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Silica aerogels used as Cherenkov radiators
NUCLEAR INSTRUMENTS AND METHODS 118 (1974)
177-182 ; 0 NORTH-HOLLAND PUBLISHING CO.
SILICA AEROGELS USED AS CHERENKOV RADIATORS M. CANTIN, M. CASSE, L. KOCH, R. JOUAN, P. MESTREAU, D. ROUSSEL Service d'Électronique Physique, Centre d'Études Nucléaires de SaclayB.P. no2,91190 Gif-sur - Yvette, France and F. BONNIN, J. MOU'rF-L, S. J . TEICHNER Laboratoire de Thereaodvnamique et Cinétique Chimique, Université Claude Bernard, 69000 Lyon, Fronce Received 3 December 1973 Silica aerogel is a porous and transparent solid material . Its index of refraction is a function of its density, and it can be freely adjusted between at least 9.01 and 1.06, andupto values as
high as at 1rast 1 .20 by heating. As a Cherenkov rad'ator it replaces gases under high pressure, eliminating all the problcms associated with their container .
1. Introduction
compressed powders are more suitable for refractive indexvalues from 1 .1 to 1.2, the aerogels are preferable for values smaller than 1.1 . This range was previously covered by gases underpressure . Silica aerogels exist as solids and are easier to use than gases.
Velocity measurements of high-energy particles are made with Cherenkov detectors . In order to obtain a good precision on the momentum of particles in different momentum ranges, it is necessary to use different values of the refractive index. Untilnow small values have required the use of gasunder pressure. It has been shown') that under certain 'conditions compressed silica powder has an index of refraction lying betweenthatof air and that of silica. Thevalue of theindex is afunction of the proportion of the volume
occupied by two components, air and silica. On this basis, we considered that aerogels of sifica 2) could also be used as Cherenkov radiators . While
2. Preparation of silica aerogels The starting material is the alcogel of silica obtained by polycondensation of orthosilicic acid, which is itself formed by hydrolysis of methyl-orthosilicate in solution in methanol in the presence of ammonia. Si(OCH3)4 + 4H 2 0 n(SiOH)4
H R 3 > (SiOH) 4
+
00 too Y. by weight, ortho Si in the mixer
60
Fig. 1 . Density of silica aerogel as a function of th;" c.;mcentration of methyl-orthosilicate in methanol. 177
4CH30H,
~ n(SiO 2 ) + 2nH20 .
M. CANTIN et al .
Fig. 2. Electron-microscope pictures with an enlargement of 60000: a) compressed silica powder, density 1, sample obtained by cutting with microtome; b) silica aerogel, density 0.3, sample obtained by grinding.
The alcogel thus formed is extremely sensitive to thermal shocks . The aerogel is obtained by eliminating the methanol which impregnates the alcogel. The elimination of the methanol is performed at atemperaturehigher than the critical temperature of the solvent. This avoids liquidgas interface where capillary action would destroy the porous texture of the "olid. Thedensity of theaerogelis controlled by theamount of methanol used as a solvent for methyl orthosilicate. Aerogels of apparent density in the range 0.04-0.3 are in this way easily prepared (fig. 1) . Thermal sintering of the aerogel raises its density (eventually up to that of bulk silica) and greatly improves its mechanical properties . 3. Properties of silica aerogels 3.1 . MECHANICAL
We, prepamd discs of 6 cm diameter, and athickness from 1 to 4 cm . Thesediscs have densities between 0.05
and 0.3 . When its density is too small the disc is very fragile and impossible to handle . The electron-microscope examination of the silica aerogel shows that microcells which make up the aerogel have diameters of 30 A and that aerogel is more homogenous than compressed powder (fig. 2) . Silica aerogels of 0.25 g/cm3 density have acrushing strength of about 10 kg/cmz and a breaking tensile strength of about 15 g/mmz. 3.2 . OPTICAL Figs 3 and 4 a) give the transmission coefficient of a silica a;rogel disc with a density oí0.16anda thickness of 4 mm . These data are obtained for wavelengths between 2600A and 25000A. A decrease in the transmission factor in the blue and violet parts of the spectrum is observed . This is due to a diffusion of light at short wavelengths . Since it is intended to use this converter in a diffusing detector, this phenomenon is not important here. Measurements with white light yield a diffusion length of the order of several cm.
