Radiation response behavior of high phosphorous doped step-index multimode optical fibers under low dose gamma irradiation M.C. Paul a, * , D. Bohra b , A. Dhar a , R. Sen a , P.K. Bhatnagar b , K. Dasgupta a a Fiber Optics Laboratory, Central Glass and Ceramic Research Institute, 196, Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India b Defence Laboratory, Jodhpur, India article info Article history: Received 9 July 2008 Received in revised form 16 April 2009 Available online 21 June 2009 PACS: 42.70.C 42.81 52.75.R 81.15.G 82.60:61.18.F 78.20.C Keywords: Optical fibers Chemical vapor deposition Radiation effects SEM S100 Defects Absorption Phosphates Silicates Radiation Defects abstract This paper revealed the role of various parameters on the radiation response behavior of P 2 O 5 doped step- index multimode (SIMM) fibers having different content of P 2 O 5 (12–6 mol%) at room temperature. Their suitability for use as a radiation sensitive fiber in fiber optic dosimeter was studied under 60 Co-gamma irradiation at different dose rates 0.01–0 Gy/h. The suitable wavelength region is found to be 500– 600 nm where the fibers shows the maximum radiation sensitivity due to formation of phosphorous– oxygen hole centers identified from their radiation induced absorption spectra. The influence of each parameter like the doping levels of P 2 O 5 , dose rates, doping region and energy of the radiation source on their sensitivities was examined. The radiation sensitivities recorded at 502, 540 and 560 nm depends strongly on P 2 O 5 content of the fiber at low dose rates within 0.01–1 Gy/h. However, the fibers shows almost dose rate independent behavior at high dose rates (>1.0 Gy/h) under 60 Co-gamma radiation source of energy 1.25 MeV with respect to all the three wavelengths. The fiber shows almost linear relation to the total dose up to saturation levels of 4.0 Gy at all dose rates. Their sensitivities could be explained through analyses of such type of glass as well as behavior of the radiation induced P-related defects gen- erated in the light guiding core region. At low dose rates the fibers becomes more radiation sensitive compared to the high dose rates due to conversion of POHC centers to P 1 (phosphorous E 0 ) defect centers as observed from their induced loss curves during gamma irradiation. At low dose rates 0.1–1 Gy/h under 502 nm transmission wavelength the fibers shows an excellent linear relation with respect to Cs-137 radiation source of energy 0.662 MeV having average sensitivity of 1.0452 ± 0.0346 dB/m/Gy and very low fading behavior at room temperature. The results suggest that P-doped SIMM fiber (40 lm core diameter) containing 16 mol% P 2 O 5 exhibit an excellent linear radiation response property of high sensi- tivity around 0.89 ± 0.09 dB/m/Gy at 502 nm wavelength with very low recovery and little dose rate dependence within 0.5–10 Gy/h region that makes them a very promising candidate as radiation sensor for use in fiber optic personal dosimeter to detect low dose gamma radiation of 0.002 Gy for human safety purpose. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The transmission characteristics of optical fiber is very much influenced in radiation environments due to generation of radia- tion induced defect centers by ionization and atomic displacement within the molecular bonding network of silica (SiO 2 ) glass. The generated defect centers [1–3] causes absorption in UV, VIS or near IR region and are responsible for radiation sensitivity [2] of the fi- ber. The radiation induced loss [4–9] in optical fiber depends on many parameters such as the radiation source, dose rates, temper- ature, fiber material, fiber dimension, dopant concentration in the core and cladding etc. In the last decade, extreme research work has been carried out to study the radiation effect on optical fibers to use them as the information transmission system in which re- quire radiation resistance [10–12] behavior and to explore their possibility for use as radiation sensor [13–19] under various radi- ation environments such as nuclear waste tanks, nuclear reactors, etc., in which require radiation softness behavior. The radiation ef- fects on P-doped [17,20,21] silica bulk materials and fibers have been reported. In the earlier some work on radiation sensitivity of P-doped fiber has been done at high dose rates 6–60 Gy/h prac- tically for use in fiber optic dosimeter regarding medical applica- tion such as cancer treatments [21,22]. Recently Regnier et al. have reported the data of low dose radiation induced attenuation at infrared wavelengths for P-doped SM fiber [23]. The P-doped fiber that has been studied earlier contain maximum 10 mol% of P 2 O 5 . No details reported data are available about the radiation response behavior of high P-doped SIMM fibers for use as a radia- tion sensor in fiber optic dosimetry to detect very low radiation 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.05.017 * Corresponding author. Tel.: +91 33 24838083; fax: +91 33 24730957. E-mail addresses: paulmukul@hotmail.com, mcpal@cgcri.res.in (M.C. Paul). Journal of Non-Crystalline Solids 355 (2009) 1496–1507 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol