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Construction of silica aerogel radiator system for Belle II RICH Counter
Nuclear Instruments and Methods in Physics Research A (xxxx) xxxx–xxxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Construction of silica aerogel radiator system for Belle II RICH Counter ⁎
I. Adachia,b, , R. Dolenecc, K. Hatayad, S. Iorie, S. Iwatad, H. Kakunod, R. Katauraf, H. Kawaig, H. Kindob, T. Kobayashif, S. Korparh,c, P. Križani,c, T. Kumitad, M. Mrvarc, S. Nishidaa,b, K. Ogawaf, S. Ogawae, R. Pestotnikc, L. Šantelja, T. Sumiyoshid, M. Tabatag, M. Yonenagad, Y. Yusaf a
Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan SOKENDAI (The Graduate University of Advanced Science), Tsukuba, Japan c Jožef Stefan Institute, Ljubljana, Slovenia d Tokyo Metropolitan University, Hachioji, Japan e Toho University, Funabashi, Japan f Niigata University, Niigata, Japan g Chiba University, Chiba, Japan h University of Maribor, Maribor, Slovenia i University of Ljubljana, Ljubljana, Slovenia b
A R T I C L E I N F O
A BS T RAC T
Keywords: Silica aerogel RICH Cherenkov radiator Belle II
We have developed a RICH counter as a new forward particle identification device for the Belle II experiment. As a Cherenkov radiator in this counter, a dual aerogel layer combination consisting of two refractive indicies, n=1.045 and 1.055, is employed. Mass production of these aerogel tiles has been done during 2013–2014 with new method improved by Chiba group. Optical qualities for them have been examined. The refractive indices of the obtained tiles were found to be in good agreement with our expectations, and the transparencies were high enough to be used for the RICH radiator.
1. Introduction Particle identification, especially π and K separation, plays a key role to perform physics analyses at the Belle II experiment. For this purpose, we have been constructing new ring RICH counter based on a silica aerogel as a Cherenkov radiator to be installed into the forward end-cap structure [1]. For this radiator system, two aerogel tiles with two refractive indices of n=1.045 and 1.055 for upstream and downstream are chosen so that Cherenkov photons emitted from each layer can be overlapped onto a photon sensor front surface after traveling a 200 mm expansion distance as shown in Fig. 1. This organizes a “focusing radiator” configuration, which allows us to increase light yield without deteriorating a Cherenkov angle resolution associated with emission point uncertainty [2]. Based on optimization studies done so far [3], the radiator tiling scheme is designed as shown in Fig. 2. In the rϕ plan, the radiator system is segmented into four rings, indicating that 4 types of wedgeshaped blocks are necessary. A wedge-shaped tile is cut out of a squareshaped 180 × 180 × 20 mm3 piece. In total, 124 tiles for each refractive index should be produced. The next section will mention aerogel production study, and mass
⁎
production and several results from quality checks we have carried out will be described in the Section 3. Then, the Section 4 will present tile machining and its installation into the radiator container. Summary will be given in the Section 5. 2. Aerogel production study The aerogel is first synthesized in alcohol solvent and the sol-gel reaction proceeds by the following two steps: Si(OCH3)4+4H2O → Si(OH)4+4 CH3OH (hydrolysis), mSi(OH)4→ (SiO2)m+2mH2O (condensation). Then, three dimensional network of SiO2 molecules, called “alco-gel”, is organized. After a hydrophobic and a rinsing processes, alco-gel is then dried by converting alcohol into air. However, surface extension force generated in a direct transition from liquid to gas phases can easily destroy its fragile structure. Therefore, we need to dry it in the supercritical (SC) phase using a special vessel, called “autoclave”, by controlling pressure and temperature. In our case, CO2 extraction method is employed instead of having the SC drying in alcohol solvent because of safety reason [4]. After alco-gel blocks are stored in the autoclave, alcohol is replaced into CO2 in the SC condition, where the critical point of CO2 is 31 °C with 7.4 MPa.
Corresponding author at: Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan.
http://dx.doi.org/10.1016/j.nima.2017.02.036 Received 14 November 2016; Received in revised form 27 December 2016; Accepted 13 February 2017 0168-9002/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Adachi, I., Nuclear Instruments and Methods in Physics Research A (2017), http://dx.doi.org/10.1016/j.nima.2017.02.036
Nuclear Instruments and Methods in Physics Research A (xxxx) xxxx–xxxx
I. Adachi et al.
n1< n2 n1 n2
Fig. 1. Schematic view of the dual focusing radiator scheme.