SILICA AEROGELS AS CHERENKOV RADIATORS
179
Fig. 3. Transmission curve, in the visible and infrared, of a silica aerogel of density 0.16 and thi , kners 4 min.
A more serious phenomenon is the absorption of light in the aerogel. This absorption varies depending on the conditions underwhich the aerogels w;,re made . Different samples showed absorption lengths of the order ofseveral cm and varying by a factor of 5 from one sample to another. These differences, related to the conditions of preparation, completely mask any differences that could be due to the density of the material') .
3.3. RESPONSE TO CHARGED PARTICLES
Measurements of the amount of light produced by the Cherenkov effect have been made at CERT',; . Geneva, with beams of protons and pions. Various samples were tested in the same diffusion box In figs 5 and 6 data are presented showing the amount of light observed by photomultipliers as a function of 1/#'. The linearity of the response with 1 /ß` corresponds well with the Cherenkov effect. The threshold gives the index of refraction of the medium Fig. 7 gives the relation between the refractive inde). and the apparent density, based on Cherenko, threshold as well as on optical measurements. It is seen that values obtained by both methods are fairly consistent. There are no evident signs of scintillation detectable with the silica aerogels . With the small volume of the samples, it was not possible to measure the holtlogeneity of the product . 4. Conclusions
Fig. 4. Transmission curve, in the ultra, iolet, of asilica aerogel of density 0.16 and thickness 4 mm.
The measurements show that silica aerogels are in principle perfectly suitable for use as solid Cherenkov radiators in the range of indices where gases under high pressure were previously used, as well as for higher indices which would have required prohibitively high gas pressures . The next step is to improve the transparency of the material and to attempt to fabricate
Fig. 5. Cherenkov signal (and number of photo-electrons) observed for singly charged particles traversing silica aerogels of various densities, as a function of 1Jß . IOD A Amplitude of observed signal (arbitrary tests)
Fig. 6. Chere.rkorv .v~: ;:^;=.1
tw,. d number of photo-electrons) observed for singly charged particles traversing the same silica aeregel before and after heating at 850`C during 30 mn as a function of 1/ß°-.
SILICA AEROGELS AS CHERENKOV RADIATORS
Fig . 7. Refractive index of silica aerogel as a function of density determined by the Cherenkov response andby optical measurements.
Fig. 8. Photograph of silica aerogel discs with I 1 cm and 6cm diameter, 2 cm thickness and 0.25 g/cm3 density. This figure shows that the 2 cm thick material can still be quite transparent.
182
M. CANTIN et al .
large volumes with acceptable mechanical properties . We are presently manufacturing discs with I I cm diameter and 2 cm thickness, of 0.25 g/cm 3 density 7). We would like to thank the Fidecaro and Yvert groups for the help and cooperation we received during our experiment at CERN. Weanso thank H . Frisby, E . Saito and M . Bourdinaud of CEN-Saclay for making the measurement on the silica aerogels. References
t) A. D. Linnet' and 'a. Peters, Nucl . (1972) 545; A. D. Linney, M. Cantin,
Instr. and Meth . 100 L . Koch, Y. Maubras,
~) a) 4) 5) s)
v)
P. Mestreau, P. Roussel, A. Soutoul, P. Valot and M. Delsery, The performance of powder Cherenkov counters, 13th International Cosmic Ray Conference, Denver 1973. G. A. Nicolaon and S. J . Teichner, Bull. Soc. Chim. (1963) 1906 ; S. J . Teichner and G. A . Nicolaon, French Patent ro. 1568817, U.S. Patent no. 3672833. H. Frisby, private communication . E. Saito, private communication . M. Bourdinaud, private communication . Note added in proof We have improved our conditions of preparation :,nd now we get more reproducible samples with good transparency. The Cherenkov yield varies by a factor of less than two from one sample to another. Hexagonal samples 18 cm across and 6 cm thick are presently manufactured . We plan to test their homogeneity with an oxygen beam at Berkely ,
177-182 ; 0 NORTH-HOLLAND PUBLISHING CO.
