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Optical and ageing studies of aerogel samples for RICH applications in space
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181 www.elsevier.com/locate/nima
Optical and ageing studies of aerogel samples for RICH applications in space A. Martı´ nez-Da´valos, E. Belmont-Moreno, A. Menchaca-Rocha Instituto de Fı´sica, UNAM, A.P. 20-364, 01000 Me´xico DF, Mexico Available online 30 August 2005
Abstract Optical and ageing properties of hydrophobic silica aerogel have been measured. The optical observations include the determination of the index of refraction of individual aerogel samples, and transmittance measurements. Concerning ageing, we investigate possible explanations for an appreciable decrease on the light yield observed in the aerogel tiles used in the AMS-01 Cherenkov detector, flown on board of the NASA Discovery Shuttle. Measurements were carried out simulating the vacuum and thermal cycle to which the aerogel was subject. These tests indicate that this material is very sensitive to residual contaminants that might be present during the vacuum pumping and/or gas admission process. r 2005 Elsevier B.V. All rights reserved. PACS: 78.20.Ci; 29.40.Ka; 98.70.Sa Keywords: Cherenkov counters; Aerogel; Cosmic rays
1. Introduction The use of silica aerogel for Ring Imaging Cherenkov (RICH) detectors requires a careful characterization of its optical properties, since the processes of absorption, scattering and reflection would limit the attainable resolution on the determination of the Cherenkov angle. When dealing with cosmic ray applications, aerogel Corresponding author.
E-mail address: menchaca@fisica.unam.mx (A. Menchaca-Rocha).
counters have been operated on board of balloon and satellite experiments, which means that they are subject to a harsh environment that might have an influence on its optical properties after a prolonged use. During the first flight of the Alpha Magnetic Spectrometer (AMS-01) in June 1998 [1], the threshold aerogel counter showed a decrease of 40% of the Cherenkov light yield with respect to a reference measurement done in November 1997, and further deteriorated by an additional 50% by November 1998. This phenomenon corresponds to an equivalent lifetime of about 300 days, and has remained largely unexplained. Although previous
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.08.045
ARTICLE IN PRESS 178
A. Martı´nez-Da´valos et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181
studies [2] had shown that the presence of contaminants can induce irreversible optical damage in aerogel, no tests had been conducted so far looking for possible ageing effects of aerogel in outer space. The main conditions that could induce ageing and probably noticeable optical changes are the vacuum and the short time extreme temperature cycle related to the orbit of the Space Shuttle. In this work we show the first experimental results of tests conducted on aerogel samples under laboratory settings that tried to reproduce the environmental conditions during the Shuttle flight. The final aim of this study has been to provide information that could be used on the design of the second generation AMS-RICH prototype [3], in order to improve the determination of particle velocities and the long-term reliability of the apparatus.
2. Materials and methods Previous research on a SP-25 aerogel sample [4] showed that the scaling rule based on the Lorentz–Lorentz formulation provides a good description of the dispersion law for this material. For this reason in this work optical studies have been restricted to the determination of the index of refraction of individual aerogel tiles at a single wavelength, and to transmittance measurements of small samples to probe for changes in its optical properties that might be due to ageing. The tests have been conducted on 40 SP-30 and SP-50 aerogel samples,1 20 of each kind. 2.1. Index of refraction measurements Refractive indices were measured by means of the minimum deflection prism technique [5], using a continuous wave 3 W argon-ion laser.2 A semiautomatic procedure based on a scanning diode and a computer controlled CCD camera attached to a frame grabber was used to measure the positions of the centroids of the light spots for both the unrefracted and refracted beams (see Fig. 1 2
Matsushita Electric Works Ltd., Japan. Spectra Physics 2020, l ¼ 514 nm.
L θ
Laser x
φ α
Aerogel
Fig. 1. Experimental setup for the index of refraction measurements.
