Gamma irradiation induced defects in different types of fused silica M. León * , P. Martín, A. Ibarra, E.R. Hodgson Fusion Materials Research Unit, Euratom/CIEMAT Fusion Association, Avda, Complutense 22, 28040 Madrid, Spain article info abstract In this work, KU1 and KS-4V silica glasses, considered as the main candidate materials for optical diag- nostic and remote handling components in ITER, have been gamma irradiated at doses from 10 kGy up to 12 MGy together with five commercial types of silica. After each irradiation the optical absorption of the different grades has been obtained, and the concentration evolution of each defect has been com- pared for the different silica types. This comparison allowed us to determine which defects are intrinsic and which depend on the impurity level or fabrication method. The results allow one a better under- standing of the relevance of the precursors present in the different silica grades. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Fused silica is a key element for optical diagnostic and remote handling components to be used in fusion devices. All these com- ponents will suffer significant neutron and gamma irradiation. The radiation induced optical absorption and light emission radio- luminescence, will affect their optical properties, and hence the component lifetime. In the case of ITER, KU1 and KS-4V silica, are being considered as the main candidate materials [1–4]. Hence a detailed knowledge of the effect of ionizing radiation and dose accumulation in these grades under irradiation is required. Previ- ous studies for gamma irradiated silica have been reported [5–9] and different results have been obtained in some of them. These differences suggest that the material origin must be an important factor in the type and concentration of gamma induced defects. As none of the studies include a complete comparison of the defect evolution with dose in several types of silica for high gamma irra- diation doses, the role of the material origin still needs to be clar- ified. The aim of this work is to understand the relevance of the manufacturing process and precursors (defects and impurities) present in the material before irradiation. For this purpose, KU1 and KS-4V have been gamma irradiated under identical conditions together with five commercial types of silica with different OH and impurity contents depending on their manufacturing process. The effect of gamma irradiation at doses from 10 kGy up to 12 MGy on the optical properties of these different silica has been studied. These results also help to complete the database on materials properties required to predict the behaviour of the transmission properties for different types of silica from the onset of operation of ITER. 2. Experimental procedure The seven silica grades studied were classified in four standard types according to the Hetherington classification [10]: type I natu- ral dry silica, obtained by fusion of natural quartz crystal in vac- uum or inert gas atmosphere; type II natural wet, prepared by flame fusion of quartz crystal in a water–vapor atmosphere; type III synthetic wet, made by the vapor–phase hydrolysis of pure sil- icon compounds such as SiCl 4 and type IV synthetic dry, obtained by the reaction of O 2 with SiCl 4 in a water–vapor–free plasma. Ta- ble 1 shows the name, origin, OH content, typical trace impurities [1,11] and classification [10] of the seven types of silica studied. In order to investigate the optical absorption evolution of these different silicas with gamma dose, several consecutive irradiations were made at doses from 10 kGy up to 12 MGy. All the samples were irradiated under identical conditions in flowing dry nitrogen gas at 27 °C, 4.8 Gy/s (CIEMAT NAYADE 60 Co gamma pool installation). Optical absorption spectra from 0.4 to 6.5 eV using a Varian Cary 5E spectrometer, were obtained at room temperature imme- diately after each irradiation. The measured absorption was cor- rected for sample thickness and the spectra given in optical density (OD) per cm. The spectra were analysed in terms of a sum of simple Gaussian bands. 3. Results and discussion 3.1. Before irradiation The optical absorption spectra of the samples before irradiation are shown in Fig. 1. No absorption is observed in any sample in the visible region (1.6–3.1 eV). However, in the ultraviolet region (3.1–6.2 eV) some silica grades show appreciable absorption 0022-3115/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2008.12.232 * Corresponding author. E-mail address: monica.leon@ciemat.es (M. León). Journal of Nuclear Materials 386–388 (2009) 1034–1037 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat