optical performances of ckov2 gh. grégoire 1. optical elements in relation with beam properties 2....
TRANSCRIPT
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 2005
3. 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
muons5527
from a simulation of the cooling channel
electrons Muon decay in Tracker 23312
Particle samples
Good muons before CKOV2
-80
-60
-40
-20
0
20
40
60
80
-80 -60 -40 -20 0 20 40 60 80
x (cm)
y (c
m)
Beam spots
-45
-25
-5
15
35
-50 -30 -10 10 30 50
Electrons Abs2
Electrons Tr1
Electrons Tr2
Aerogel
0
500
1000
-60 -40 -20 0 20 40 60
X (cm)
N
Muons
Electrons Tr2
Muons at CKOV2 entrance
Projection on X-axis
3
Good muons after CKOV2
-80
-60
-40
-20
0
20
40
60
80
-80 -60 -40 -20 0 20 40 60 80
x ( cm )
y (
cm )
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
0
1000
2000
-40 -20 0 20 40
Theta XZ (degrees)
Electrons Tr2
Muons
1
10
100
1000
0 10 20 30 40 50 60 70
Theta (degrees)
N
Electrons Abs2
Electrons Tr1
Electrons Tr2
Muons
0
200
400
600
800
1000
1200
1400
1600
0 100 200 300 400Momentum (MeV/c)
Muons
Electrons
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).
0
200
400
600
800
0 100 200 300 400
Momentum (MeV/c)
NElectrons Abs2
Electrons Tr1
Electrons Tr2
Muons
1
10
100
1000
0 100 200 300 400
Momentum (MeV/c)
Electrons Abs2
Electrons Tr1
Electrons Tr2
Muons
T. Roberts T. Roberts
5
Light yield for muons
0
200
400
600
800
0 100 200 300 400
Momentum (MeV/c)
NElectrons Abs2
Electrons Tr1
Electrons Tr2
Muons
1
10
100
1000
0 100 200 300 400
Momentum (MeV/c)
Electrons Abs2
Electrons Tr1
Electrons Tr2
Muons
No light produced by muons for 1.02 < n < 1.04
0
10
20
30
40
50
60
0 100 200 300 400 500 600 700
Muon momentum (MeV/c)
Nr
ph
oto
elec
tro
ns
n=1.02
n=1.03
n=1.04
n=1.05
n=1.06
Range chosen for this talk !
6
Threshold curve for muons
0
200
400
600
800
1.01 1.02 1.03 1.04 1.05 1.06
Index of refraction
Th
resh
old
mo
men
tum
fo
r m
uo
ns
( M
eV/c
)
Exp. distribution of index for aerogel n=1.03
FWHM 0.002
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 Matsushita Each 130 mm x 130 mm x 10 mmHydrophobic aerogel (i.e. chemically modified)
Number of tiles 320 ( Cost 260 € /tile )
Aerogel container Polygonal honeycomb box
10
Aerogel type n=1.02 n=1.04
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
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
4
1
C
Lscat
« Clarity » C = 0.01 m4 cm-1
Photon production
222
2 11
2
nddz
Nd
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
4exp
LC
AT Experimentally
A ~ 0.96( in the UV and visible ranges )
Here (MICE) Lscat = 25.6 mm at = 400 nm
(Hermes)( = not scattered ! )
Experimentally very small (Hermes)
Dipole-type angular distribution
cos1)( P
Here 4% per cm11
Simulation of aerogel
scatL
uuAuP exp96.0)(1)(
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.
scatL
uxT exp96.0)(4. Transmission probability varies as
where u is the distance along the ray path
5. Absorption
with Lscat = 25.6 mm
3. Isotropic angular distribution for scattering (approximation of the dipole distribution)
104.0)( cmuA
All simulations performed at = 400 nm for aerogels with n = 1.02 and n = 1.04
12
Labsorption = 245 mm
Optical properties
Also taken into account
1. Reflectivities of mirrors Front mirror
Reflecting 12-sided pyramidWinston cones
2. Bulk transmittance Optical windows
3. Reflectances and transmittances at interfaces
Aerogel-air
Opt. windows - airPMT - air
4. Diffuse scattering (Lambertian for 50% of the rays) Walls of the aerogel box
(angle dependent / unpolarized light)
90 % Labsorption = 94.9 mm
i.e. 50 % undergo specular reflection50 % are diffused around specular with a cos
distribution13
96 % 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 + SiO2 + Hf O2 ]
Very good reflectivity (A. Braem/CERN)
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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 + SiO2 )
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 107 (at 2300 V)
Positive HV supply ! (i.e. photocathode at ground and anode at HV !)
