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Investigation into Cherenkov light scattering and refraction on aerogel surface
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Investigation into Cherenkov light scattering and refraction on aerogel surface A.Yu. Barnyakova,b,c, M.Yu. Barnyakova,b,c,e, V.S. Bobrovnikova,b, A.R. Buzykaeva,b, A.F. Danilyukb,d, A.A. Katcina,c, P.S. Kirilenkob, S.A. Kononova,b,e, D.V. Kordaa,b,e, ⁎ E.A. Kravchenkoa,b, , V.N. Kudryavtseva,b, I.A. Kuyanova,b,e, A.P. Onuchina,b,c, I.V. Ovtina,b,c, N.A. Podgornova,b,e, A.Yu. Predeind, V.G. Prisekina, R.S. Protsenkod, L.I. Shekhtmana,b a
Budker Institute of Nuclear Physics SB RAS, Novosibirsk, Russia Novosibirsk State University, Novosibirsk, Russia Novosibirsk State Technical University, Novosibirsk, Russia d Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia e FSBI “SSC RF ITEP” of NRC “Kurchatov Institute”, Moscow, Russia b c
A R T I C L E I N F O
A BS T RAC T
Keywords: Aerogel Ring imaging Cherenkov counter Particle identification
The work concerns the development of aerogel radiators for RICH detectors. Aerogel tiles with a refractive index of 1.05 were tested with a RICH prototype on the electron beam on the VEPP-4M collider. It has been shown that polishing with silk tissue yields good surface quality, the amount of light loss at this surface being about 5– 7%. The Cherenkov angle resolution was measured for a tile in two conditions: with a clean exit face and with a polished exit face. The number of photons detected was 13.3 and 12.7 for the clean and polished surfaces, respectively. The Cherenkov angle resolution for the polished surface is 55% worse, which can be explained with the forward scattering on the polished surface. A tile with a crack inside was also tested. The experimental data show that the Cherenkov angle resolution is the same for tracks crossing the crack area and in a crack-free area.
1. Introduction
2. Light scattering on aerogel surface
The particle identification power of the modern aerogel RICH detectors strongly depends on the optical quality of the radiators. In this work we investigate two factors which affect the optical quality of an aerogel Cherenkov radiator. The development of multilayer focusing aerogel radiators for the RICH detectors [1–4] requires the entrance and exit surfaces to be flat and optically clean. This is most important for radiators composed of 3–4 aerogel layers. The first aim of this work was to find a method of aerogel polishing. Tiles with a polished surface were tested using a RICH prototype on the electron test beam on the VEPP-4M collider in Novosibirsk. Measurements of optical parameters of polished aerogel tiles are presented below. The second aim of the research was to find out what will happen with the Cherenkov angle resolution of a RICH detector if a crack appears inside the aerogel radiator.
The current technology of aerogel synthesis used at Boreskov Institute of Catalysis in Novosibirsk cannot produce aerogel tiles with an optically clean flat bottom (entrance) surface. To prevent gel adhesion to stainless steel molds during production, the molds are covered with paraffin. After contacting with the paraffin, the aerogel surface becomes slightly matte. The existing technologies of optical polishing of materials use liquids. A liquid has to wet the polished material for aerogel fragments to be removed from the processed plane. Since the capillary forces on the liquid-air interface destroy the aerogel, such technology cannot be used for aerogel polishing. A quantitative method to characterize the light scattering on an aerogel surface was developed [5] for examination of different polishing procedures. The Hunt formula was modified to describe the scattering in a thin layer of silica dust on the surface of aerogel tile. The following additional term with M coefficient and inverse square dependence on the wavelength was added:
⁎
Corresponding author at: Budker Institute of Nuclear Physics SB RAS, Novosibirsk, Russia. E-mail address: [email protected] (E.A. Kravchenko).
http://dx.doi.org/10.1016/j.nima.2017.03.051 Received 30 November 2016; Received in revised form 16 March 2017; Accepted 28 March 2017 0168-9002/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Barnyakov, A.Y., Nuclear Instruments and Methods in Physics Research A (2017), http://dx.doi.org/10.1016/j.nima.2017.03.051
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
A.Y. Barnyakov et al.
Table 1 Results of transparency fit using Formula (1) for aerogel tile before and after polishing by different techniques.
A, % Lsc, mm M
Initial tile
Abrasive paper
Natural silk
94.9 ± 0.8 51.3 ± 0.9 0.01 ± 0.01
91.2 ± 1.7 56.8 ± 2.2 0.36 ± 0.03
92.4 ± 1.2 50.3 ± 1.5 0.05 ± 0.02
Table 2 Results of test beam measurements of aerogel tiles with different exit surfaces. R – Cherenkov ring radius, σ R – Cherenkov ring radius resolution, Npe – number of photons detected in ring. Surface conditions
R, mm
σ R , mm
Npe
Clean Natural silk
69.33 ± 0.05 69.01 ± 0.05
1.18 ± 0.03 1.83 ± 0.06
13.3 ± 0.1 12.7 ± 0.1
Clean Abrasive paper
69.78 ± 0.05 68.01 ± 0.05
1.22 ± 0.03 2.67 ± 0.09
12.6 ± 0.1 12.0 ± 0.1
Clean Raw bottom
70.95 ± 0.05 71.27 ± 0.05
1.08 ± 0.04 1.39 ± 0.06
13.3 ± 0.1 13.3 ± 0.1
Fig. 3. Cherenkov ring radius resolution for single hits (photons) at different points of tile. The vertical lines isolate points in the crack.
ency of polished aerogel tiles. Lsc from the fit does not differ significantly after different polishing procedures. Tests of polishing with different fine abrasive papers gave no positive results. We concluded that dry polishing with an abrasive cannot remove aerogel fragments from the surface, these fragments remaining inside the aerogel pores. We decided to use some porous material and tried several tissues: wool, natural silk, and synthetic silk. The best result was obtained with the natural silk tissue [5]. The transparency of a single aerogel tile was measured using spectrophotometer several times. The results of using Formula (1) to fit the measurements of initial transparency of aerogel tile and its transparency after polishing with abrasive paper and with natural silk are presented in Table 1. For direct measurement of the quality of surface polishing of aerogel, three tiles were tested on the electron beam on the VEPP4M collider [6] using a DPC-RICH prototype equipped with a matrix of DPC photon detectors [7]. The DPC RICH detector is a square array of 24×24 DPC-3200-22 sensors (chips). Each sensor is furnished with a TDC and yields the time of the first cell hit with a least significant bit value of about 20 ps. The sensor is divided into 4 pixels of 3.2×3.9 mm2 size. Each pixel has 3200 cells and constitutes a single amplitude channel. The pixels cover the area of the detector with a 69% geometrical efficiency. The whole setup contains 2304 pixels, covering a total area of 20×20 cm2. The photon detector with the front-end electronics is placed in a thermally insulated box and cooled down to −40 °C for the dark count rate to be reduced by two orders of magnitude as compared with the room temperature. The tests were performed twice for each aerogel tile. In the first measurement, the Cherenkov photons left the radiator through the clean (top) surface, and in the second one, through the polished (bottom) surface. The results are presented in Table 2. The number of detected photons is 13.3 and 12.7 for the clean and silk-polished surface, respectively. The numbers of Cherenkov photons detected for the ‘clean’ and ‘polished’ surfaces are consistent with the transparency measurements of the tiles. We observed strong degradation of the Cherenkov ring radius resolution when photons exited through the ‘polished’ surface. This can be explained with the strong small angle scattering inside the thin subsurface polished layer, and thus we conclude that polishing of aerogel with silk does not work well.
Fig. 1. Photo of aerogel tile with crack. The rectangles show the location of the experimental track areas.
Fig. 2. Cherenkov ring radius at different points of tile. The vertical lines isolate points in the crack.
⎛ d M ⎞ T (λ ) = A·exp ⎜ − − ⎟, ⎝ Lsc·(λ /400)4 (λ /400)2 ⎠
3. Effect of cracks on Cherenkov angle resolution in Aerogel RICH detectors
(1)
where T is the transparency of aerogel tile, A is the surface scattering coefficient, d is the thickness of the tile in mm, Lsc is the light scattering length in mm at 400 nm, λ is the light wavelength in nm. Subsequent tests showed that Formula (1) describes well the transpar-
Aerogel is a very specific optical material since its refractive index is close to 1. There are several factors that minimize the deflection of a photon passing through a crack: 2
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
A.Y. Barnyakov et al.
•
•
4. Conclusion
A crack is a thin air gap inside the aerogel. As the curvature radius of the crack is much larger than the gap width, the crack can be considered as a parallel gap which does not deflect photons. In addition, as sin Θagl × 1.05 = sin Θair , the difference between the angle of incidence and the angle of refraction shall be small in the case of nonparallel gap. The Fresnel reflection coefficient for an aerogel-air interface at normal incidence is R = (n − 1)2 /(n + 1)2 ≈ 0.0006 . Thus, the loss of photons on the crack due to reflection is negligible.
The previously found method of aerogel polishing with silk cannot be used for production of radiators for the RICH detectors because of the strong effect of forward scattering on the polished surface. The search for aerogel polishing technology should be continued. In single-layer aerogel RICH detectors, a non-destructive crack in the aerogel radiator does not influence substantially the accuracy of the Cherenkov angle measurement. The experimental data show that the Cherenkov angle resolution is the same for tracks in a crack area of radiator and for tracks in a normal area.
To verify these considerations we have tested an aerogel tile with a crack using the DPC-RICH prototype [7]. The index of refraction of the tile was 1.05; its thickness was 30 mm. We measured the Cherenkov emission angle from electrons with a 3 GeV/c momentum in 18 rectangular areas, 5×20 mm2 each. Fig. 1presents a photo of the aerogel tile with the crack and the scheme of the selected track areas. Fig. 2 shows experimental results obtained on the Cherenkov ring radius at different points of the tile. Fig. 3 presents experimental results for the Cherenkov ring radius resolution for a single hit (photon) at different points. The increase in the radius of the ring is explained with the variation of the refractive index of aerogel Δn = 0.001, which corresponds to a 2% density variation. We see no degradation or peculiarities in the Cherenkov ring radius resolution for points which correspond to the crack area. Under certain conditions it is possible to have a reflection of a photon at the crack surface that would lead to loss of photons. The probability of these conditions to be met must be rather small as no signal loss is observed when an area of 5×20 mm2 on the tile is used.
Acknowledgments This work was supported in part by the Ministry of Education and Science of the Russian Federation, grant Sci. School-2479.2014.2, RFBR grant 14-02-00984-a and the FAIR-Russia Research Center. References [1] [2] [3] [4] [5] [6]
T. Iijima, et al., Nucl. Instrum. Methods A 548 (2005) 383. S. Korpar, et al., Nucl. Instrum. Methods A 553 (2005) 64. A.Yu. Barnyakov, et al., Nucl. Instrum. Methods A 553 (2005) 70. A.Yu. Barnyakov, et al., Proceedings of SNIC 2006, eConf C0604032, 2006, p. 0045. A.Yu. Barnyakov, et al., Nucl. Instrum. Methods A 824 (2016) 123. G.N. Abramov, et al., J. Instrum. 9 (2014) C08022 http://iopscience.iop.org/article/ 10.1088/1748-0221/9/08/C08022. [7] A.Yu. Barnyakov, et al., Nucl. Instrum. Methods A 732 (2013) 352.
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Investigation into Cherenkov light scattering and refraction on aerogel surface A.Yu. Barnyakova,b,c, M.Yu. Barnyakova,b,c,e, V.S. Bobrovnikova,b, A.R. Buzykaeva,b, A.F. Danilyukb,d, A.A. Katcina,c, P.S. Kirilenkob, S.A. Kononova,b,e, D.V. Kordaa,b,e, ⁎ E.A. Kravchenkoa,b, , V.N. Kudryavtseva,b, I.A. Kuyanova,b,e, A.P. Onuchina,b,c, I.V. Ovtina,b,c, N.A. Podgornova,b,e, A.Yu. Predeind, V.G. Prisekina, R.S. Protsenkod, L.I. Shekhtmana,b a
Budker Institute of Nuclear Physics SB RAS, Novosibirsk, Russia Novosibirsk State University, Novosibirsk, Russia Novosibirsk State Technical University, Novosibirsk, Russia d Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia e FSBI “SSC RF ITEP” of NRC “Kurchatov Institute”, Moscow, Russia b c
A R T I C L E I N F O
A BS T RAC T
Keywords: Aerogel Ring imaging Cherenkov counter Particle identification
The work concerns the development of aerogel radiators for RICH detectors. Aerogel tiles with a refractive index of 1.05 were tested with a RICH prototype on the electron beam on the VEPP-4M collider. It has been shown that polishing with silk tissue yields good surface quality, the amount of light loss at this surface being about 5– 7%. The Cherenkov angle resolution was measured for a tile in two conditions: with a clean exit face and with a polished exit face. The number of photons detected was 13.3 and 12.7 for the clean and polished surfaces, respectively. The Cherenkov angle resolution for the polished surface is 55% worse, which can be explained with the forward scattering on the polished surface. A tile with a crack inside was also tested. The experimental data show that the Cherenkov angle resolution is the same for tracks crossing the crack area and in a crack-free area.
1. Introduction
2. Light scattering on aerogel surface
The particle identification power of the modern aerogel RICH detectors strongly depends on the optical quality of the radiators. In this work we investigate two factors which affect the optical quality of an aerogel Cherenkov radiator. The development of multilayer focusing aerogel radiators for the RICH detectors [1–4] requires the entrance and exit surfaces to be flat and optically clean. This is most important for radiators composed of 3–4 aerogel layers. The first aim of this work was to find a method of aerogel polishing. Tiles with a polished surface were tested using a RICH prototype on the electron test beam on the VEPP-4M collider in Novosibirsk. Measurements of optical parameters of polished aerogel tiles are presented below. The second aim of the research was to find out what will happen with the Cherenkov angle resolution of a RICH detector if a crack appears inside the aerogel radiator.
The current technology of aerogel synthesis used at Boreskov Institute of Catalysis in Novosibirsk cannot produce aerogel tiles with an optically clean flat bottom (entrance) surface. To prevent gel adhesion to stainless steel molds during production, the molds are covered with paraffin. After contacting with the paraffin, the aerogel surface becomes slightly matte. The existing technologies of optical polishing of materials use liquids. A liquid has to wet the polished material for aerogel fragments to be removed from the processed plane. Since the capillary forces on the liquid-air interface destroy the aerogel, such technology cannot be used for aerogel polishing. A quantitative method to characterize the light scattering on an aerogel surface was developed [5] for examination of different polishing procedures. The Hunt formula was modified to describe the scattering in a thin layer of silica dust on the surface of aerogel tile. The following additional term with M coefficient and inverse square dependence on the wavelength was added:
⁎
Corresponding author at: Budker Institute of Nuclear Physics SB RAS, Novosibirsk, Russia. E-mail address: [email protected] (E.A. Kravchenko).
http://dx.doi.org/10.1016/j.nima.2017.03.051 Received 30 November 2016; Received in revised form 16 March 2017; Accepted 28 March 2017 0168-9002/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Barnyakov, A.Y., Nuclear Instruments and Methods in Physics Research A (2017), http://dx.doi.org/10.1016/j.nima.2017.03.051
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
A.Y. Barnyakov et al.
Table 1 Results of transparency fit using Formula (1) for aerogel tile before and after polishing by different techniques.
A, % Lsc, mm M
Initial tile
Abrasive paper
Natural silk
94.9 ± 0.8 51.3 ± 0.9 0.01 ± 0.01
91.2 ± 1.7 56.8 ± 2.2 0.36 ± 0.03
92.4 ± 1.2 50.3 ± 1.5 0.05 ± 0.02
Table 2 Results of test beam measurements of aerogel tiles with different exit surfaces. R – Cherenkov ring radius, σ R – Cherenkov ring radius resolution, Npe – number of photons detected in ring. Surface conditions
R, mm
σ R , mm
Npe
Clean Natural silk
69.33 ± 0.05 69.01 ± 0.05
1.18 ± 0.03 1.83 ± 0.06
13.3 ± 0.1 12.7 ± 0.1
Clean Abrasive paper
69.78 ± 0.05 68.01 ± 0.05
1.22 ± 0.03 2.67 ± 0.09
12.6 ± 0.1 12.0 ± 0.1
Clean Raw bottom
70.95 ± 0.05 71.27 ± 0.05
1.08 ± 0.04 1.39 ± 0.06
13.3 ± 0.1 13.3 ± 0.1
Fig. 3. Cherenkov ring radius resolution for single hits (photons) at different points of tile. The vertical lines isolate points in the crack.
ency of polished aerogel tiles. Lsc from the fit does not differ significantly after different polishing procedures. Tests of polishing with different fine abrasive papers gave no positive results. We concluded that dry polishing with an abrasive cannot remove aerogel fragments from the surface, these fragments remaining inside the aerogel pores. We decided to use some porous material and tried several tissues: wool, natural silk, and synthetic silk. The best result was obtained with the natural silk tissue [5]. The transparency of a single aerogel tile was measured using spectrophotometer several times. The results of using Formula (1) to fit the measurements of initial transparency of aerogel tile and its transparency after polishing with abrasive paper and with natural silk are presented in Table 1. For direct measurement of the quality of surface polishing of aerogel, three tiles were tested on the electron beam on the VEPP4M collider [6] using a DPC-RICH prototype equipped with a matrix of DPC photon detectors [7]. The DPC RICH detector is a square array of 24×24 DPC-3200-22 sensors (chips). Each sensor is furnished with a TDC and yields the time of the first cell hit with a least significant bit value of about 20 ps. The sensor is divided into 4 pixels of 3.2×3.9 mm2 size. Each pixel has 3200 cells and constitutes a single amplitude channel. The pixels cover the area of the detector with a 69% geometrical efficiency. The whole setup contains 2304 pixels, covering a total area of 20×20 cm2. The photon detector with the front-end electronics is placed in a thermally insulated box and cooled down to −40 °C for the dark count rate to be reduced by two orders of magnitude as compared with the room temperature. The tests were performed twice for each aerogel tile. In the first measurement, the Cherenkov photons left the radiator through the clean (top) surface, and in the second one, through the polished (bottom) surface. The results are presented in Table 2. The number of detected photons is 13.3 and 12.7 for the clean and silk-polished surface, respectively. The numbers of Cherenkov photons detected for the ‘clean’ and ‘polished’ surfaces are consistent with the transparency measurements of the tiles. We observed strong degradation of the Cherenkov ring radius resolution when photons exited through the ‘polished’ surface. This can be explained with the strong small angle scattering inside the thin subsurface polished layer, and thus we conclude that polishing of aerogel with silk does not work well.