Fig. 2. Cross section of the Belle II RICH radiator system.
Fig. 4. Refractive index distributions for n=1.045 (top) and n=1.055 (bottom) tiles.
produced using their facility in 2011. A company, the Japan Fine Ceramics Center (JFCC),2 was newly contacted since this center had some experience in producing various types of aerogel samples, and there was joint effort between JFCC and Panasonic. A necessary technology transfer to JFCC was done and we had carried out several production tests during 2012–2013 to establish detailed procedures and to maintain tile quality at JFCC [6].
Fig. 3. Aerogel tile from mass production with dimensions of 180 × 180 × 20 mm3 . Small tile (100 × 100 × 20 mm3) produced for Belle aerogel Cherenkov counter is also shown for comparison.
3. Mass production and quality check Fundamental optical parameters, for instance refractive index, in aerogels obtained after the SC drying are basically defined in the synthesis step. On the other hand, the SC drying step mainly gives serious effect on physical condition of the resultant aerogel. In particular, crack in the aerogel tile volume is of our prime concern since to make a crack-free tile is one of the most important issues. This was carefully investigated before mass production, and the conditions in the SC drying process was examined by Chiba university group [5]. It was found that a duration of the pressure change after the SC condition is attained gives a certain impact on crack generation for the tiles. Therefore, we expanded this period by 3 times longer than before. By introducing this, a crack-free probability was improved by about 30%. It is noted that we had been working aerogel production many years with Panasonic,1 but they decided that new aerogel tiles were no longer
Mass production of the aerogel tiles started from September 2013, and was completed in May 2014. During this period, 16 batches were submitted to JFCC, where a production recipe was provided from our group. One batch contains 28 tiles, which comes from volume capability of the autoclave in the SC drying system. In this mass production, 87% crack free rate was achieved. Fig. 3 shows a photograph of our aerogel tile produced. After new tiles were delivered to KEK, the following quality checks have been performed. (1) visual inspection, (2) dimension and weight measurements, (3) optical quality measurements. As a first step, 1 2
2
1048, Kadoma, Osaka 57l-8686, Japan 2–4-1 Mutsuno, Atsuta-ku, Nagoya, 456–8587 Japan
Nuclear Instruments and Methods in Physics Research A (xxxx) xxxx–xxxx
I. Adachi et al.
Fig. 7. The distribution of refractive index difference Δn = nup − n down for obtained combinations.
Fig. 8. Wedge-shaped tile after a water jet cutting.
Fig. 5. Transmission length distributions for n=1.045 (top) and n=1.055 (bottom) tiles for λ = 400 nm .
Fig. 9. The radiator container.
Fig. 6. Transmission length at λ = 400 nm as a function of the refractive index for all tiles.
existence of any cracks in the tile is visually scanned. At the same time, milky area, which is usually caused by wrong synthesis process in solgel polymerization, is also examined. Once these failure points were found, the tile was rejected from the candidates and subsequent measurements were not proceeded for the faulty sample. From this measurements a crack-free propability of ∼87% was achieved, and the
Fig. 10. Photograph of the aerogel boxes after the installation.
3
Nuclear Instruments and Methods in Physics Research A (xxxx) xxxx–xxxx
I. Adachi et al.
candidate. The container was designed to realize the tiling scheme as shown in Fig. 2. It was delivered in December 2015, as shown in Fig. 9. To reduce material budget, aluminum is used for all components. Two aerogel tiles are placed into each box in the container, and one box is surrounded with thin septum walls, where 0.3 mm and 0.5 mm thick plates are used for the radial and ϕ directions, respectively. These septum walls are connected to the 1 mm thick bottom plane using spot welding technique. The welding conditions were optimized so that each spot welding should keep tight connections between thick plates with minimizing thermal deformations. As can be seen in Fig. 9, each box is mechanically isolated. Therefore, Cherenkov photons emitted at the tile boundary can not enter into the adjacent radiator. The installation of aerogel tiles into the container started from May 2016. Each space, where two aerogel tiles are installed, are filled with white foams to avoid possible deformation during the transportation from the company to KEK. We first remove these white foams, and the black sheets cover aluminum surface of each box, where tiles are stacked. Then, two layers of tiles of n=1.045 and 1.055 are installed, and glass fiber are stringed to fix the tiles. These procedures are repeated one-by-one until all tiles are placed into the container. At present, 70% of tiles have been installed, and it will be completed in 2016 (Fig. 10).
