- Email: [email protected]
Metal multi-dielectric mirror coatings for Cherenkov detectors
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186 www.elsevier.com/locate/nima
Metal multi-dielectric mirror coatings for Cherenkov detectors A. Braem, C. David, C. Joram PH Department, CERN, CH-1211 Geneva, Switzerland
Abstract Application-specific reflective coatings have been developed and are being implemented in LHC experiments currently under construction. The broadband reflective coating consists of an aluminum film combined with one or two pairs of low- and high-index dielectric layers. The layer stacks are designed and optimized using commercial thin film software and verified on small mirror samples. The wavelength of peak reflectivity is tuned to maximize the light yield, taking into account the emission spectrum (e.g., Cherenkov) and the photosensor characteristics. We report about coatings of Mylar foil-based light guides for the Hadronic Forward calorimeter of CMS and spherical mirrors for the RICH2 counter of LHCb. r 2005 Elsevier B.V. All rights reserved. PACS: 78.40; 78.20.Ci; 81.15.Ef; 29.40.Ka Keywords: Mirror; Reflectivity; Multi-dielectric film; Vacuum deposition; Sputtering
1. Introduction Cherenkov radiation is a weak light source. Good detector performance depends therefore on optimized light transport and collection. Highreflectivity mirrors are key elements in Cherenkov detectors, both in imaging and non-imaging designs. The importance of the mirror reflectivity grows with the number of reflections the Cherenkov photons undergo from source to detection. Reflectivity enhancing coatings on top of a highquality metallic film provide the possibility to Corresponding author.
E-mail address: [email protected] (A. Braem).
tailor the mirror characteristics to the specific needs of the experiment, taking into account the form of the Cherenkov emission spectrum dN=dE ¼ const:; dN=dl ¼ 1=l2 , the chromaticity of the radiator, the wavelength/ energy-dependent sensitivity of the photodetector eQ ðEÞ, and possible environmental constraints (temperature, humidity). The quantity to be optimized can be expressed as Z
E2 E1
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.08.074
Rn ðEÞ eQ ðEÞ TðEÞ dE,
ARTICLE IN PRESS A. Braem et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186
where R describes the mirror reflectivity, n is the number of reflections and T accounts for any other energy-dependent transport efficiencies (e.g., transparency of a radiator window). For the choice of the energy band, particularly the upper limit E2, parameters like the chromaticity of the radiator medium, Rayleigh scattering, or radiation driven aging of components may play a role. This article describes two developments of multi-layer dielectric coatings on aluminum films to produce high-reflectivity mirrors: 1. The flat and spherical mirrors of the RICH2 detector of LHCb [1] require optimized reflectivity in a broad band centered around 275 nm (driven by the sensitivity characteristics of the HPD detector) for incidence angles of 211 (spherical mirror) and 481 (flat mirror). The substrate is a 6 mm borosilicate glass. The goal is to achieve R494% for 250olo300 nm and R480% for 210olo500 nm. 2. In the CMS Forward Hadron Calorimeter [2], Cherenkov light produced in quartz fibers needs to be transported via a 40 cm long air core light guide (2.5 cm Ø) from the fiber bundle to the PMTs. The inner surface of the light guide has to provide maximum reflectivity in the range 400olo650 nm for incidence angles of about 701. On average the photons undergo six reflections. Suppression of the UV component below 400 nm is desired because its yield is subject to radiation-related aging and annealing of the quartz fibers. The substrate is a 75 mm thick polyester film which is rolled to form a cylindrical light guide.
2. Metal multi-dielectric mirror coatings The basic principle, illustrated in Fig. 1, consists in enhancing (or decreasing) the reflectivity of a metal film at a given wavelength band by exploiting interference effects in thin films. A highreflectivity (R) metal layer (Al: R92%; Ag: R96%) is overcoated with one or several pairs of transparent films of high (H) and low (L) refractive index. For this purpose, dielectric films like SiO2, MgF2 (L-materials) or HfO2, Nb2O5,
183
hν θin High Low High Low
n pairs
Al reflector Cr adherence layer
substrate
Fig. 1. Schematic representation of a metal multi-dielectric mirror.