Fig. 1. Photo of aerogel tile with crack. The rectangles show the location of the experimental track areas.
Fig. 2. Cherenkov ring radius at different points of tile. The vertical lines isolate points in the crack.
⎛ d M ⎞ T (λ ) = A·exp ⎜ − − ⎟, ⎝ Lsc·(λ /400)4 (λ /400)2 ⎠
3. Effect of cracks on Cherenkov angle resolution in Aerogel RICH detectors
(1)
where T is the transparency of aerogel tile, A is the surface scattering coefficient, d is the thickness of the tile in mm, Lsc is the light scattering length in mm at 400 nm, λ is the light wavelength in nm. Subsequent tests showed that Formula (1) describes well the transpar-
Aerogel is a very specific optical material since its refractive index is close to 1. There are several factors that minimize the deflection of a photon passing through a crack: 2
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
A.Y. Barnyakov et al.
•
•
4. Conclusion
A crack is a thin air gap inside the aerogel. As the curvature radius of the crack is much larger than the gap width, the crack can be considered as a parallel gap which does not deflect photons. In addition, as sin Θagl × 1.05 = sin Θair , the difference between the angle of incidence and the angle of refraction shall be small in the case of nonparallel gap. The Fresnel reflection coefficient for an aerogel-air interface at normal incidence is R = (n − 1)2 /(n + 1)2 ≈ 0.0006 . Thus, the loss of photons on the crack due to reflection is negligible.
The previously found method of aerogel polishing with silk cannot be used for production of radiators for the RICH detectors because of the strong effect of forward scattering on the polished surface. The search for aerogel polishing technology should be continued. In single-layer aerogel RICH detectors, a non-destructive crack in the aerogel radiator does not influence substantially the accuracy of the Cherenkov angle measurement. The experimental data show that the Cherenkov angle resolution is the same for tracks in a crack area of radiator and for tracks in a normal area.
To verify these considerations we have tested an aerogel tile with a crack using the DPC-RICH prototype [7]. The index of refraction of the tile was 1.05; its thickness was 30 mm. We measured the Cherenkov emission angle from electrons with a 3 GeV/c momentum in 18 rectangular areas, 5×20 mm2 each. Fig. 1presents a photo of the aerogel tile with the crack and the scheme of the selected track areas. Fig. 2 shows experimental results obtained on the Cherenkov ring radius at different points of the tile. Fig. 3 presents experimental results for the Cherenkov ring radius resolution for a single hit (photon) at different points. The increase in the radius of the ring is explained with the variation of the refractive index of aerogel Δn = 0.001, which corresponds to a 2% density variation. We see no degradation or peculiarities in the Cherenkov ring radius resolution for points which correspond to the crack area. Under certain conditions it is possible to have a reflection of a photon at the crack surface that would lead to loss of photons. The probability of these conditions to be met must be rather small as no signal loss is observed when an area of 5×20 mm2 on the tile is used.
Acknowledgments This work was supported in part by the Ministry of Education and Science of the Russian Federation, grant Sci. School-2479.2014.2, RFBR grant 14-02-00984-a and the FAIR-Russia Research Center. References [1] [2] [3] [4] [5] [6]
T. Iijima, et al., Nucl. Instrum. Methods A 548 (2005) 383. S. Korpar, et al., Nucl. Instrum. Methods A 553 (2005) 64. A.Yu. Barnyakov, et al., Nucl. Instrum. Methods A 553 (2005) 70. A.Yu. Barnyakov, et al., Proceedings of SNIC 2006, eConf C0604032, 2006, p. 0045. A.Yu. Barnyakov, et al., Nucl. Instrum. Methods A 824 (2016) 123. G.N. Abramov, et al., J. Instrum. 9 (2014) C08022 http://iopscience.iop.org/article/ 10.1088/1748-0221/9/08/C08022. [7] A.Yu. Barnyakov, et al., Nucl. Instrum. Methods A 732 (2013) 352.
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