standard deviations of the tile length measurements was found to be 0.6 mm and 1.0 mm for 1.045 and 1.055 samples, respectively. Two important parameters of transmittance and refractive index were measured as an optical quality assessment. The refractive index was extracted from a deflection angle in the Fraunhofer method by injecting a 405 nm laser into the block corner. The measured values are plotted in Fig. 4. The averages over all samples are obtained to be 1.0451 ± 0.0007 and 1.0547 ± 0.0007 for 1.045 and 1.055 samples, respectively. These numbers are consistent with our targets. The transmittance can be expressed as; T = T0 exp(−d / Λ (λ )), where T0 and T are the initial and the measured intensities of the light with a wave-length of λ, d is the thickness of the sample, and Λ is the transmission length, which depends on the wave-length. We measured the transmittance using a spectro-photometer from UV to visible wavelength region. Fig. 5 shows the transmission length for two refractive indices at λ = 400 nm . The average transmission lengths at 400 nm wave-length were obtained to be 47.3 ± 3.1 mm and 36.0 ± 2.7 mm for n=1.045 and 1.055, respectively. Compared to the previous study [3], high optical qualities of our tiles from our mass production were confirmed. The measured transmission length at λ = 400 nm as a function of the refractive index for all tiles is plotted in Fig. 6. Some batch dependences can be found, however they are within our tolerance. From these assessments, 182 tiles (n=1.045) and 151 tiles (n=1.055) can be used for the radiator candidates. The yield with respect to the total number of tiles produced is 74%, which is consistent with our estimation of 70%. From those tiles, combinations of upstream and downstream tiles were assembled to form a focusing radiator system. To focus two Cherenkov images from two different refractive index layers, our condition is the refractive index difference (Δn ) between upstream (n up ) and downstream (ndown ) should not exceed 0.012. Fig. 7 indicates obtained combinations. As can be seen in this figure, our radiator matching in refractive index are well under our requirement, and the Cherenkov angle resolution due to the refractive index matching uncertainty is expected to be small enough.
5. Summary We have produced aerogel tiles having n=1.045 and 1.055 for the Cherenkov radiator system in the Belle II RICH counter. The quality assessments for all tiles delivered have been done, and it was confirmed that mass production for aerogel tiles has been successfully completed. As a result, sufficient number of tile combinations can be arranged for the installation. Then, machining using a water-jet cutter was done to make a wedge-shaped block, and optical quality was found to be kept in this process. The mechanical structure for the radiator system has been built and delivered in 2015. The aerogel tile installation into this container is now in progress and it will be completed by the end of 2016. With this Cherenkov radiator system, ∼5.5σ π / K separation at 4 GeV/c is expected.
4. Tile machining and installation The tiles passing through the quality check were followed by machining process, by which wedge-shaped tiles are made. This cutting step was done using a water jet machine, where highly pressurized water is injected through a small needle onto a tile. A movement of the needle is programmable via a control device and a speed was optimized. Fig. 8 is a photograph of a wedge-shape tile after a water jet machining. The difference in the tile transmittance before and after this machining process was examined, and very tiny deterioration, less than 2% degradation in transmittance, was observed. Here, it is emphasized that this machining is possible since our aerogel tile posses highly hydrophobic feature. One drawback due to this step was that small chip was found, in particular, at the corner of the tile. In this case, missing area (ΔS ) was measured and if ΔS is beyond ∼1.0 cm2 corresponding to 0.4% with respect to the total tile surface, this sample is excluded from the
Acknowledgements This work was partially supported by Japan Society for the Promotion of Science KAKENHI Grant Numbers JP24244035 and JP17540284. References [1] Belle II Collaboration, Technical Design Report, KEK-REPORT-2010-1, arXiv:1011. 0352. [2] T. Iijima, S. Korpar, et al., Nucl. Instr. MethodsA 548 (2005) 383. [3] M. Tabata, et al., Nucl. Instrum. Methods A 697 (2013) 52. [4] I. Adachi, et al., Nucl. Instrum. Methods A 355 (1995) 390. [5] M. Tabata, et al., J. Supercrit. Fluids 110 (2016) 183. [6] M. Tabata, et al., Nucl. Instrum. Methods A 766 (2014) 212.
4
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Construction of silica aerogel radiator system for Belle II RICH Counter ⁎
I. Adachia,b, , R. Dolenecc, K. Hatayad, S. Iorie, S. Iwatad, H. Kakunod, R. Katauraf, H. Kawaig, H. Kindob, T. Kobayashif, S. Korparh,c, P. Križani,c, T. Kumitad, M. Mrvarc, S. Nishidaa,b, K. Ogawaf, S. Ogawae, R. Pestotnikc, L. Šantelja, T. Sumiyoshid, M. Tabatag, M. Yonenagad, Y. Yusaf a
Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan SOKENDAI (The Graduate University of Advanced Science), Tsukuba, Japan c Jožef Stefan Institute, Ljubljana, Slovenia d Tokyo Metropolitan University, Hachioji, Japan e Toho University, Funabashi, Japan f Niigata University, Niigata, Japan g Chiba University, Chiba, Japan h University of Maribor, Maribor, Slovenia i University of Ljubljana, Ljubljana, Slovenia b
A R T I C L E I N F O
A BS T RAC T
Keywords: Silica aerogel RICH Cherenkov radiator Belle II
We have developed a RICH counter as a new forward particle identification device for the Belle II experiment. As a Cherenkov radiator in this counter, a dual aerogel layer combination consisting of two refractive indicies, n=1.045 and 1.055, is employed. Mass production of these aerogel tiles has been done during 2013–2014 with new method improved by Chiba group. Optical qualities for them have been examined. The refractive indices of the obtained tiles were found to be in good agreement with our expectations, and the transparencies were high enough to be used for the RICH radiator.
1. Introduction Particle identification, especially π and K separation, plays a key role to perform physics analyses at the Belle II experiment. For this purpose, we have been constructing new ring RICH counter based on a silica aerogel as a Cherenkov radiator to be installed into the forward end-cap structure [1]. For this radiator system, two aerogel tiles with two refractive indices of n=1.045 and 1.055 for upstream and downstream are chosen so that Cherenkov photons emitted from each layer can be overlapped onto a photon sensor front surface after traveling a 200 mm expansion distance as shown in Fig. 1. This organizes a “focusing radiator” configuration, which allows us to increase light yield without deteriorating a Cherenkov angle resolution associated with emission point uncertainty [2]. Based on optimization studies done so far [3], the radiator tiling scheme is designed as shown in Fig. 2. In the rϕ plan, the radiator system is segmented into four rings, indicating that 4 types of wedgeshaped blocks are necessary. A wedge-shaped tile is cut out of a squareshaped 180 × 180 × 20 mm3 piece. In total, 124 tiles for each refractive index should be produced. The next section will mention aerogel production study, and mass
⁎
production and several results from quality checks we have carried out will be described in the Section 3. Then, the Section 4 will present tile machining and its installation into the radiator container. Summary will be given in the Section 5. 2. Aerogel production study The aerogel is first synthesized in alcohol solvent and the sol-gel reaction proceeds by the following two steps: Si(OCH3)4+4H2O → Si(OH)4+4 CH3OH (hydrolysis), mSi(OH)4→ (SiO2)m+2mH2O (condensation). Then, three dimensional network of SiO2 molecules, called “alco-gel”, is organized. After a hydrophobic and a rinsing processes, alco-gel is then dried by converting alcohol into air. However, surface extension force generated in a direct transition from liquid to gas phases can easily destroy its fragile structure. Therefore, we need to dry it in the supercritical (SC) phase using a special vessel, called “autoclave”, by controlling pressure and temperature. In our case, CO2 extraction method is employed instead of having the SC drying in alcohol solvent because of safety reason [4]. After alco-gel blocks are stored in the autoclave, alcohol is replaced into CO2 in the SC condition, where the critical point of CO2 is 31 °C with 7.4 MPa.
Corresponding author at: Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan.
http://dx.doi.org/10.1016/j.nima.2017.02.036 Received 14 November 2016; Received in revised form 27 December 2016; Accepted 13 February 2017 0168-9002/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Adachi, I., Nuclear Instruments and Methods in Physics Research A (2017), http://dx.doi.org/10.1016/j.nima.2017.02.036
Nuclear Instruments and Methods in Physics Research A (xxxx) xxxx–xxxx
I. Adachi et al.
n1< n2 n1 n2
Fig. 1. Schematic view of the dual focusing radiator scheme.