1) which allowed us to determine the distance x. Using f ¼ arctanðL=xÞ and Snell’s law, the following equation applies for the experimental setup shown on Fig. 1: f ¼ sin1 fn sin½Sg a þ ðy þ dÞ sinðy þ dÞ S ¼ a sin1 : n
(1)
(square sample), In our case a ¼ 90
L ¼ 2000 1 mm, and d is a parameter reflecting the uncertainty associated to the y ¼ 0 position. Since f has a well defined minimum for a given value of n, Eq. (1) can be used to determine n by a w2 fitting of the f vs. x data for a set of rotations of the sample that goes through the minimum deflection. This method has the advantage that it does not require a precise determination of the tilting angle y, and can be performed reasonably quickly on a large number of samples. For this report all four corners of every aerogel tile were measured. Further details of this technique are given elsewhere [4]. 2.2. Ageing effects A very important property of aerogel, in terms of its use as a RICH radiator, is that it minimizes scattering, absorption and reflection of the Cherenkov light produced within the medium [6]. Amongst these processes it is light diffusion which mostly limits its performance. A simple way to characterize any variations on optical quality is to find the clarity coefficient C. This can be done by measuring the transmittance T of a small sample
ARTICLE IN PRESS A. Martı´nez-Da´valos et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181
179
16
Number of aerogel corners
14
SP-30
12 10 8 6 4 2 0 1.030
1.031
(a)
1.032 1.033 Refractive index
1.034
16 Fig. 2. Ageing chamber with the cooling/heating coils and clean vacuum system.
4
TðlÞ ¼ AeCt=l .
(2)
Here A and C are parameters related to surface and bulk (Rayleigh) scattering, respectively. In this work transmittance measurements were done with a spectrophotometer3 in the 200–900 nm range, and the experimental data were fitted with Eq. (2) to obtain the clarity coefficient. The main idea of the ageing measurements was to see any possible variation in the clarity coefficient of aerogel when subject to aggressive temperature variations while in vacuum. For this purpose a small clean chamber was constructed where a piece of aerogel could be encapsulated in vacuum in order to sustain hot–cold cycles (see Fig. 2). After a period of time, the transmittance was measured and compared with the initial value. The vacuum system was kept to minimum in volume to have more control on possible dirt. All the system was made of stainless steel and prior to assembly all pieces underwent ultrasonic cleaning. 3
Milton Roy 3000.
Number of aerogel corners
of thickness t as a function of wavelength l, which can be parameterized in terms of the Hunt formula as follows:
SP-50
14 12 10 8 6 4 2 0 1.050 (b)
1.051
1.052 1.053 Refractive index
1.054
1.055
Fig. 3. Refractive index data from all corners of the aerogel tiles. (a) SP-30, nominal index n ¼ 1:03, (b) SP-50, nominal index n ¼ 1:05.
Metal gaskets were used in all the joints to support the temperature gradient and minimize the leaks. The vacuum was obtained with a cryogenic adsorption pump to ensure a hydrocarbon free vacuum. The working vacuum was 0.133 Pa and the thermal cycle was done by heating and cooling from outside with heating and cooling
ARTICLE IN PRESS 180
A. Martı´nez-Da´valos et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181
liquid4 coils around the small sample chamber. The temperature range was from 0 to 100 C in 1 h cycle. The air admission to the sample chamber was done slowly and using dry clean nitrogen.
3.1. Index of refraction To illustrate the overall variations on the resulting n values, in Fig. 3 we show the statistics obtained for SP-30 and SP-50, respectively. For the first one we observe a peak centered near n ¼ 1:032, and a smaller group, of only four counts, near n ¼ 1:0335. To get a better understanding about the origin of the latter, we obtained the mean n values, averaged over the four corners, for each SP-30 aerogel sample. In this way we found out that the little group at n ¼ 1:0335 was associated to only one tile. With respect to SP-50, we observe a double-humped group, which is also present (and more clearly separated) if one plots the tiles’ mean n. Presumably, both cases (SP-30 and SP-50) reflect systematic density variations originated during the manufacturing process. 3.2. Ageing Initial test were performed on a standard vacuum system that, after finding several sources of contamination, developed into a very clean system. With these tests it was clear that at vacuum, aerogel is much more sensitive to catch fumes and odors than at atmospheric pressure. Moreover, it was found that repeated evacuation and air admission to the sampling cell can also damage the quality of the samples. After we fixed all sources of contamination, we observed that optical quality of aerogel remained constant, within the spectrophotometer error, even after several weeks undergoing the thermal cycle. Fig. 4 shows a set of transmittance curves, measured before (solid line) and after (open circles) a week long thermal treatment. The clarity factor obtained from a fit of Eq. (2) was 0:0149 mm4 =cm at 4
Freon F-22, chlorodifluoromethane.
0.8 Transmission fraction
3. Results and analysis
1.0
0.6
0.4 Before After (clean) After (contaminated)
0.2
0.0 200
300
400
500 600 λ [nm]
700
800
900
Fig. 4. Transmission fraction as function of wavelength, before (solid line) and after (open circles) one week of thermal 1 h, 0–100 C cycle. The corresponding values of C are 0.0149 and 0:0151 mm4 =cm, respectively.
the beginning and 0:0151 mm4 =cm at the end of the test. The same kind of results were obtained for approximately 10 tests with different samples, some of them lasting up to 1 month. Therefore, we can conclude that there is no evidence of ageing effects due to the thermal cycle under clean vacuum conditions. For comparison, Fig. 4 also shows a transmittance curve from an aerogel sample taken from the same tile, that was contaminated by oil fumes; in this case C ¼ 0:0193 mm4 =cm and it can be observed that transmittance values dropped by about 20% on average over the whole wavelength range.
4. Conclusions We have performed optical and ageing tests on a set of 40 hydrophobic silica aerogel tiles, types SP30 and SP-50, which are suitable for the development of a RICH detector for space applications. Optical tests included the determination of the index of refraction on the four corners of every aerogel tile. The measured mean refractive
ARTICLE IN PRESS A. Martı´nez-Da´valos et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181
indices of both aerogel types are systematically higher ð 0:3%Þ than the nominal values quoted by the manufacturer. Similar deviations were reported by Gougas et al. [2] for SP-30, and by our group [4] for SP-25. These deviations from nominal values are a clear justification for this type of measurements. Moreover, having this information for every tile makes it possible to correct for these effects, without the introduction of a tilerejection criterion. A clean vacuum chamber with an integrated system for subjecting small aerogel samples to hot/cold cycles has been build. Determination of the clarity coefficient by means of transmittance measurements in the wavelength range from 200 to 900 nm indicate that, within the experimental precision, no appreciable effects on the optical quality of the samples can be observed after repeated 1 h, 0–100 C cycles. Concerning the decaying yield observed on the AMS-01 prototype, our results indicate that it was most probably due to contamination in the laboratory by the vacuum system, or in-flight contamination due to fumes from any nearby material not stable at high temperatures.
181
Acknowledgements The tests were conducted on aerogel samples kindly lent to us by J. Berdugo, from the AMSCIEMAT group (Spain). The technical support of J.M. Lo´pez, R. Nu´n˜ez, M. Rangel and E. Camarillo is gratefully acknowledged. This project has been carried out with financial support from CONACYT projects G39091 and F44380, and PAPIIT-UNAM IN101501.
References [1] D. Barancourt, et al., Nucl. Instr. and Meth. A 465 (2001) 306 M. Buenerd, T. Thuillier, AMS Internal Report ISN Grenoble 99/122, 1999. [2] A.K. Gougas, et al., Nucl. Instr. and Meth. A 421 (1999) 249. [3] E. Lanciotti, for the AMS02-RICH Collaboration, Nucl. Instr. and Meth. A 518 (2004) 150. [4] M. Villoro, et al., Nucl. Instr. and Meth. A 480 (2002) 456. [5] T. Bellunato, et al., Nucl. Instr. and Meth. A 527 (2004) 319. [6] R. De Leo, et al., Nucl. Instr. and Meth. A 457 (2001) 52.
Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181 www.elsevier.com/locate/nima
Optical and ageing studies of aerogel samples for RICH applications in space A. Martı´ nez-Da´valos, E. Belmont-Moreno, A. Menchaca-Rocha Instituto de Fı´sica, UNAM, A.P. 20-364, 01000 Me´xico DF, Mexico Available online 30 August 2005
Abstract Optical and ageing properties of hydrophobic silica aerogel have been measured. The optical observations include the determination of the index of refraction of individual aerogel samples, and transmittance measurements. Concerning ageing, we investigate possible explanations for an appreciable decrease on the light yield observed in the aerogel tiles used in the AMS-01 Cherenkov detector, flown on board of the NASA Discovery Shuttle. Measurements were carried out simulating the vacuum and thermal cycle to which the aerogel was subject. These tests indicate that this material is very sensitive to residual contaminants that might be present during the vacuum pumping and/or gas admission process. r 2005 Elsevier B.V. All rights reserved. PACS: 78.20.Ci; 29.40.Ka; 98.70.Sa Keywords: Cherenkov counters; Aerogel; Cosmic rays
1. Introduction The use of silica aerogel for Ring Imaging Cherenkov (RICH) detectors requires a careful characterization of its optical properties, since the processes of absorption, scattering and reflection would limit the attainable resolution on the determination of the Cherenkov angle. When dealing with cosmic ray applications, aerogel Corresponding author.
E-mail address: menchaca@fisica.unam.mx (A. Menchaca-Rocha).
counters have been operated on board of balloon and satellite experiments, which means that they are subject to a harsh environment that might have an influence on its optical properties after a prolonged use. During the first flight of the Alpha Magnetic Spectrometer (AMS-01) in June 1998 [1], the threshold aerogel counter showed a decrease of 40% of the Cherenkov light yield with respect to a reference measurement done in November 1997, and further deteriorated by an additional 50% by November 1998. This phenomenon corresponds to an equivalent lifetime of about 300 days, and has remained largely unexplained. Although previous
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.08.045
ARTICLE IN PRESS 178
A. Martı´nez-Da´valos et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181
studies [2] had shown that the presence of contaminants can induce irreversible optical damage in aerogel, no tests had been conducted so far looking for possible ageing effects of aerogel in outer space. The main conditions that could induce ageing and probably noticeable optical changes are the vacuum and the short time extreme temperature cycle related to the orbit of the Space Shuttle. In this work we show the first experimental results of tests conducted on aerogel samples under laboratory settings that tried to reproduce the environmental conditions during the Shuttle flight. The final aim of this study has been to provide information that could be used on the design of the second generation AMS-RICH prototype [3], in order to improve the determination of particle velocities and the long-term reliability of the apparatus.
2. Materials and methods Previous research on a SP-25 aerogel sample [4] showed that the scaling rule based on the Lorentz–Lorentz formulation provides a good description of the dispersion law for this material. For this reason in this work optical studies have been restricted to the determination of the index of refraction of individual aerogel tiles at a single wavelength, and to transmittance measurements of small samples to probe for changes in its optical properties that might be due to ageing. The tests have been conducted on 40 SP-30 and SP-50 aerogel samples,1 20 of each kind. 2.1. Index of refraction measurements Refractive indices were measured by means of the minimum deflection prism technique [5], using a continuous wave 3 W argon-ion laser.2 A semiautomatic procedure based on a scanning diode and a computer controlled CCD camera attached to a frame grabber was used to measure the positions of the centroids of the light spots for both the unrefracted and refracted beams (see Fig. 1 2
Matsushita Electric Works Ltd., Japan. Spectra Physics 2020, l ¼ 514 nm.
L θ
Laser x
φ α
Aerogel
Fig. 1. Experimental setup for the index of refraction measurements.
1) which allowed us to determine the distance x. Using f ¼ arctanðL=xÞ and Snell’s law, the following equation applies for the experimental setup shown on Fig. 1: f ¼ sin1 fn sin½Sg a þ ðy þ dÞ sinðy þ dÞ S ¼ a sin1 : n
(1)
(square sample), In our case a ¼ 90
L ¼ 2000 1 mm, and d is a parameter reflecting the uncertainty associated to the y ¼ 0 position. Since f has a well defined minimum for a given value of n, Eq. (1) can be used to determine n by a w2 fitting of the f vs. x data for a set of rotations of the sample that goes through the minimum deflection. This method has the advantage that it does not require a precise determination of the tilting angle y, and can be performed reasonably quickly on a large number of samples. For this report all four corners of every aerogel tile were measured. Further details of this technique are given elsewhere [4]. 2.2. Ageing effects A very important property of aerogel, in terms of its use as a RICH radiator, is that it minimizes scattering, absorption and reflection of the Cherenkov light produced within the medium [6]. Amongst these processes it is light diffusion which mostly limits its performance. A simple way to characterize any variations on optical quality is to find the clarity coefficient C. This can be done by measuring the transmittance T of a small sample
ARTICLE IN PRESS A. Martı´nez-Da´valos et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181
179
16
Number of aerogel corners
14
SP-30
12 10 8 6 4 2 0 1.030
1.031
(a)
1.032 1.033 Refractive index
1.034
16 Fig. 2. Ageing chamber with the cooling/heating coils and clean vacuum system.
4
TðlÞ ¼ AeCt=l .
(2)
Here A and C are parameters related to surface and bulk (Rayleigh) scattering, respectively. In this work transmittance measurements were done with a spectrophotometer3 in the 200–900 nm range, and the experimental data were fitted with Eq. (2) to obtain the clarity coefficient. The main idea of the ageing measurements was to see any possible variation in the clarity coefficient of aerogel when subject to aggressive temperature variations while in vacuum. For this purpose a small clean chamber was constructed where a piece of aerogel could be encapsulated in vacuum in order to sustain hot–cold cycles (see Fig. 2). After a period of time, the transmittance was measured and compared with the initial value. The vacuum system was kept to minimum in volume to have more control on possible dirt. All the system was made of stainless steel and prior to assembly all pieces underwent ultrasonic cleaning. 3
Milton Roy 3000.
Number of aerogel corners
of thickness t as a function of wavelength l, which can be parameterized in terms of the Hunt formula as follows:
SP-50
14 12 10 8 6 4 2 0 1.050 (b)
1.051
1.052 1.053 Refractive index
1.054
1.055
Fig. 3. Refractive index data from all corners of the aerogel tiles. (a) SP-30, nominal index n ¼ 1:03, (b) SP-50, nominal index n ¼ 1:05.
Metal gaskets were used in all the joints to support the temperature gradient and minimize the leaks. The vacuum was obtained with a cryogenic adsorption pump to ensure a hydrocarbon free vacuum. The working vacuum was 0.133 Pa and the thermal cycle was done by heating and cooling from outside with heating and cooling
ARTICLE IN PRESS 180
A. Martı´nez-Da´valos et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181
liquid4 coils around the small sample chamber. The temperature range was from 0 to 100 C in 1 h cycle. The air admission to the sample chamber was done slowly and using dry clean nitrogen.
3.1. Index of refraction To illustrate the overall variations on the resulting n values, in Fig. 3 we show the statistics obtained for SP-30 and SP-50, respectively. For the first one we observe a peak centered near n ¼ 1:032, and a smaller group, of only four counts, near n ¼ 1:0335. To get a better understanding about the origin of the latter, we obtained the mean n values, averaged over the four corners, for each SP-30 aerogel sample. In this way we found out that the little group at n ¼ 1:0335 was associated to only one tile. With respect to SP-50, we observe a double-humped group, which is also present (and more clearly separated) if one plots the tiles’ mean n. Presumably, both cases (SP-30 and SP-50) reflect systematic density variations originated during the manufacturing process. 3.2. Ageing Initial test were performed on a standard vacuum system that, after finding several sources of contamination, developed into a very clean system. With these tests it was clear that at vacuum, aerogel is much more sensitive to catch fumes and odors than at atmospheric pressure. Moreover, it was found that repeated evacuation and air admission to the sampling cell can also damage the quality of the samples. After we fixed all sources of contamination, we observed that optical quality of aerogel remained constant, within the spectrophotometer error, even after several weeks undergoing the thermal cycle. Fig. 4 shows a set of transmittance curves, measured before (solid line) and after (open circles) a week long thermal treatment. The clarity factor obtained from a fit of Eq. (2) was 0:0149 mm4 =cm at 4
Freon F-22, chlorodifluoromethane.
