2852 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 49, NO. 6, DECEMBER 2002 Origin of the Radiation-Induced OH Vibration Band in Polymer-Coated Optical Fibers Irradiated in a Nuclear Fission Reactor B. Brichard, Member, IEEE, A. Fernandez Fernandez, Member, IEEE, F. Berghmans, Member, IEEE, and M. Decréton, Member, IEEE Abstract—We measured in situ the radiation-induced absorp- tion of pure silica core fibers exposed to a fission nuclear radia- tion. We observed the growth of the 1.38- m OH vibration band in polymer coated fiber. Two contributions, a gamma-induced hy- drogen diffusion and the recoil protons, are compared. The major contribution to the OH content growth is identified to originate from the gamma-induced hydrogen diffusion. The identification of the parameters governing the mechanism has important implica- tions on the design of a fast-neutron fiber optic monitor. Index Terms—Compaction, gamma, hydrogen diffusion, neu- tron, optical fiber sensor. I. INTRODUCTION I ONIZING or particle radiation affect the optical properties of silicon dioxide glasses through a defect creation mech- anism, not yet fully understood. In the last decade, a common effort to assess optical fibers under radiation was mainly driven by the fusion plasma diagnostic community [1]–[3]. Next-step fusion machines will operate at a relative high radiation level, implying the need to develop radiation-resistant glasses [4]. Such R&D work may also contribute to the development of fiber-optic radiation monitors. A neutron detection system generally relies on a conversion mechanism by ionization, material activation, or recoil protons. As an example, the latter process is used in polymer materials to perform neutron spec- trometry [5]. In an early work [6], we observed the growth of the 1.38- m associated OH vibration bands in polymer-coated fiber, whereas this effect was absent in aluminum-jacketed fiber. This observation was the starting point to use the recoil proton effect as a fiber-optic neutron monitor. In this paper, we report new measurements of the radiation-induced absorption (RIA) of step-index pure silica optical fiber. In Section IV, we focus on the understanding of the kinetic of the OH absorption band growth. We attribute this effect to mainly a hydrogen diffusion mechanism. Manuscript received August 1, 2002; revised September 7, 2002. This work was supported by the European Fusion Development Agreement. B. Brichard and M. Decréton are with SCK•CEN, Belgian Nuclear Research Centre, B-2400 Mol, Belgium. A. F. Fernandez is with SCK•CEN, Belgian Nuclear Research Centre, B-2400 Mol, Belgium, and the Université Libre de Bruxelles, B-1050 Brussels, Belgium (e-mail: afernand@sckcen.be). F. Berghmans is with SCK•CEN, Belgian Nuclear Research Centre, B-2400 Mol, Belgium, and Vrije Universiteit Brussel, B-1050 Brussels, Belgium. Digital Object Identifier 10.1109/TNS.2002.805986 TABLE I FIBER SIZES AS SPECIFIED BY THE MANUFACTURERS. KS4V ISA PURE SILICA CORE FIBER,WHEREAS MF ISA FLUORINE-DOPED CORE FIBER II. BACKGROUND To explain the growth of the OH vibration bands, it is con- venient to review two important concepts: the creation of non- bridging oxygen hole defect (NBOH) [7] and the cracking of molecular hydrogen on an NBOH site. The first important reac- tion is the well-known creation of NBOH defect Si OH Si O H (1) This reaction proceeds to the right upon irradiation. Above 130 K, the radiolytic hydrogen H combines very efficiently to form molecular hydrogen. At a temperature higher than 230 K, the bleaching of the NBOH may take place according to the diffusion-limited reaction Si O H Si OH H (2) Recent ab initio calculations confirmed that the reaction (2) is exothermic by 0.4 eV, whereas the hydrogen dissociation process on NBOH sites requires 0.1 eV only [8]. Therefore, NBOH centers are quite efficient to crack molecular hydrogen. From a technological point of view, (2) is used as a basic process to improve the dielectric response of silicon-dioxide film. Beside the formation of defect in irradiated silica, the neutron bombardment also affects the coating material. The fast neutrons produce recoil protons from the polymer coating through neutron elastic scattering [9]. In the case of neu- tron energy lower than 10 MeV (isotropic scattering), the energy distribution of recoil protons is equiprobable in the center-of-mass system and spreads out from zero to the incident neutron energy. An order of magnitude of the neutron-proton conversion efficiency [10] can be quickly estimated by (3) where is the number density of hydrogen, is the neutron elastic scattering cross-section, and is the coating thickness. 0018-9499/02$17.00 © 2002 IEEE