Fig. 2. Cross section of the Belle II RICH radiator system.
Fig. 4. Refractive index distributions for n=1.045 (top) and n=1.055 (bottom) tiles.
produced using their facility in 2011. A company, the Japan Fine Ceramics Center (JFCC),2 was newly contacted since this center had some experience in producing various types of aerogel samples, and there was joint effort between JFCC and Panasonic. A necessary technology transfer to JFCC was done and we had carried out several production tests during 2012–2013 to establish detailed procedures and to maintain tile quality at JFCC [6].
Fig. 3. Aerogel tile from mass production with dimensions of 180 × 180 × 20 mm3 . Small tile (100 × 100 × 20 mm3) produced for Belle aerogel Cherenkov counter is also shown for comparison.
3. Mass production and quality check Fundamental optical parameters, for instance refractive index, in aerogels obtained after the SC drying are basically defined in the synthesis step. On the other hand, the SC drying step mainly gives serious effect on physical condition of the resultant aerogel. In particular, crack in the aerogel tile volume is of our prime concern since to make a crack-free tile is one of the most important issues. This was carefully investigated before mass production, and the conditions in the SC drying process was examined by Chiba university group [5]. It was found that a duration of the pressure change after the SC condition is attained gives a certain impact on crack generation for the tiles. Therefore, we expanded this period by 3 times longer than before. By introducing this, a crack-free probability was improved by about 30%. It is noted that we had been working aerogel production many years with Panasonic,1 but they decided that new aerogel tiles were no longer
Mass production of the aerogel tiles started from September 2013, and was completed in May 2014. During this period, 16 batches were submitted to JFCC, where a production recipe was provided from our group. One batch contains 28 tiles, which comes from volume capability of the autoclave in the SC drying system. In this mass production, 87% crack free rate was achieved. Fig. 3 shows a photograph of our aerogel tile produced. After new tiles were delivered to KEK, the following quality checks have been performed. (1) visual inspection, (2) dimension and weight measurements, (3) optical quality measurements. As a first step, 1 2
2
1048, Kadoma, Osaka 57l-8686, Japan 2–4-1 Mutsuno, Atsuta-ku, Nagoya, 456–8587 Japan
Nuclear Instruments and Methods in Physics Research A (xxxx) xxxx–xxxx
I. Adachi et al.
Fig. 7. The distribution of refractive index difference Δn = nup − n down for obtained combinations.
Fig. 8. Wedge-shaped tile after a water jet cutting.
Fig. 5. Transmission length distributions for n=1.045 (top) and n=1.055 (bottom) tiles for λ = 400 nm .
Fig. 9. The radiator container.
Fig. 6. Transmission length at λ = 400 nm as a function of the refractive index for all tiles.
existence of any cracks in the tile is visually scanned. At the same time, milky area, which is usually caused by wrong synthesis process in solgel polymerization, is also examined. Once these failure points were found, the tile was rejected from the candidates and subsequent measurements were not proceeded for the faulty sample. From this measurements a crack-free propability of ∼87% was achieved, and the
Fig. 10. Photograph of the aerogel boxes after the installation.
3
Nuclear Instruments and Methods in Physics Research A (xxxx) xxxx–xxxx
I. Adachi et al.
candidate. The container was designed to realize the tiling scheme as shown in Fig. 2. It was delivered in December 2015, as shown in Fig. 9. To reduce material budget, aluminum is used for all components. Two aerogel tiles are placed into each box in the container, and one box is surrounded with thin septum walls, where 0.3 mm and 0.5 mm thick plates are used for the radial and ϕ directions, respectively. These septum walls are connected to the 1 mm thick bottom plane using spot welding technique. The welding conditions were optimized so that each spot welding should keep tight connections between thick plates with minimizing thermal deformations. As can be seen in Fig. 9, each box is mechanically isolated. Therefore, Cherenkov photons emitted at the tile boundary can not enter into the adjacent radiator. The installation of aerogel tiles into the container started from May 2016. Each space, where two aerogel tiles are installed, are filled with white foams to avoid possible deformation during the transportation from the company to KEK. We first remove these white foams, and the black sheets cover aluminum surface of each box, where tiles are stacked. Then, two layers of tiles of n=1.045 and 1.055 are installed, and glass fiber are stringed to fix the tiles. These procedures are repeated one-by-one until all tiles are placed into the container. At present, 70% of tiles have been installed, and it will be completed in 2016 (Fig. 10).