Quantum efficiency (Electron Tubes Ltd)
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100
100150200250300350400
80
60
40
20
0
Wavelength (nm)
Tra
nsm
issi
on
(%
)
UV
100 150 200 250 300 350 400
Transmission through window
Theoretical efficiency
For a single particle loosing an energy E in the radiator,
Review of particle properties, July 2004
dEEEKLN cep )(sin)( 2.. 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)
cep NLN 20.. sin
where (E) = efficiency for collecting light and converting it in photoelectrons
with dEEKN )(0
For a typical PMT working in the visible and near UV N0 = 90-100 cm-1
Threshold: 1)(
1sin
nc or 12
n
mpp threshold
N0 already “contains” the Q.E. of a (typical) PMT and assumes all photons are collected !18
Realistic efficiency
physgeom
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 usecep NLN 2
0.. sin
where
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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!
emittedraysofNr
detectedraysofNrdef
geom
- Minimal attenuation Minimize number of reflections/transmissions
( or ray path length!)
i.e. maximize phys
20
Geometries
# 1 # 2 # 3
12 flat faces at 45°
12 cylindrical faces 12 spherical faces
(R = 843 mm)
(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
Incr
easi
ng
focu
sing
pow
er
(sti
ll keepin
g m
ech
an
ical
sim
plic
ity …
)
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Sequence of operations
Mechanical design
Autocad 2000
Optical performances
Zemax Engineering v. Feb 2005
Detection performance
s
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
Reflections
0
50
100
150
200
250
300
350
400
450
500
0 10 20 30
Nr of reflections
Path length
0
200
400
600
800
1000
0 1000 2000 3000 4000 5000 6000
Path length (mm)
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 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 .
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No scattering
Performances of configuration #2
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
geom = 0.67
Pyramid with curved cylindrical faces faces
Reflections
1
10
100
1000
0 10 20 30
Nr of reflections
Cylindrical faces
Flat faces
Path length
1
10
100
1000
1 1001 2001 3001 4001 5001
Path length (mm)
Cylindrical faces
Flat faces
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Geometrical efficiency geom
Tracking 2000 rays
Type of reflecting pyramid Flat facesCylindrical
facesSpherical
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
geom
0.61 0.67 0.67
best ! 30
Physical attenuation (no scattering)
0
100
200
300
400
500
600
700
800
900
0 0.2 0.4 0.6 0.8 1
Physical attenuation factor
Fre
qu
ency
Cylindrical faces
Flat faces
Spherical faces
for = 400 nm
31
phys
Type of reflecting pyramid Flat facesCylindrical
facesSpherical
faces
Average attenuation factor 0.685 0.711 0.689
Most probable attenuation factor 0.750 0.750 0.750
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)
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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 -4.029 -4.21 4220.7 1.603 0.685 41.818
-40
-20
0
20
40
-40 -20 0 20 40
2. Additional particle and Cherenkov effect data
x y z Px Py Pz
23.13817 2.38705 0.999925 35 11.344
0
10
20
30
40
50
60
70
0 10 20 30 40 50
the
ta (
de
gre
es
)
Nc
c
3. Generation of Nc photons around a cone of angle c
….. 35 lines
(cm, MeV/c)
(mm)
i.e. generate 1 light ray of unit intensity (I0 = 1) per photoelectron and then track each ray.
-40.290 -42.100 87.098 -0.219 0.125 0.968 1
x y z l m n I (a.u)
-40.290 -42.100 47.333 -0.203 0.145 0.968 1
35
….. ….. ….. …..…..
Ray 7 Wavelength 1 8 segments 1 branchSeg# XRTS BZ X Y Z Path Intensity Comment
0 ---- -- -40.290 -42.100 51.601 - 1.000 Source1 ---- *- -38.311 -45.773 69.130 18.018 0.929 Bulk scattered2 --*- -- -74.371 -43.763 102.145 48.932 0.761 AEROGEL.STL3 -*-- -- -421.545 -24.403 398.609 456.942 0.760 PYRAMID_3.STL4 --*- -- -670.000 -13.855 386.137 248.991 0.726 WINDOW_095 --*- -- -680.000 -13.579 385.811 10.009 0.693 WINDOW_096 -*-- -- -700.622 -12.703 384.776 20.667 0.661 CONE_7.STL7 --*- -- -736.662 -3.989 342.931 55.910 0.631 DETECTOR_098 *--- -- -871.611 28.640 186.244 209.348 0.631 Missed
Optical tracking database
R = reflected
T= transmitted
S = scattered
X= terminated
Optical elements hitPhysical path
length between successive elements
Physical attenuation
A typical good event
… ending on the PMT at 9 o’clock
Starting point of 7th photon (see previous transparency)
B = bulk scattered
Z = tracking error
36
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)
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
0det II
Total detected intensity37
Analysis of database (2)
- geometrical detection efficiency
- global detection efficiency
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
cdefgeom N
Ndet
0
det
IN
I
cdef
- relative individual PMT signal0det
.
