Aerogel - University of Glasgownuclear.gla.ac.uk/.../Talks/kaiser_aerogel.pdf · aerogel. There may be large differences in the properties of single-step and two-step aerogel. Two-step

Aerogel

Ralf Kaiser, University of Glasgow

What is Aerogel ?

A Brief History of Aerogel

Some Properties

Using Aerogel in a RICH

Aerogel – p.1/16

What is Aerogel ?Sol: Solution of various reactants that undergohydrolysis (forming of a hydroxid) and condensationM-OH + HO-M → M-O-M .The oxid begins to form a 3d-network.

Typical starting points for silica aerogel tetraethylorthosilicate (TEOS, Si(OCH2CH3)4) or tetramethylorthosilicate (TMOS, Si(OCH3)4).

Alcogel: When the network of linked oxids fills theentire container it becomes an alcogel consisting ofthe solid 3d-network and the liquid solvents.

Aerogel: What remains of the alcogel when the liquidpart is removed.

Chemically silica aerogel is SiO2 !

R.Kaiser - Aerogel – p.2/16

What is Aerogel ?

The reactions forming aerogel are typically slow ontheir own. Acid or base catalysts are used, e.g. HCl.

Typical acid or base catalyzed TEOS aerogels areoften refered to as ‘single step’ aerogel.

Using pre-polymerized TEOS one obtains ‘two-step’aerogel.

There may be large differences in the properties ofsingle-step and two-step aerogel. Two-step aerogelare typically clearer due to their smaller pores sizeand they can be hydrophobic , where single-stepaerogel is hygroscopic .

R.Kaiser - Aerogel – p.3/16

What is Aerogel ?

Supercritical drying: In an autoclave, under pressure,the alcohol is exchanged for CO2, then the CO2 isremoved at supercritical temperatures and pressures.

As the gel is formed, aerogel already is given it’s finalshape. A mold is required for the formation of aerogelpieces.

The length and width of the aerogel pieces isprimarily determined by the size of the equipmentused (e.g. the autoclave).

The transport of material into and out of the gel isdiffusion controlled. The thicker the material, theslower. Therefore, for practical purposes thethickness of aerogel is limited to a few cm.

R.Kaiser - Aerogel – p.4/16

What is Aerogel ?

Silca Aerogel basically is glass - therefore it canbreak like glass, only much more easily.

It can be cut with a diamond coated saw. Holding byvacuum suction. Hydrophobic aerogel can also becut with a water jet cutter.

Rapid changes in ambient pressure may shatter theaerogel. E.g. putting it into high vacuum should bedone slowly.

When cutting aerogel, one should work with a maskor under a fume hood.

R.Kaiser - Aerogel – p.5/16

A Brief History of Aerogel1931 Steve S. Kistler discovers aerogel. Monsanto

produces it until the 1960s as an additive fortoothpaste.

1970s Stanislaus Teichner (one of his students) producesaerogel based on tetrametyl orthosilicate (TMOS)

1980 TASSO at PETRA - first aerogel Cerenkov detector.1700 liters are produced at DESY (G.Poelz et al.).

1983 Arlon Hunt et al. at Berkeley replace TMOS byTEOS, which is much safer.

1995 I.Adachi et al. and H.Yokogawa et al. developtwo-step process

1998 HERMES at HERA - first aerogel RICH detector(Matsushita, also in space experiment AMS.

R.Kaiser - Aerogel – p.6/16

Aerogel - Properties

Aerogel holds 15 entries in the Guiness book,including best insulator and lowest density solid.

Index of refraction 1.008 to 1.055. This closes thegap between traditional gas radiators (C5F12 -1.0018) and liquid radiators (C6F14 - 1.27).

Simple relation between density and index ofrefraction:

n = 1 + 0.21ρ

R.Kaiser - Aerogel – p.7/16

Aerogel - Optical Properties

(highest and lowest cur-ve out of 200 - HERMESRICH aerogel)

Transmittance:

T = A · e−C·t/λ4

Hunt parameters:C: clarity coefficient,measured in µm4/cm,proportional to theradiation scattered perunit lengthA: fraction of lightthat’s not absorbed

ideal tile: C=0, A=1R.Kaiser - Aerogel – p.8/16

Aerogel - Rayleigh Scattering

unscattered and scatteredCerenkov light calculated for5cm of aerogel from Huntparameters

number of photoelectrons forunscattered and scatteredphotons

using average Huntparameteres ± 1 standarddeviation

integral of p.e. from unscatte-red photons: 19±2

R.Kaiser - Aerogel – p.9/16

Aerogel - Improved Transmittance

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.3 0.4 0.5 0.6 0.7 0.8

Matsushita 2004HERMES RICH

λ [µm]

Tra

nsm

itta

nce

Transmittance allows to use photons down to 200 nmand improves number of photoelectrons.

R.Kaiser - Aerogel – p.10/16

Aerogel - Dispersion Relation

dispersion relationfrom linearcombination of airand SiO2

singlemeasurement at633 nmdetermines mixingparameter A

important input forMC simulation

R.Kaiser - Aerogel – p.11/16

Aerogel Radiator - HERMES

R.Kaiser - Aerogel – p.12/16

Aerogel - Tile Effects

30

35

40

45

50

-30 -20 -10 0 10 20 300

2

4

6

8

10

12

14

x [cm]

y [c

m]

0

1

2

3

4

5

6

7

8

-30 -20 -10 0 10 20 30x [cm]

aver

age

nu

mb

er o

f h

its

0.241

0.2415

0.242

0.2425

0.243

0.2435

0.244

0.2445

0.245

0 1 2 3 4 5 6

d [cm]

< θ

> [r

ad]

d [cm]

σ <θ>

[rad

]

0.25

0.275

0.3

0.325

0.35

0.375

0.4

0.425

0.45

0.475

0.5

x 10-2

0 1 2 3 4 5 6

R.Kaiser - Aerogel – p.13/16

Aerogel Radiator - Refraction

track

aerogel tile stack

photon

aerogel-gas-aerogel

aerogel-gas

parallel shift

changed angle

(Refraction at last aerogel/air interface requires correction

in reconstruction algorithm.)

R.Kaiser - Aerogel – p.14/16

Aerogel Radiator - NPMT

(Number of fired PMTs for electrons and pions

with/without acceptance and tile effects. Measured at

HERMES.)R.Kaiser - Aerogel – p.15/16

Summary

Aerogel can be an excellent radiator for a RICH detector,but a number of points must be considered:

As clear as possible !

Tiles as large as possible and with the correctthickness.

Hygroscopic or hydrophobic ?

It all has to be measured and tested and the bestmaterial selected.

There are few commercial manufacturers in theworld: Matsushita Electric Works and the BorskovInstitute for Catalysis in Novosibirsk.

R.Kaiser - Aerogel – p.16/16