standard deviations of the tile length measurements was found to be 0.6 mm and 1.0 mm for 1.045 and 1.055 samples, respectively. Two important parameters of transmittance and refractive index were measured as an optical quality assessment. The refractive index was extracted from a deflection angle in the Fraunhofer method by injecting a 405 nm laser into the block corner. The measured values are plotted in Fig. 4. The averages over all samples are obtained to be 1.0451 ± 0.0007 and 1.0547 ± 0.0007 for 1.045 and 1.055 samples, respectively. These numbers are consistent with our targets. The transmittance can be expressed as; T = T0 exp(−d / Λ (λ )), where T0 and T are the initial and the measured intensities of the light with a wave-length of λ, d is the thickness of the sample, and Λ is the transmission length, which depends on the wave-length. We measured the transmittance using a spectro-photometer from UV to visible wavelength region. Fig. 5 shows the transmission length for two refractive indices at λ = 400 nm . The average transmission lengths at 400 nm wave-length were obtained to be 47.3 ± 3.1 mm and 36.0 ± 2.7 mm for n=1.045 and 1.055, respectively. Compared to the previous study [3], high optical qualities of our tiles from our mass production were confirmed. The measured transmission length at λ = 400 nm as a function of the refractive index for all tiles is plotted in Fig. 6. Some batch dependences can be found, however they are within our tolerance. From these assessments, 182 tiles (n=1.045) and 151 tiles (n=1.055) can be used for the radiator candidates. The yield with respect to the total number of tiles produced is 74%, which is consistent with our estimation of 70%. From those tiles, combinations of upstream and downstream tiles were assembled to form a focusing radiator system. To focus two Cherenkov images from two different refractive index layers, our condition is the refractive index difference (Δn ) between upstream (n up ) and downstream (ndown ) should not exceed 0.012. Fig. 7 indicates obtained combinations. As can be seen in this figure, our radiator matching in refractive index are well under our requirement, and the Cherenkov angle resolution due to the refractive index matching uncertainty is expected to be small enough.
5. Summary We have produced aerogel tiles having n=1.045 and 1.055 for the Cherenkov radiator system in the Belle II RICH counter. The quality assessments for all tiles delivered have been done, and it was confirmed that mass production for aerogel tiles has been successfully completed. As a result, sufficient number of tile combinations can be arranged for the installation. Then, machining using a water-jet cutter was done to make a wedge-shaped block, and optical quality was found to be kept in this process. The mechanical structure for the radiator system has been built and delivered in 2015. The aerogel tile installation into this container is now in progress and it will be completed by the end of 2016. With this Cherenkov radiator system, ∼5.5σ π / K separation at 4 GeV/c is expected.
4. Tile machining and installation The tiles passing through the quality check were followed by machining process, by which wedge-shaped tiles are made. This cutting step was done using a water jet machine, where highly pressurized water is injected through a small needle onto a tile. A movement of the needle is programmable via a control device and a speed was optimized. Fig. 8 is a photograph of a wedge-shape tile after a water jet machining. The difference in the tile transmittance before and after this machining process was examined, and very tiny deterioration, less than 2% degradation in transmittance, was observed. Here, it is emphasized that this machining is possible since our aerogel tile posses highly hydrophobic feature. One drawback due to this step was that small chip was found, in particular, at the corner of the tile. In this case, missing area (ΔS ) was measured and if ΔS is beyond ∼1.0 cm2 corresponding to 0.4% with respect to the total tile surface, this sample is excluded from the
Acknowledgements This work was partially supported by Japan Society for the Promotion of Science KAKENHI Grant Numbers JP24244035 and JP17540284. References [1] Belle II Collaboration, Technical Design Report, KEK-REPORT-2010-1, arXiv:1011. 0352. [2] T. Iijima, S. Korpar, et al., Nucl. Instr. MethodsA 548 (2005) 383. [3] M. Tabata, et al., Nucl. Instrum. Methods A 697 (2013) 52. [4] I. Adachi, et al., Nucl. Instrum. Methods A 355 (1995) 390. [5] M. Tabata, et al., J. Supercrit. Fluids 110 (2016) 183. [6] M. Tabata, et al., Nucl. Instrum. Methods A 766 (2014) 212.
4