)(
IN
I
I knrPMThittingraysall
ki
k
(Total detected intensity)
- accepted individual PMT signalthresholdelectronick II
38
Global efficiency
0
50
100
150
200
250
300
350
400
450
0.00 0.25 0.50 0.75 1.00
Geometrical eff iciency
0
50
100
150
200
250
300
350
400
450
500
550
600
650
0.00 0.25 0.50 0.75 1.00
Global eff iciency
20 % of the Cherenkov light intensity (in relative photoelectron units) reach the PMTs
About 50 % of the Cherenkov photons reach the PMTs
n = 1.04
n = 1.04
39
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
0.00
0.10
0.20
0.30
0.40
0.50
0 100 200 300
Momentum ( MeV/c )
Glo
bal
eff
icie
ncy
0.00
0.10
0.20
0.30
0.40
0.50
0 100 200 300
Momentum ( MeV/c )
Glo
bal
eff
icie
ncy
n = 1.04
n = 1.02
40
Efficiency versus particle impact
Radial dependence
- Trend of smaller efficiencies for larger impact radius
0.00
0.10
0.20
0.30
0.40
0.50
0 100 200 300 400 500
Radial distance (mm)
Glo
bal
eff
icie
ncy
0.00
0.10
0.20
0.30
0.40
0.50
-4 -3 -2 -1 1 2 3 4
Azimutal angle ( radians )
Glo
bal
eff
icie
ncy
Azimutal dependence
- Very sensitive to axial misalignments
n = 1.04
n = 1.04
41
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
PM_1 PM_2 PM_3 PM_4 PM_5 PM_6Hit Intens. Hit Intens. Hit Intens. Hit Intens. Hit Intens. Hit Intens.0 0.00 1 0.00 1 0.15 1 0.02 2 1.69 3 1.940 0.00 1 0.16 0 0.00 1 0.10 2 1.40 2 0.210 0.00 2 0.62 1 0.62 1 0.32 6 5.04 2 0.762 0.91 1 0.56 0 0.00 0 0.00 8 5.02 6 3.43
PM_7 PM_8 PM_9 PM_10 PM_11 PM_12Hit Intens. Hit Intens. Hit Intens. Hit Intens. Hit Intens. Hit Intens.4 1.41 2 0.64 0 0.00 2 0.14 0 0.00 2 1.035 3.06 4 2.54 1 0.57 0 0.00 0 0.00 2 0.381 0.46 1 0.03 0 0.00 0 0.00 1 0.02 0 0.002 1.13 1 0.40 0 0.00 2 0.68 1 0.18 1 0.34
PM_1 PM_2 PM_3 PM_4 PM_5 PM_6Hit Intens. Hit Intens. Hit Intens. Hit Intens. Hit Intens. Hit Intens.0 0.00 1 0.00 1 0.00 1 0.00 2 0.00 3 0.000 0.00 1 0.00 0 0.00 1 0.00 2 0.00 2 0.000 0.00 2 0.00 1 0.00 1 0.00 6 5.04 2 0.002 0.00 1 0.00 0 0.00 0 0.00 8 5.02 6 3.43
PM_7 PM_8 PM_9 PM_10 PM_11 PM_12Hit Intens. Hit Intens. Hit Intens. Hit Intens. Hit Intens. Hit Intens.4 0.00 2 0.00 0 0.00 2 0.00 0 0.00 2 0.005 3.06 4 2.54 1 0.00 0 0.00 0 0.00 2 0.001 0.00 1 0.00 0 0.00 0 0.00 1 0.00 0 0.002 0.00 1 0.00 0 0.00 2 0.00 1 0.00 1 0.00
detected
not detected
42
Preliminary optical assessment
IndexPhoton 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 %
- 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
-500
-400
-300
-200
-100
0
100
200
300
400
500
-500 -400 -300 -200 -100 0 100 200 300 400 500
-500
-400
-300
-200
-100
0
100
200
300
400
500
-500 -400 -300 -200 -100 0 100 200 300 400 500
0
10
20
0 100 200
Momentum (MeV/c)
n=1.04
n=1.02
n = 1.02
n = 1.04
x-y
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 = 1.02 n = 1.04
0 0.002 0.002
1 0.011 0.003
2 0.214 0.025
3 0.513 0.146
45
What’s next ?
46
1. Update the CKOV2 part of the Technical Reference Document
2. Resume the final (?) mechanical drawings
0. Comments, questions and criticisms