IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 48, NO. 6, DECEMBER 2001 2069 Radiation Effect in Silica Optical Fiber Exposed to Intense Mixed Neutron–Gamma Radiation Field B. Brichard, P. Borgermans, A. Fernandez Fernandez, K. Lammens, and M. Decréton Abstract—We measured in situ the radiation-induced absorp- tion of pure silica core fibers exposed to a fission nuclear reactor. We observed the growth of the 1.39- m OH vibration band in polymer coated fiber. Three mechanisms are responsible for this effect: recoil protons, hydrogen diffusion, and a probable compaction effect. Based on this experiment, a fiber-optic neutron monitor prototype is proposed. Index Terms—Compaction, gamma, hydrogen diffusion, neu- tron, optical fiber sensor. I. INTRODUCTION A MONG the wide range of application of silicon dioxide, the optical waveguide property of the silica fiber is of in- terest because of the data multiplexing and sensing capabilities. This particularity explains why optical fibers are widely studied under radiation. Most of the studies deal with radiation effects at low as well as moderate accumulated dose (see references in [1] and [2]). Fewer publications are available concerning high gamma dose and neutron fluence [3]–[8]. However, sensing ca- pability and data transmission could be also conceived under in- tense radiation field, e.g., inside a fission reactor. Moreover, the optical diagnostic system of future fusion reactors must operate in high temperature and a severe radiation environment. The use of optical fibers is expected to significantly simplify the design of such a system [9]. In this paper, we report the radiation-induced absorption measurements of pure silica optical fibers irradiated in the material testing fission reactor BR2 (Mol, Belgium). In the infrared wavelength window, the experiences clearly evidenced a neutron-induced growth of the 1.39- m OH vibration band in polymer coated fiber. The origin of this phenomenon is discussed in Section IV. To conclude, a neutron sensing system prototype is proposed based on a differential measurement method. II. BACKGROUND Unlike gamma radiation, neutrons above the threshold dis- placement energy may potentially displace the oxygen and sil- icon atoms. Generally speaking, the net effect of neutrons on the Manuscript received July 17, 2001. B. Brichard, K. Lammens, and M. Decréton are with SCK CEN, Belgian Nu- clear Research Centre, B-2400 Mol, Belgium (e-mail: bbrichar@sckcen.be). P. Borgermans is with SCK CEN, Belgian Nuclear Research Centre, B-2400 Mol, Belgium and Vrije Universiteit Brussel, B-1050, Brussels, Belgium. A. Fernandez Fernandez is with SCK CEN, Belgian Nuclear Research Centre, B-2400 Mol, Belgium and Université Libre de Bruxelles, B-1050 Brussels, Belgium. Publisher Item Identifier S 0018-9499(01)10645-3. Fig. 1. Neutron-induced compaction and refractive index change in amorphous bulk silica. After [13]. change of optical absorption is believed to be negligible at low fluence, although this observation is still uncertain, especially because of the difficulty to separate experimentally the gamma field from the neutron bombardment. As soon as the fluence reaches a significant value, the displaced atoms will affect the microstructure of the material and in turn the optical proper- ties. Probably, the most complete studies of neutron-irradiated silica have been achieved in [10]. A comprehensive and more recent review is also available in [11]. When subject to neutron or particle bombardment, amorphous silica, as well as its crys- talline counterpart, gradually transforms into a different amor- phous state, known as the metamict phase. This phase results from a radiation-induced loss of the long-range translational pe- riodicity and orientational order (loss of topological order) [12]. The exact nature of this amorphous phase is still debated but has its origin in the change of the tetrahedral geometry and the ring-size arrangement distribution. Under fast neutron irradia- tion, quartz undergoes a swelling characterized by a decrease of its density up to 14%. On the other hand, amorphous irradiated silica shows the reversed process, the so-called compaction with a density increase up to about 4%. In the case of compaction, the refractive index of the material must increase according to the Lorentz–Lorentz law. This has been experimentally observed [13]. Fig. 1 shows the change of refractive index in neutron amorphous irradiated silica and the corresponding compaction effect. The Kramers–Kronig relationship indicates that a change in absorption should be related to a refractive index change [14]. 0018–9499/01$10.00 © 2001 IEEE