0.8 Transmission fraction
3. Results and analysis
1.0
0.6
0.4 Before After (clean) After (contaminated)
0.2
0.0 200
300
400
500 600 λ [nm]
700
800
900
Fig. 4. Transmission fraction as function of wavelength, before (solid line) and after (open circles) one week of thermal 1 h, 0–100 C cycle. The corresponding values of C are 0.0149 and 0:0151 mm4 =cm, respectively.
the beginning and 0:0151 mm4 =cm at the end of the test. The same kind of results were obtained for approximately 10 tests with different samples, some of them lasting up to 1 month. Therefore, we can conclude that there is no evidence of ageing effects due to the thermal cycle under clean vacuum conditions. For comparison, Fig. 4 also shows a transmittance curve from an aerogel sample taken from the same tile, that was contaminated by oil fumes; in this case C ¼ 0:0193 mm4 =cm and it can be observed that transmittance values dropped by about 20% on average over the whole wavelength range.
4. Conclusions We have performed optical and ageing tests on a set of 40 hydrophobic silica aerogel tiles, types SP30 and SP-50, which are suitable for the development of a RICH detector for space applications. Optical tests included the determination of the index of refraction on the four corners of every aerogel tile. The measured mean refractive
ARTICLE IN PRESS A. Martı´nez-Da´valos et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 177–181
indices of both aerogel types are systematically higher ð 0:3%Þ than the nominal values quoted by the manufacturer. Similar deviations were reported by Gougas et al. [2] for SP-30, and by our group [4] for SP-25. These deviations from nominal values are a clear justification for this type of measurements. Moreover, having this information for every tile makes it possible to correct for these effects, without the introduction of a tilerejection criterion. A clean vacuum chamber with an integrated system for subjecting small aerogel samples to hot/cold cycles has been build. Determination of the clarity coefficient by means of transmittance measurements in the wavelength range from 200 to 900 nm indicate that, within the experimental precision, no appreciable effects on the optical quality of the samples can be observed after repeated 1 h, 0–100 C cycles. Concerning the decaying yield observed on the AMS-01 prototype, our results indicate that it was most probably due to contamination in the laboratory by the vacuum system, or in-flight contamination due to fumes from any nearby material not stable at high temperatures.
181
Acknowledgements The tests were conducted on aerogel samples kindly lent to us by J. Berdugo, from the AMSCIEMAT group (Spain). The technical support of J.M. Lo´pez, R. Nu´n˜ez, M. Rangel and E. Camarillo is gratefully acknowledged. This project has been carried out with financial support from CONACYT projects G39091 and F44380, and PAPIIT-UNAM IN101501.
References [1] D. Barancourt, et al., Nucl. Instr. and Meth. A 465 (2001) 306 M. Buenerd, T. Thuillier, AMS Internal Report ISN Grenoble 99/122, 1999. [2] A.K. Gougas, et al., Nucl. Instr. and Meth. A 421 (1999) 249. [3] E. Lanciotti, for the AMS02-RICH Collaboration, Nucl. Instr. and Meth. A 518 (2004) 150. [4] M. Villoro, et al., Nucl. Instr. and Meth. A 480 (2002) 456. [5] T. Bellunato, et al., Nucl. Instr. and Meth. A 527 (2004) 319. [6] R. De Leo, et al., Nucl. Instr. and Meth. A 457 (2001) 52.