SILICA AEROGELS USED AS CHERENKOV RADIATORS M. CANTIN, M. CASSE, L. KOCH, R. JOUAN, P. MESTREAU, D. ROUSSEL Service d'Électronique Physique, Centre d'Études Nucléaires de SaclayB.P. no2,91190 Gif-sur - Yvette, France and F. BONNIN, J. MOU'rF-L, S. J . TEICHNER Laboratoire de Thereaodvnamique et Cinétique Chimique, Université Claude Bernard, 69000 Lyon, Fronce Received 3 December 1973 Silica aerogel is a porous and transparent solid material . Its index of refraction is a function of its density, and it can be freely adjusted between at least 9.01 and 1.06, andupto values as
high as at 1rast 1 .20 by heating. As a Cherenkov rad'ator it replaces gases under high pressure, eliminating all the problcms associated with their container .
1. Introduction
compressed powders are more suitable for refractive indexvalues from 1 .1 to 1.2, the aerogels are preferable for values smaller than 1.1 . This range was previously covered by gases underpressure . Silica aerogels exist as solids and are easier to use than gases.
Velocity measurements of high-energy particles are made with Cherenkov detectors . In order to obtain a good precision on the momentum of particles in different momentum ranges, it is necessary to use different values of the refractive index. Untilnow small values have required the use of gasunder pressure. It has been shown') that under certain 'conditions compressed silica powder has an index of refraction lying betweenthatof air and that of silica. Thevalue of theindex is afunction of the proportion of the volume
occupied by two components, air and silica. On this basis, we considered that aerogels of sifica 2) could also be used as Cherenkov radiators . While
2. Preparation of silica aerogels The starting material is the alcogel of silica obtained by polycondensation of orthosilicic acid, which is itself formed by hydrolysis of methyl-orthosilicate in solution in methanol in the presence of ammonia. Si(OCH3)4 + 4H 2 0 n(SiOH)4
H R 3 > (SiOH) 4
+
00 too Y. by weight, ortho Si in the mixer
60
Fig. 1 . Density of silica aerogel as a function of th;" c.;mcentration of methyl-orthosilicate in methanol. 177
4CH30H,
~ n(SiO 2 ) + 2nH20 .
M. CANTIN et al .
Fig. 2. Electron-microscope pictures with an enlargement of 60000: a) compressed silica powder, density 1, sample obtained by cutting with microtome; b) silica aerogel, density 0.3, sample obtained by grinding.
The alcogel thus formed is extremely sensitive to thermal shocks . The aerogel is obtained by eliminating the methanol which impregnates the alcogel. The elimination of the methanol is performed at atemperaturehigher than the critical temperature of the solvent. This avoids liquidgas interface where capillary action would destroy the porous texture of the "olid. Thedensity of theaerogelis controlled by theamount of methanol used as a solvent for methyl orthosilicate. Aerogels of apparent density in the range 0.04-0.3 are in this way easily prepared (fig. 1) . Thermal sintering of the aerogel raises its density (eventually up to that of bulk silica) and greatly improves its mechanical properties . 3. Properties of silica aerogels 3.1 . MECHANICAL
We, prepamd discs of 6 cm diameter, and athickness from 1 to 4 cm . Thesediscs have densities between 0.05
and 0.3 . When its density is too small the disc is very fragile and impossible to handle . The electron-microscope examination of the silica aerogel shows that microcells which make up the aerogel have diameters of 30 A and that aerogel is more homogenous than compressed powder (fig. 2) . Silica aerogels of 0.25 g/cm3 density have acrushing strength of about 10 kg/cmz and a breaking tensile strength of about 15 g/mmz. 3.2 . OPTICAL Figs 3 and 4 a) give the transmission coefficient of a silica a;rogel disc with a density oí0.16anda thickness of 4 mm . These data are obtained for wavelengths between 2600A and 25000A. A decrease in the transmission factor in the blue and violet parts of the spectrum is observed . This is due to a diffusion of light at short wavelengths . Since it is intended to use this converter in a diffusing detector, this phenomenon is not important here. Measurements with white light yield a diffusion length of the order of several cm.
