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)

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 + 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)

17

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

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!

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 …

)

22

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 .

28

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

29

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)

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 -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