Study of a Silica Aerogel for a Cherenkov Radiator

Ichiro Adachi

KEKrepresenting for the Belle Aerogel RICH R&D group

2007 October 15-20

RICH2007, Trieste, Italy

RICH2007, Trieste, Italy2

Outline

• Introduction

• Silica Aerogel Production

• Optical Quality Improvements & Studies Transparancy Refractive Index Uniformity

• Machining Possibility

• Further Developments

• Conclusions

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Introduction

• Proximity focusing RICH with silica aerogel as Cherenkov radiator for new Belle forward PID upgrade program going on to replace the present threshold-type

aerogel Cherenkov counter

• Requirements for radiator Refractive index ~ 1.05 High transparency Hydrophobic

for long term stability

Reasonable block size Aerogel radiator

Position sensitive PDwith B=1.5Tesla

Readout electronics

Cherenkov photon

200mm

n=1.05

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• Baseline aerogel tiling configuration

Cover ~3.6m2 area Use hexagonal-shape aerogel

block• Reduce possible photon loss a

t corner

Hexagon with 75-mm side ~220 tiles in total Make square shape block first Then, make it hexagon with wa

ter-jet cutting device, making full advantage of hydrophobic nature

Radiator Tiling Layout

420mm

1145mm

Hexagon shape

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Silica Aerogel Production

• Production Method Sol-gel process

nSi(OR)4 + 4nH2O nSi(OH)4 + 4nH2O hydrolysisnSi(OH)4 (SiO2)n + 2nH2O condensation

Chemical treatment to make hydrophobic Supercritical drying

CO2 extraction method 31 degree Celsius and 7.5 MPa

• Optical Quality Transparency

T = T0*exp(-d/) where T is light intensity and d sample thickness Refractive index measured with Fraunhofer method These properties are strongly related to:

Chemical solvent Mixing ratio between them

3 dimensional network

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History of Aerogel Production

20

50

tran

smis

sion

leng

th a

t 40

0nm

(m

m)

refractive index

1.010 1.040 1.070 1.100

1st generation:1970’s-1980’sTASSO/PETRA1.025 ~ 1.055

2nd generation:1992-2002Belle Aerogel counter/KEKB1.010 ~ 1.030new production methodhydrophobic

3rd generation:2002- A-RICH for Belle upgrade1.030 ~ 1.080new solventI

II

III

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Optical Transparency

0

2

4

6

8

10

12

57 59 61 63 65 67

n = 1.050 sample20 mm thickness

Transmission at 400nm (%)

transmission measurement for 20 mm thickness samples

n = 1.045 20 mm thickness

Target indexAveraged transmission

length at 400nm

1.045 46.6 1.4

1.050 40.4 1.1

1.055 32.8 1.1

1.060 28.9 0.7

T = T0 exp(–d/): trans.length

2 times higher than previous samples

C = 0.005 m4/cm

C ~ 0.005-6 m4/cm

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Transmission Length

• Transparency for index ~ 1.04-1.06 samples almost doubled• Confirmed in a series of test beam experiments

2nd generation

0

10

20

30

40

50

60

1.02 1.04 1.06 1.08Refractive index

Transmision length(mm)

◆2005-2006▲2004■Before 2003

Transmission length at = 400nmprototype result with 3 GeV/c pions

2005 sample

2001 sample

n~1.050

photon yield is not limited by radiator transparency up to ~50mm

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Index Measurement

• Refractive index Measured with Fraunhofer method using 405nm laser

0

5

10

15

Measured index

1.050

1.045 1.055 1.060

Target index Measured

1.045 1.0446 ± 0.0002

1.050 1.0488 ± 0.0001

1.055 1.0533 ± 0.0003

1.060 1.0614 ± 0.0002

screen

deflection angle

405nm laser

aerogel sample

only edge of aerogel block is usedCheck other area with an independent way

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0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

108 108.5 109 109.5 110 110.5 111 111.5

Angle

Intensity

Index Scan Study (1)

• Relative weight for each composition in an aerogel was examined with XRF (X-ray fluorescence) analysis

• X-ray tomography device was used to scan relative aerogel density difference

X-ray =0.156nm

beam spot < 1mm

element Si O C

weight(%) 43.4% 50.6% 6.0%

Si

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Index Scan Study (2)

109mm

109mm

• density relative uniformity

Distance from edge(mm)

Den

sity

rat

io(%

)

edge

center

middle

(n-1)/(n-1) ~ +/-0.02

Index (Fraunhofer method at 405nm) = 1.0577 +/- 0.0006

10.7mmt

need further studies

preliminary value:

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Block Size

• Large sample produced Can be used for real detector 150 x 150 mm2 cross section Thickness: 10 mm and 20 mm

0% 50% 100%

1.045

1.050

1.055

1.060

150x150x20

150x150x10

100x100x10

“crack-free” rate by visual scan

110x110x20mm3 150x150x20mm3

n =1.050

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Machining Possibility

• Hydrophobic feature allows us to use “water-jet” cutter for machining

highly pressurized water injected via very small hole to a sample

hexagonal shape for two samples

110mm150mm

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Multiple-Layer Sample

two-layer sample with 160x160x20 mm3 has been successfully producedone can use two aerogel layers as one unit

n = 1.045

n = 1.050

160mm

transmission length(400nm): 46mm

old new

stress inside a tile well controlled

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High Density Aerogel

• Challenge to produce transparent aerogel with high density index ~ 1.10-1.20 ( ~ 0.4-0.8g/cc ). Fill a “gap” between gas and li

quid. Very difficult to make high density aerogel. Aerogel gets milky and i

t can not be used due to low transparency in a normal way. new method invented

n = 1.22

60x35x10mm3

transmission length: 18mm at 400nm

clear enough to detect Cherenkov photonsNpe ~ 9 for 3 GeV/c pions

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Conclusions

• Aerogel in the 3rd generation has been produced. index : 1.03 - 1.08 transmission length at 400 nm ~ 40 mm clarity factor ~ 0.005-6 m4/cm transparent sufficiently to employ Cherenkov radiator uniformity of index examined with X-ray tomography device

• Various aspects in aerogel production as well as handling possibility have been investigated machining two layer samples with big size of 160x160x20 cm3

• Further attempt for the 4th generation high density aerogels

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KEK - J. Stefan Institute - Univ. Ljubljana - Nagoya - Chiba - Tokyo Metro. Univ. - Toho

Acknowledgements to Matstushita Electric Works

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Backup Slide

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Aerogel Production Procedure

PreparationAging ~2 weeks

Rinse 1Hydrophobic treatment

Rinse 2-1Rinse 2-2

Rinse 2-3(Rinse 2-4)

3 days3 days

2 days2 days

2 days(2 days)

3 daysSupercritical drying

total 1 month