elements
Back mirror
Particle entrance window
Optical windows, Winston cones, PMs
Aerogel box
Reflecting pyramid
Front mirror
various small elements (clamping pieces for
windows)
Particle exit window
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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
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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 )
Aerogel type n1.02 n1.04
Average nr of photoelectrons/electron 37 71
Density (g cm-3) 0.072 0.1434
Average Cherenkov angle (degrees) 11.4 15.9
Nr of tracked light rays 121 125 235 615
for 100 mm thickness
Aerogel container
Polygonal honeycomb box
Inside walls covered with a diffusing paint
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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
 Clarity  C 0.01 mm4 cm-1
Experimentally
( 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
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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
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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
Opt. windows - air
(angle dependent / unpolarized light)
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
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Bulk transmission of optical windows
Schott optical glasses
For 10 mm thickness
BK7
B270 choosen
90 transmission
Cost !
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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)
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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
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( Reflecting layer Al SiO2 )
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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
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Theoretical efficiency e
Review of particle properties, July 2004
For a single particle loosing an energy DE in the
radiator,
K 370 cm-1 eV-1
with
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 !
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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)
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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
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Geometries
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12 flat faces at 45
12 cylindrical faces
12 spherical faces
(R 843 mm)
(R 843 mm)
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Optical configurations
Tested three optical configurations
externally identical (with the same external
envelope! )
with the same optical elements except the
reflecting pyramid
12-sided pyramid with flat faces and back mirror
1
12-sided pyramid with cylindrical faces (no back
mirror needed)
2
12-sided pyramid with spherical faces (no back
mirror needed)
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Sequence of operations
Mechanical constraints
Optical/material constraints
Physical constraints
This presentation
Optical performances
Detection performances
Analysis of ray data base
Mechanical design
Zemax Engineering v. Feb 2005
Autocad 2000
Mathematica 3.0
iteration
Generation of Cherenkov photons
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MC files (T.J. Roberts)
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Typical event (config. 1)
Lots of rays
No scattering
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
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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
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Performances of configuration 1
Pyramid with flat faces
Rays emitted 2000
Rays detected 1210
Average nr of reflections 4.68
Most probable nr of reflections 2
Average path length 2065 mm
Most probable path length 1100 mm
egeom 0.61
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Typical event (config. 2)
35 photons from a single electron ( same event as
for config. 1 )
No scattering
Only one ray is lost !
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Same event (config.2)
No scattering
3 detectors hit !
Some ring imaging clearly visible on the screen
display .
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Performances of configuration 2
Pyramid with curved cylindrical faces faces
Rays emitted 2000
Rays detected 1347
Average nr of reflections 3.31
Most probable nr of reflections 2
Average path length 1647 mm
Most probable path length 1100 mm
egeom 0.67
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Geometrical efficiency egeom
Tracking 2000 rays
Type of reflecting pyramid Flat faces Cylindrical faces Spherical faces
Average nr of reflections 4.68 3.31 3.69
Most probable nr of reflections 2 2 2
Average path length 2065 mm 1647 mm 1774 mm
Most probable path length 1100 mm 1100 mm 1100 mm
Geometrical light collection efficiency egeom 0.61 0.67 0.67
best !
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Physical attenuation (no scattering)
for l 400 nm
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ephys
Type of reflecting pyramid Flat faces Cylindrical faces Spherical faces
Average attenuation factor 0.685 0.711 0.689
Most probable attenuation factor 0.750 0.750 0.750
best !
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Conclusions of optimization process
The configuration with cylindrical faces for the
reflecting pyramid is better
lt egeom gt 0.67
lt ephys gt 0.75
(most probable values)
lt e gt lt egeom gt lt ephys gt 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 !
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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)
!
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Structure of data
1. Toms 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
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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
Physical path length between successive elements
B bulk scattered Z tracking error
ending on the PMT at 9 oclock
Physical attenuation
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Analysis of database (1)
For a given electron with (xe, ye, ze, px, py,
pz) which generates Nc photelectrons of intensity
I01, fill a row for each detected photon (i.e.
one which reaches a PMT)
Light ray detected Index of PMT hit Detected intensity
1 h1 I1
2 h2 I2
3 h3 I3
4 h4 I4

i hi Ii

n hn In
Ndet n
Total number of photons detected
Total detected intensity
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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
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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).
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Efficiency versus momentum
Momentum dependence
n 1.04
n 1.02
- There is no obvious momentum dependence
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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
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PMT response table
For example, for the first four particles (with
n1.02), we get
not detected
detected
Assume that at least one PMT gets a signal equal
to or greater than a given electronic threshold
of, lets say 2 photoelectrons
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Preliminary optical assessment
Assuming each PMT has a detection threshold of 2
photoelectrons
Index Photon sample Electrons not detected Particle Sample Detection inefficiency
n 1.02 121 125 710 3324 21
n 1.04 235 615 82 3324 2.5
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 ?
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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.
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Electron inefficiency versus threshold
Electronic Threshold (p.e.) n 1.02 n 1.04
0 0.002 0.002
1 0.011 0.003
2 0.214 0.025
3 0.513 0.146
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Whats next ?
0. Comments, questions and criticisms
1. Update the CKOV2 part of the Technical
Reference Document
2. Resume the final (?) mechanical drawings
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