TiO2 (H-materials) are used. For the selection of the coating materials, properties like residual stress, adherence, resistivity to abrasion and humidity, and coating yield are of importance. The optical thickness of the layers d opt ¼ nd cos yinc is usually chosen to be l=4. The dielectric coatings lead, therefore, to a wavelength- and angledependent modulation of the reflectivity. The larger the ratio of the refractive index of the LH pair, the higher is the peak reflectivity and width of the enhanced region. Adding LH pairs, optimized for the same wavelength, will increase the reflectivity but narrow down the useful range. The layer stack terminates usually with a H-layer. The mathematical treatment of multi-layer coatings relies on elaborated matrix methods (see, e.g., text books [3,4]). Simulation codes like FTG FilmStar [5] (used by us) and SCI Film Wizard [6] allow to predict the behavior of multi-layer films with an accuracy which depends on the knowledge of the relevant optical properties (refractive index nðlÞ and absorption coefficient kðlÞ) of the deposited layers. Those are generally dependent on the deposition method and process parameters. The simulation is therefore useful to define a concept and a promising starting point. The optimum result can, however, only be achieved by systematically varying the experimental parameters in a series of test coatings. The simulations help to interpret the results and narrow down the parameter space. Fig. 2 gives examples of simulated reflectivity curves. It
ARTICLE IN PRESS A. Braem et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186
184 100 95
Reflectivity [%]
90 85 80 75
Aluminium Aluminium + MgF2 Al + 1 pair SiO2/HfO2 Al + 2 pairs SiO2/HfO2
70 65 60 200
300
400 Wavelength [nm]
500
600
Fig. 2. Simulated reflectivity at yinc ¼ 301 of pure aluminum and Al coated with various layers. MgF2 serves as a protective layer of the Al coating without significant reflection enhancement. The thickness of the SiO2/HfO2 pairs was optimized for l ¼ 300 nm.
demonstrates the effect of protective and reflection enhancing coatings on top of an aluminum reflective layer.
3. Fabrication and characterization of metal multidielectric coatings at CERN A dedicated thin film coating plant is available at CERN for the production of coatings on substrates up to 0.9 m diameter. It consists of a high vacuum evaporation chamber of 1 m diameter evacuated with a cryogenic pump (3500 l/s). The end pressure is about 1 105 Pa. The plant is equipped with 2 Joule evaporation sources and an electron gun with four crucibles. Uniform layer thickness (75%) is achieved by rotating the substrate. The layer thickness is controlled with a calibrated quartz crystal balance. The typical production rate is 2 full process cycles per day. 3.1. Evaporation process As an example, we describe the evaporation process for the production of the LHCb mirrors on a glass substrate. The two metals, namely a 20 nm thick chrome adherence layer and the aluminum main reflectance layer, are evaporated from tungsten filaments. The substrate is at
ambient temperature. The dielectrics are evaporated from an electron gun source in an oxygen atmosphere (p(O2)103 Pa) to compensate for partial dissociation and oxygen loss during the evaporation process. TiO2 is obtained from Ti3O5 tablets of 99.5% purity. The thickness values are derived from the quartz crystal balance. The reflectivity can be measured only after the process is completed. A spectrophotometer1 allows to determine the absolute reflectivity at an angle of incidence (yinc) of 301 in the wavelength range 200–800 nm. The essential process parameters are summarized in Table 1.
4. Industrial fabrication of reflective films A large number of light guides (1800) needed for the CMS Hadron Forward calorimeter are required to produce 100 m2 of reflective film. The optimization of layer stack and substrate choice was done in collaboration with FEP Dresden.2 FEP also carried out the final production. The pilot sputter roll coater3 at FEP can coat polymer webs of 600 mm width with six different materials at a speed of 0.3–3 m/min. The applied technique is dual magnetron sputtering (DMS) for metals and reactive DMS for dielectric coatings. The chosen layer stack consists of Cr adhesion layer (10 nm)/ Al reflectance layer (70 nm)/2 pairs of SiO2/Nb2O5 (66/53 nm). A 75 mm thick polyester film (Melinex 400 from DuPont Teijing Films)4 was found appropriate as a substrate from the mechanical, thermal, and surface properties point of view.