SILICA AEROGELS AS CHERENKOV RADIATORS
179
Fig. 3. Transmission curve, in the visible and infrared, of a silica aerogel of density 0.16 and thi , kners 4 min.
A more serious phenomenon is the absorption of light in the aerogel. This absorption varies depending on the conditions underwhich the aerogels w;,re made . Different samples showed absorption lengths of the order ofseveral cm and varying by a factor of 5 from one sample to another. These differences, related to the conditions of preparation, completely mask any differences that could be due to the density of the material') .
3.3. RESPONSE TO CHARGED PARTICLES
Measurements of the amount of light produced by the Cherenkov effect have been made at CERT',; . Geneva, with beams of protons and pions. Various samples were tested in the same diffusion box In figs 5 and 6 data are presented showing the amount of light observed by photomultipliers as a function of 1/#'. The linearity of the response with 1 /ß` corresponds well with the Cherenkov effect. The threshold gives the index of refraction of the medium Fig. 7 gives the relation between the refractive inde). and the apparent density, based on Cherenko, threshold as well as on optical measurements. It is seen that values obtained by both methods are fairly consistent. There are no evident signs of scintillation detectable with the silica aerogels . With the small volume of the samples, it was not possible to measure the holtlogeneity of the product . 4. Conclusions
Fig. 4. Transmission curve, in the ultra, iolet, of asilica aerogel of density 0.16 and thickness 4 mm.
The measurements show that silica aerogels are in principle perfectly suitable for use as solid Cherenkov radiators in the range of indices where gases under high pressure were previously used, as well as for higher indices which would have required prohibitively high gas pressures . The next step is to improve the transparency of the material and to attempt to fabricate
Fig. 5. Cherenkov signal (and number of photo-electrons) observed for singly charged particles traversing silica aerogels of various densities, as a function of 1Jß . IOD A Amplitude of observed signal (arbitrary tests)
Fig. 6. Chere.rkorv .v~: ;:^;=.1
tw,. d number of photo-electrons) observed for singly charged particles traversing the same silica aeregel before and after heating at 850`C during 30 mn as a function of 1/ß°-.
SILICA AEROGELS AS CHERENKOV RADIATORS
Fig . 7. Refractive index of silica aerogel as a function of density determined by the Cherenkov response andby optical measurements.
Fig. 8. Photograph of silica aerogel discs with I 1 cm and 6cm diameter, 2 cm thickness and 0.25 g/cm3 density. This figure shows that the 2 cm thick material can still be quite transparent.
182
M. CANTIN et al .
large volumes with acceptable mechanical properties . We are presently manufacturing discs with I I cm diameter and 2 cm thickness, of 0.25 g/cm 3 density 7). We would like to thank the Fidecaro and Yvert groups for the help and cooperation we received during our experiment at CERN. Weanso thank H . Frisby, E . Saito and M . Bourdinaud of CEN-Saclay for making the measurement on the silica aerogels. References
t) A. D. Linnet' and 'a. Peters, Nucl . (1972) 545; A. D. Linney, M. Cantin,
Instr. and Meth . 100 L . Koch, Y. Maubras,
~) a) 4) 5) s)
v)
P. Mestreau, P. Roussel, A. Soutoul, P. Valot and M. Delsery, The performance of powder Cherenkov counters, 13th International Cosmic Ray Conference, Denver 1973. G. A. Nicolaon and S. J . Teichner, Bull. Soc. Chim. (1963) 1906 ; S. J . Teichner and G. A . Nicolaon, French Patent ro. 1568817, U.S. Patent no. 3672833. H. Frisby, private communication . E. Saito, private communication . M. Bourdinaud, private communication . Note added in proof We have improved our conditions of preparation :,nd now we get more reproducible samples with good transparency. The Cherenkov yield varies by a factor of less than two from one sample to another. Hexagonal samples 18 cm across and 6 cm thick are presently manufactured . We plan to test their homogeneity with an oxygen beam at Berkely ,