Effect of γ irradiation on optical components

Transcription

1 Effect of γ irradiation on optical components S. Baccaro, A. Piegari, I. Di Sarcina, A. Cecilia Abstract Optical components operating in radiation environments, like nuclear facilities, High Energy Physics and space experiments, are exposed to fluxes of energetic particles, which may deteriorate the image quality. In order to avoid unwanted failure of such components that is due to changes produced in the optical properties of the materials by the hostile conditions, it is useful to perform a preliminary investigation of the radiation induced damage on each type of optical component. In this work, coated optical components are investigated. A set of optical coatings were submitted to γ irradiation at the Calliope 60 Co radioisotope source (Research Centre ENEA-Casaccia, Rome) in order to simulate the hostile radiation environment in which they could be employed. The behavior of different substrates, single-layer materials and multilayer optical coatings was investigated by comparing both transmittance and reflectance measurements before and after the γ-ray exposure. O I. INTRODUCTION ptical components are widely used in many different optical systems. Either single components or more complex optical instruments are used in operating conditions where some kind of radiation impinges on their surface [1]. In some cases (as for instance nuclear facilities, High Energy Physics and space experiments) they are exposed to fluxes of energetic particles [2, 3, 4, 5, 6, 7]. Depending on the particle type and the flux value, a significant damage can be induced on the optical components. It is important to know the behavior of such components that is the sensitivity to radiation damage, to properly select the most resistant materials and devices. A class of optical components, of large use, is represented by coated optics. That means optical bulk materials coated by layers of different materials, which optical behavior is based on interference phenomena. In this work, a set of optical coatings were submitted to γ irradiation at the 60 Co radioisotope source Calliope (ENEA Research Centre) to simulate the hostile radiation environment in which they can be employed and to analyze their radiation S. Baccaro is with ENEA Advanced Techonological Physics/ION & INFN, R.C. Casaccia, Via Anguillarese 301, S. Maria di Galeria, Rome, Italy (telephone: , stefania.baccaro@casaccia.enea.it). A. Piegari is with ENEA Optical Coatings Group, R.C. Casaccia, Via Anguillarese 301, S. Maria di Galeria, Rome, Italy (telephone: , angela.piegari@casaccia.enea.it). I. Di Sarcina is with INOA-CNR (Istituto Nazionale di Ottica Applicata) & ENEA, Largo E.Fermi 6, Florence, Italy (telephone: , ilaria.disarcina@casaccia.enea.it). A. Cecilia is with ENEA Advanced Techonological Physics/ION & INFN, R.C. Casaccia, Via Anguillarese 301, S. Maria di Galeria, Rome, Italy (telephone: , angelica.cecilia@casaccia.enea.it). resistance. All samples were irradiated at the Calliope plant at the dose of 50 Gy. This radiation dose was chosen because it corresponds to the dose absorbed during five years operation of astronomical telescopes in the polar sun-synchronous orbit at an altitude of about 700 km [8]. Indeed, this is only one of the possible applications, similar dose can be used even for other experiments in hostile environments to study the components reliability. In order to investigate optical coating performance, an optical characterization of each component was performed before and after In particular transmittance and reflectance measurements were carried out by spectrophotometry in the ultraviolet, visible and near infrared spectrum. II. COMPONENTS AND TECHNIQUES The selected samples are reported in Table 1 and include different optical substrates, single-layer and multilayer coatings typically used in optical instrumentation. All coatings were prepared by PVD (Physical Vapour Deposition) [9]: thermal evaporation was used for aluminum mirror while radiofrequency sputtering for the other coatings. Both techniques are widely used for optical coating fabrication. In thermal evaporation the selected material is heated in a crucible by Joule effect and produced vapors condense on the substrate. In the radiofrequency sputtering technique the target material is bombarded by energetic ions that cause the extraction of target molecules that are deposited on substrates. Standard process parameters were used for sample prepared in both techniques. Samples were irradiated at the Calliope plant [10] which is located at the Research Centre ENEA-Casaccia (Rome, Italy). It is a pool-type irradiation facility equipped with the 60 Co γ source in a high-volume ( m 3 ) shielded cell and the present source has a cylindrical geometry with 60 Co pencils in the rack circumference. The emitted radiation consists of two γ photons with energy 1.17 MeV and 1.32 MeV, mean energy being 1.25 MeV. The maximum licensed activity is Bq and this plant offers the possibility to select the dose rate for sample Samples were irradiated at the dose of 50 Gy in the dosimetric point corresponding to the dose rate of 45 Gy/h (in water). During irradiation, materials were kept in the dark in order to avoid unwanted recovery processes induced by the visible light. Standard reflectance and transmittance measurements were carried out on the selected samples by a UV-VIS-NIR