5. Results The layer stack developed for the mirrors of LHCb RICH2 consists of Al+1 pair SiO2/HfO2. The hafnium oxide layer which terminates the 1
Perkin Elmer Lambda 15. Fraunhofer Institute for Electron Beam and Plasma Technology (FEP), Winterbergstrasse 28, 01277 Dresden, Germany (contact person: Dr. Matthias Fahland). 3 FOSA 600 - FOil Sputter Apparatus, 600 mm foil width. 4 DuPont Teijin Films, Luxembourg S.A., P.O. Box 1681, L-1016 Luxembourg. 2
ARTICLE IN PRESS A. Braem et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186
185
Table 1 Typical process parameters for LHCb mirror (Al+SiO2/HfO2), optimized for l ¼ 275 nm Purity (%)
Chamber pressure
Deposition rate (nm/s)
Thickness (geom.) (nm)
Cr Al SiO2 HfO2
99.98 99.999 99.99 99.9
2 105 Pa 2 105 Pa 103 Pa O2 103 Pa O2
1 5 0.2 0.2
20 85 28 38
100 95
98
90 85
96
97
95 R [%]
R [%]
Material
80 75 70
93 92
65 60 55 50 200
94
300
400 W.L. [nm]
Measurement
91
Simulation
90
500
0
600
Fig. 3. Reflectivity of the LHCb mirror coating Al+1 pair SiO2/HfO2 on glass, yinc ¼ 301. The layer stack was optimized for maximum reflectivity at l ¼ 275 nm.
5
10 15 20 25 Process number
30
0
5 10 15 20 25 Nb of mirrors
Fig. 4. Measured reflectivity (averaged in the interval 250olo350 nm, yinc ¼ 301) of the first batch of 32 LHCb mirror coatings. The statistical analysis (right plot) finds an average reflectivity of the batch of 95.570.9%.
λ [nm]
stack is known for its hard abrasion-resistant finish. Wiping tests with a class 100 tissue did not produce any visible micro-scratches on the surface. Together with each LHCb mirror, we produced a small witness sample. The reflectivity measurement of such a sample is shown in Fig. 3. At 275 nm, the wavelength for which the coating was optimized, reflectivity of about 96% is reached. The agreement with the computation is good, except for wavelengths below 250 nm, where the absorption parameter kðlÞ of HfO2 seems to be larger than the one we used in the simulation. Fig. 4 summarizes the results of the first batch of 32 (of 100) mirrors which have been coated so far. The mean reflectivity, averaged from 250 to 350 nm, is 95.570.9%. The wavelength dependence of the reflectivity is well matched to the quantum efficiency of the multi-alkali photocathode of the LHCb Pixel HPD photodetectors.
620
413
310
248
206
0.30 0.25 0.20 0.15 0.10 = 0.176 = 0.149
0.05 0.00 2.00
3.00
4.00 Energy [eV]
5.00
6.00
Fig. 5. Detection efficiency of an HPD detector (quartz window), with and without double reflection from the coated mirror, yinc ¼ 301.
As shown in Fig. 5, reflection from the two mirrors leads to a reduction in the number of photons of only 15%, averaged over the relevant energy range from 2 to 6 eV.
ARTICLE IN PRESS A. Braem et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186
R [%]
186 100 95 90 85 80 75 70 65 60 55 50 350
Measurement inc. = 30˚ Simulation inc. = 30˚ Simulation inc. = 70˚ 400
450
500 550 W.L. [nm]
600
650
replaced TiO2 for its higher sputtering yield. This minimizes the heat load and, therefore, reduces the stress in the layer stack. The resulting film has good adherence on the polyester and can easily be rolled to a light guide of 2.5 cm diameter. Measured at yinc ¼ 301, the film reaches a peak reflectivity at 500 nm of about 98% (see Fig. 6). For comparison, in Fig. 7, we also show results which have been obtained at CERN by vacuum evaporation on glass substrates. The sample with the two-layer SiO2/TiO2 stack achieves excellent reflectivity, in good agreement with the simulation.
