Effect of thermal neutron radiation on an optical fiber, 2

Experimental studies was made on the radiation-induced loss of light transmission of an optical fiber in the wavelength region from 400 to 1100 nm and on the effect of moisture content or OH radical on thermal neutron irradiation. Optical fibers employed contained 50 to 960 ppm of OH and less than 400 ppm of Ci in a core, and boron and fluorine were doped in a cladding. Thermal neutrons were irradiated by the condition of 3.4 x 104 to 3.8 x 104 n/cm2.sec of the neutron flux and of 50 R/h of γ-ray dose rate in a research reactor. The induced loss of light transmission was determined by measuring output through the employed fibers with an optical power meter. As for the induced loss by thermal neutron, it was significant in shorter wavelength side between regions from 400 to 900 nm especially for no LiF shield of a neutron flux entrance. Induced loss increase curve was steep in the initial one hour of irradiation period and increase rate became smaller as irradiation time increased. Restoration after stopping of irradiation was wavelength-dependent and fast restoration was observed in the wavelength region from 700 to 800 nm, and slow restoration in the vicinity of 500 nm. Peaks of induced loss spectra were observed in such spectrum bands as 420 nm, 470 nm, 630 nm and 730 nm. Characteristics of the absorption bands are not identified and detailed examination of the absorption bands should be further carried out. (Takagi, S.)