2 spectrophotometer (Perkin Elmer Lambda 900) in order to investigate their optical performance. TABLE I: SAMPLES III. MEASUREMENTS AND ANALYSIS Both reflectance and transmittance measurements were carried out before and after irradiation on substrates, singlelayer materials and multi-layer optical coatings. Fig. 1. Transmittance measurements of B270 glass (a) and BK7-918 (b) glasses before and after γ-rays exposure. A. Substrates The aim of this work is to study the radiation effects induced on substrates typically used for optical coatings and compare the results with data on bulk materials published during last years [11]. Transmittance measurements of B270 and BK7-918 glasses, transmittance and reflectance of BK7B glass, transmittance of Suprasil (fused silica) quartz and reflectance measurement of Silicon substrate, performed before and after irradiation, are shown in Figures 1, 2, 3 and 4, respectively. Fig. 2. Transmittance (a) and reflectance (b) measurements of BK7B glass before and after γ-ray exposure.

3 Fig. 3. Transmittance measurements of Suprasil quartz before and after γ- ray exposure. irradiation test. The thickness of each layer was chosen in the range nm. In the following figures the performances of different materials, frequently used in optical coating structures, are shown. By comparing transmittance measurements, performed before and after irradiation on silicon oxide (SiO 2 ), hafnium oxide (HfO 2 ), tantalum oxide (Ta 2 O 5 ) and yttrium oxide (Y 2 O 3 ), it can be deduced that these coatings do not undergo relevant changes after exposure to ionizing radiation. Indeed, Silicon oxide (Figure 5), Hafnium oxide (Figure 6) and Tantalum oxide (Figure 7) appear practically unaffected by γ irradiation, while Yttrium oxide (Figure 8) shows a slight change. In particular in this case the whole transmittance curve is slightly shifted towards longer wavelengths, this effect could indicate a possible increase of the optical thickness of this layer. Fig. 5. Transmittance curves of SiO 2 single-layer coating before and after Fig. 4. Reflectance measurements of Silicon before and after γ-ray exposure. The irradiation test on the selected substrates shows that damage on BK7B glass causes both a transmittance and reflectance reduction at wavelengths shorter than 500 nm (about 2% in transmittance and 0.3% in reflectance, at 400 nm). These effects indicate an increasing of absorption losses in the analyzed material. For B270 glass the transmittance variation is smaller than 1% at 400 nm, while for the BK7-918, Suprasil and Silicon substrates the corresponding transmittance and reflectance measurements before and after exposure are perfectly overlapped. B. Single layer material Multilayer optical coatings, selected for this work, contain dielectric and/or metallic material layers; therefore in order to understand their optical behavior under irradiation, a study of each single layer coating material has been made. Some single films were deposited on Suprasil substrates that, as shown before, do not present relevant variation after Fig. 6. Transmittance curves of HfO 2 single-layer coating before and after

4 Fig. 7. Transmittance curves of Ta 2O 5 single-layer coating before and after Fig. 9. Reflectance curves of Aluminum (a) and Silver (b) coatings before and after 50 Gy irradiation dose. Fig. 8. Transmittance curves of Y 2O 3 single-layer coating before and after Concerning metals, aluminum and silver were analyzed because they are widely used for mirrors and other metaldielectric coatings in optical instrumentation. The metal layers were deposited by different techniques, thermal evaporation for the aluminum coating (390 nm) and sputtering for the silver coating (50 nm). By comparing reflectance measurements for both materials before and after γ irradiation (Figure 9), in the spectrum where they are highly reflecting, it can be observed that these coatings do not show any relevant modification. Fig. 10. Reflectance of MgF 2 coated aluminum mirror on quartz (a) and SiO 2 coated silver mirror on glass (b).

