Study of a Silica Aerogel for a Cherenkov Radiator Ichiro Adachi

Study of a Silica Aerogel for a Cherenkov Radiator

• Ichiro Adachi

• KEK

• representing for the Belle Aerogel RICH R&D group

• 2007 October 15-20

• RICH2007, Trieste, Italy

Outline

• Introduction

• Silica Aerogel Production

• Optical Quality Improvements & Studies

• Transparancy
• Refractive Index Uniformity

• Machining Possibility

• Further Developments

• Conclusions

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

Radiator Tiling Layout

• Baseline aerogel tiling configuration

• Cover ~3.6m2 area
• Use hexagonal-shape aerogel block
• Reduce possible photon loss at corner
• Hexagon with 75-mm side
• ~220 tiles in total
• Make square shape block first
• Then, make it hexagon with water-jet cutting device, making full advantage of hydrophobic nature

Silica Aerogel Production

• Production Method

• Sol-gel process

• nSi(OR)4 + 4nH2O  nSi(OH)4 + 4nH2O hydrolysis

• nSi(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

History of Aerogel Production

Optical Transparency

Transmission Length

• Transparency for index ~ 1.04-1.06 samples almost doubled

• Confirmed in a series of test beam experiments

Index Measurement

• Refractive index

• Measured with Fraunhofer method using 405nm laser

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

Index Scan Study (2)

• density relative uniformity

Block Size

• Large sample produced

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

Machining Possibility

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

Multiple-Layer Sample

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 liquid.
• Very difficult to make high density aerogel. Aerogel gets milky and it can not be used due to low transparency in a normal way.
• new method invented

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

Backup Slide

Aerogel Production Procedure