Fig. 6. Reflectivity of Al+2 pairs SiO2/Nb2O5 on Melinex 400 Polyester film.
Acknowledgments 100
We would like to thank M. Fahland from FEP, Dresden, Germany, for his competent and efficient support during the development of the reflective coatings on the polymer films. Thanks also to our colleagues from the CMS-HF and LHCb RICH projects for many clarifying discussions and a fruitful collaboration.
95
R [%]
90 85 80 2 pairs SiO2 / TiO2 measurement 2 pairs SiO2 / TiO2 simulation 1 pair SiO2 / TiO2 simulation
75
References
70 350
400
450
500 550 W.L. [nm]
600
650
Fig. 7. Reflectivity of an evaporated layer stack on a glass substrate. Al+1 or 2 pairs of SiO2/TiO2, yinc ¼ 301.
For CMS, prototype films with one and two pairs SiO2/TiO2, evaporated at CERN on glass substrates, gave excellent reflectivity values in the desired 400–600 nm range. For the final production at FEP, a stack of two pairs of SiO2/Nb2O5 was sputtered onto the polyester film. Nb2O5
[1] LHCb Collaboration, RICH Technical Design Report No. CERN/LHCC/2000-037, 2000. [2] CMS Collaboration, HCAL Technical Design Report No. CERN/LHCC/97-31, 1997. [3] H. Angus Macleod, Thin-Film Optical Filters, third ed., IOP, ISBN:0-7503-0688-2, 2001. [4] O.S. Heavens, Optical Properties of Thin Solid Films, Dover Publications Inc, New York, ISBN 0-486-66924-6, 1991. [5] FilmStar Design, FTG Software Associates, Princeton, NJ, 2004. [6] SCI Film Wizard, Scientific Computing International, Carlsbad, CA, 2004.
Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186 www.elsevier.com/locate/nima
Metal multi-dielectric mirror coatings for Cherenkov detectors A. Braem, C. David, C. Joram PH Department, CERN, CH-1211 Geneva, Switzerland
Abstract Application-specific reflective coatings have been developed and are being implemented in LHC experiments currently under construction. The broadband reflective coating consists of an aluminum film combined with one or two pairs of low- and high-index dielectric layers. The layer stacks are designed and optimized using commercial thin film software and verified on small mirror samples. The wavelength of peak reflectivity is tuned to maximize the light yield, taking into account the emission spectrum (e.g., Cherenkov) and the photosensor characteristics. We report about coatings of Mylar foil-based light guides for the Hadronic Forward calorimeter of CMS and spherical mirrors for the RICH2 counter of LHCb. r 2005 Elsevier B.V. All rights reserved. PACS: 78.40; 78.20.Ci; 81.15.Ef; 29.40.Ka Keywords: Mirror; Reflectivity; Multi-dielectric film; Vacuum deposition; Sputtering
1. Introduction Cherenkov radiation is a weak light source. Good detector performance depends therefore on optimized light transport and collection. Highreflectivity mirrors are key elements in Cherenkov detectors, both in imaging and non-imaging designs. The importance of the mirror reflectivity grows with the number of reflections the Cherenkov photons undergo from source to detection. Reflectivity enhancing coatings on top of a highquality metallic film provide the possibility to Corresponding author.
E-mail address: [email protected] (A. Braem).
tailor the mirror characteristics to the specific needs of the experiment, taking into account the form of the Cherenkov emission spectrum dN=dE ¼ const:; dN=dl ¼ 1=l2 , the chromaticity of the radiator, the wavelength/ energy-dependent sensitivity of the photodetector eQ ðEÞ, and possible environmental constraints (temperature, humidity). The quantity to be optimized can be expressed as Z
E2 E1
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.08.074
Rn ðEÞ eQ ðEÞ TðEÞ dE,
ARTICLE IN PRESS A. Braem et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186
where R describes the mirror reflectivity, n is the number of reflections and T accounts for any other energy-dependent transport efficiencies (e.g., transparency of a radiator window). For the choice of the energy band, particularly the upper limit E2, parameters like the chromaticity of the radiator medium, Rayleigh scattering, or radiation driven aging of components may play a role. This article describes two developments of multi-layer dielectric coatings on aluminum films to produce high-reflectivity mirrors: 1. The flat and spherical mirrors of the RICH2 detector of LHCb [1] require optimized reflectivity in a broad band centered around 275 nm (driven by the sensitivity characteristics of the HPD detector) for incidence angles of 211 (spherical mirror) and 481 (flat mirror). The substrate is a 6 mm borosilicate glass. The goal is to achieve R494% for 250olo300 nm and R480% for 210olo500 nm. 2. In the CMS Forward Hadron Calorimeter [2], Cherenkov light produced in quartz fibers needs to be transported via a 40 cm long air core light guide (2.5 cm Ø) from the fiber bundle to the PMTs. The inner surface of the light guide has to provide maximum reflectivity in the range 400olo650 nm for incidence angles of about 701. On average the photons undergo six reflections. Suppression of the UV component below 400 nm is desired because its yield is subject to radiation-related aging and annealing of the quartz fibers. The substrate is a 75 mm thick polyester film which is rolled to form a cylindrical light guide.
2. Metal multi-dielectric mirror coatings The basic principle, illustrated in Fig. 1, consists in enhancing (or decreasing) the reflectivity of a metal film at a given wavelength band by exploiting interference effects in thin films. A highreflectivity (R) metal layer (Al: R92%; Ag: R96%) is overcoated with one or several pairs of transparent films of high (H) and low (L) refractive index. For this purpose, dielectric films like SiO2, MgF2 (L-materials) or HfO2, Nb2O5,
183
hν θin High Low High Low
n pairs
Al reflector Cr adherence layer
substrate
Fig. 1. Schematic representation of a metal multi-dielectric mirror.
TiO2 (H-materials) are used. For the selection of the coating materials, properties like residual stress, adherence, resistivity to abrasion and humidity, and coating yield are of importance. The optical thickness of the layers d opt ¼ nd cos yinc is usually chosen to be l=4. The dielectric coatings lead, therefore, to a wavelength- and angledependent modulation of the reflectivity. The larger the ratio of the refractive index of the LH pair, the higher is the peak reflectivity and width of the enhanced region. Adding LH pairs, optimized for the same wavelength, will increase the reflectivity but narrow down the useful range. The layer stack terminates usually with a H-layer. The mathematical treatment of multi-layer coatings relies on elaborated matrix methods (see, e.g., text books [3,4]). Simulation codes like FTG FilmStar [5] (used by us) and SCI Film Wizard [6] allow to predict the behavior of multi-layer films with an accuracy which depends on the knowledge of the relevant optical properties (refractive index nðlÞ and absorption coefficient kðlÞ) of the deposited layers. Those are generally dependent on the deposition method and process parameters. The simulation is therefore useful to define a concept and a promising starting point. The optimum result can, however, only be achieved by systematically varying the experimental parameters in a series of test coatings. The simulations help to interpret the results and narrow down the parameter space. Fig. 2 gives examples of simulated reflectivity curves. It
ARTICLE IN PRESS A. Braem et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186
184 100 95
Reflectivity [%]
90 85 80 75
Aluminium Aluminium + MgF2 Al + 1 pair SiO2/HfO2 Al + 2 pairs SiO2/HfO2
70 65 60 200
300
400 Wavelength [nm]
500
600
Fig. 2. Simulated reflectivity at yinc ¼ 301 of pure aluminum and Al coated with various layers. MgF2 serves as a protective layer of the Al coating without significant reflection enhancement. The thickness of the SiO2/HfO2 pairs was optimized for l ¼ 300 nm.
demonstrates the effect of protective and reflection enhancing coatings on top of an aluminum reflective layer.
3. Fabrication and characterization of metal multidielectric coatings at CERN A dedicated thin film coating plant is available at CERN for the production of coatings on substrates up to 0.9 m diameter. It consists of a high vacuum evaporation chamber of 1 m diameter evacuated with a cryogenic pump (3500 l/s). The end pressure is about 1 105 Pa. The plant is equipped with 2 Joule evaporation sources and an electron gun with four crucibles. Uniform layer thickness (75%) is achieved by rotating the substrate. The layer thickness is controlled with a calibrated quartz crystal balance. The typical production rate is 2 full process cycles per day. 3.1. Evaporation process As an example, we describe the evaporation process for the production of the LHCb mirrors on a glass substrate. The two metals, namely a 20 nm thick chrome adherence layer and the aluminum main reflectance layer, are evaporated from tungsten filaments. The substrate is at
ambient temperature. The dielectrics are evaporated from an electron gun source in an oxygen atmosphere (p(O2)103 Pa) to compensate for partial dissociation and oxygen loss during the evaporation process. TiO2 is obtained from Ti3O5 tablets of 99.5% purity. The thickness values are derived from the quartz crystal balance. The reflectivity can be measured only after the process is completed. A spectrophotometer1 allows to determine the absolute reflectivity at an angle of incidence (yinc) of 301 in the wavelength range 200–800 nm. The essential process parameters are summarized in Table 1.
4. Industrial fabrication of reflective films A large number of light guides (1800) needed for the CMS Hadron Forward calorimeter are required to produce 100 m2 of reflective film. The optimization of layer stack and substrate choice was done in collaboration with FEP Dresden.2 FEP also carried out the final production. The pilot sputter roll coater3 at FEP can coat polymer webs of 600 mm width with six different materials at a speed of 0.3–3 m/min. The applied technique is dual magnetron sputtering (DMS) for metals and reactive DMS for dielectric coatings. The chosen layer stack consists of Cr adhesion layer (10 nm)/ Al reflectance layer (70 nm)/2 pairs of SiO2/Nb2O5 (66/53 nm). A 75 mm thick polyester film (Melinex 400 from DuPont Teijing Films)4 was found appropriate as a substrate from the mechanical, thermal, and surface properties point of view.
5. Results The layer stack developed for the mirrors of LHCb RICH2 consists of Al+1 pair SiO2/HfO2. The hafnium oxide layer which terminates the 1
Perkin Elmer Lambda 15. Fraunhofer Institute for Electron Beam and Plasma Technology (FEP), Winterbergstrasse 28, 01277 Dresden, Germany (contact person: Dr. Matthias Fahland). 3 FOSA 600 - FOil Sputter Apparatus, 600 mm foil width. 4 DuPont Teijin Films, Luxembourg S.A., P.O. Box 1681, L-1016 Luxembourg. 2
ARTICLE IN PRESS A. Braem et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186
185
Table 1 Typical process parameters for LHCb mirror (Al+SiO2/HfO2), optimized for l ¼ 275 nm Purity (%)
Chamber pressure
Deposition rate (nm/s)
Thickness (geom.) (nm)
Cr Al SiO2 HfO2
99.98 99.999 99.99 99.9
2 105 Pa 2 105 Pa 103 Pa O2 103 Pa O2
1 5 0.2 0.2
20 85 28 38
100 95
98
90 85
96
97
95 R [%]
R [%]
Material
80 75 70
93 92
65 60 55 50 200
94
300
400 W.L. [nm]
Measurement
91
Simulation
90
500
0
600
Fig. 3. Reflectivity of the LHCb mirror coating Al+1 pair SiO2/HfO2 on glass, yinc ¼ 301. The layer stack was optimized for maximum reflectivity at l ¼ 275 nm.
5
10 15 20 25 Process number
30
0
5 10 15 20 25 Nb of mirrors
Fig. 4. Measured reflectivity (averaged in the interval 250olo350 nm, yinc ¼ 301) of the first batch of 32 LHCb mirror coatings. The statistical analysis (right plot) finds an average reflectivity of the batch of 95.570.9%.