5 C. Multilayer optical coatings In order to investigate the behavior of optical components operating in radiation environment, widely used coatings were chosen to perform the radiation test. In this paper two protected metal mirrors, a wideband antireflection coating and a narrow band transmission filter have been analyzed. By evaluating the results for both protected metal mirrors, MgF 2 coated aluminum and SiO 2 coated silver (Figure 10), it turns out that the protected Al mirror is not affected by irradiation and the protected Ag mirror shows no relevant change in the range of wavelengths where is highly reflecting while a reflectance decrease can be noticed at shorter wavelengths. The irradiation test on the second type of optical component, a wideband antireflection coating for the visible spectrum (Figure 11), shows that the sample undergoes a small reflectance decrease (<0.1%), essentially due to the glass substrate damage. This coating contains two layers of SiO 2 and two alternate layers of HfO 2, on a B270 glass substrate. Each layer thickness is in the range nm. The low (SiO 2 ) and high (HfO 2 ) index materials were selected as the most insensitive to radiation effect. Fig. 12. Transmittance curve of an induced transmission filter after irradiation test. IV. CONCLUSIONS Optical components are largely used in different instruments operating in hostile environments. The instrument performance can be worsened if the optical components are damaged, therefore such components should be tested in advance under the condition of use. In this work the effect of γ irradiation on coated optical components is investigated. A number of typical optical coatings were selected (antireflection coatings, mirrors, filters) and each substrate and single material, used in the coating, was analyzed before and after It was shown that a proper choice of the coating materials allows the fabrication of optical components which performance is not affected by Fig. 11. Wideband antireflection coating reflectance before and after irradiation test. The last sample described in this paper is a narrow-band transmission filter (Figure 12), known as induced transmission filter [12], made by 12 alternate layers of HfO 2 and SiO 2 ( nm thick) and one silver layer (50 nm thick) in between. By comparing transmittance curves before and after irradiation, is evident that they are practically overlapped. V. REFERENCES [1] M. Fernández-Rodríguez et al., Ellipsometric analysis of gamma radiation effects on standard optical coatings used in aerospace applications, Thin Solid Films , (2004), [2] E.R. Benton, E.V. Benton, Space radiation dosimetry in low-earth orbit and beyond, Nuclear Instrument and Methods in Phys. Research B 184, (2001), p [3] F. Spurny, Radiation doses at high altitudes and during space flights, Radiation Physics and Chemistry 61 (2001) [4] [5] B.K. Ridley, The physical environment, Ellis Horwood Series in Environmental Science, (John Wiley & Sons, 1979). [6] [7] [8] S. Baccaro, A. Cecilia, I. Di Sarcina, A. Piegari, Optical coatings behavior under γ irradiation for space applications, SPIE Proceeding Vol (2004) p [9] R.F.Bunshah et al., Deposition Technologies for Films and Coatings, (Noyes Publications, New Jersey, 1982). [10] S. Baccaro, A. Festinesi, B. Borgia, Gamma and neutron irradiation facilities at ENEA-Casaccia Center (Rome), Cern CMS Technical Note 1995/192. [11] P. Beynel, P. Maier, H. Schombacher, Compilation of radiation damage test data, part III: Materials used around high-energy accelerators, CERN82-10 Health and Safety Department (Geneva 1982). [12] H.A. Macleod, Thin Film Optical Filters, (Macmillan, London, 1986).