λ [nm]
stack is known for its hard abrasion-resistant finish. Wiping tests with a class 100 tissue did not produce any visible micro-scratches on the surface. Together with each LHCb mirror, we produced a small witness sample. The reflectivity measurement of such a sample is shown in Fig. 3. At 275 nm, the wavelength for which the coating was optimized, reflectivity of about 96% is reached. The agreement with the computation is good, except for wavelengths below 250 nm, where the absorption parameter kðlÞ of HfO2 seems to be larger than the one we used in the simulation. Fig. 4 summarizes the results of the first batch of 32 (of 100) mirrors which have been coated so far. The mean reflectivity, averaged from 250 to 350 nm, is 95.570.9%. The wavelength dependence of the reflectivity is well matched to the quantum efficiency of the multi-alkali photocathode of the LHCb Pixel HPD photodetectors.
620
413
310
248
206
0.30 0.25 0.20 0.15 0.10
0.05 0.00 2.00
3.00
4.00 Energy [eV]
5.00
6.00
Fig. 5. Detection efficiency of an HPD detector (quartz window), with and without double reflection from the coated mirror, yinc ¼ 301.
As shown in Fig. 5, reflection from the two mirrors leads to a reduction in the number of photons of only 15%, averaged over the relevant energy range from 2 to 6 eV.
ARTICLE IN PRESS A. Braem et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 182–186
R [%]
186 100 95 90 85 80 75 70 65 60 55 50 350
Measurement inc. = 30˚ Simulation inc. = 30˚ Simulation inc. = 70˚ 400
450
500 550 W.L. [nm]
600
650
replaced TiO2 for its higher sputtering yield. This minimizes the heat load and, therefore, reduces the stress in the layer stack. The resulting film has good adherence on the polyester and can easily be rolled to a light guide of 2.5 cm diameter. Measured at yinc ¼ 301, the film reaches a peak reflectivity at 500 nm of about 98% (see Fig. 6). For comparison, in Fig. 7, we also show results which have been obtained at CERN by vacuum evaporation on glass substrates. The sample with the two-layer SiO2/TiO2 stack achieves excellent reflectivity, in good agreement with the simulation.
Fig. 6. Reflectivity of Al+2 pairs SiO2/Nb2O5 on Melinex 400 Polyester film.
Acknowledgments 100
We would like to thank M. Fahland from FEP, Dresden, Germany, for his competent and efficient support during the development of the reflective coatings on the polymer films. Thanks also to our colleagues from the CMS-HF and LHCb RICH projects for many clarifying discussions and a fruitful collaboration.
95
R [%]
90 85 80 2 pairs SiO2 / TiO2 measurement 2 pairs SiO2 / TiO2 simulation 1 pair SiO2 / TiO2 simulation
75
References
70 350
400
450
500 550 W.L. [nm]
600
650
Fig. 7. Reflectivity of an evaporated layer stack on a glass substrate. Al+1 or 2 pairs of SiO2/TiO2, yinc ¼ 301.
For CMS, prototype films with one and two pairs SiO2/TiO2, evaporated at CERN on glass substrates, gave excellent reflectivity values in the desired 400–600 nm range. For the final production at FEP, a stack of two pairs of SiO2/Nb2O5 was sputtered onto the polyester film. Nb2O5
[1] LHCb Collaboration, RICH Technical Design Report No. CERN/LHCC/2000-037, 2000. [2] CMS Collaboration, HCAL Technical Design Report No. CERN/LHCC/97-31, 1997. [3] H. Angus Macleod, Thin-Film Optical Filters, third ed., IOP, ISBN:0-7503-0688-2, 2001. [4] O.S. Heavens, Optical Properties of Thin Solid Films, Dover Publications Inc, New York, ISBN 0-486-66924-6, 1991. [5] FilmStar Design, FTG Software Associates, Princeton, NJ, 2004. [6] SCI Film Wizard, Scientific Computing International, Carlsbad, CA, 2004.