An irradiation test was performed for polymethylmethacrylate plastic optical fibers under gamma-ray irradiation in order
to use the fiber in low-level radiation environments. Under gamma-ray irradiation at a high dose rate, only a large
radiation-induced transmission loss at wavelengths less than 700 nm was observed. Under irradiation at a low dose rate,
the loss was small and other two characteristic effects were observed. One is a radiation-induced loss that has peaks in
the near infrared wavelength range, and the other is an improvement in the optical transmission over an entire
wavelength range. The peak intensity depends on the absorbed dose of the gamma ray. Therefore, it is considered that
the absorbed dose can be measured using the peak intensity. Irradiation tests were performed by changing the
configuration of the fiber sample to observe the effect of a bending loss. It was found that the improvement in the optical
transmission is dependent on the configuration of the irradiation sample and that the bending loss can be recovered by
gamma-ray irradiation at a low dose rate.
Thermo-luminescence behavior of fused silica (SiO2) core optical fibers was studied in the air under the &ggr; ray irradiation.
The thermo-luminescence peak at 1390nm, being attributed to the oxyhydrate (OH) in the fused silica had an expected
temperature dependence of the intensity and could be used as a temperature monitor. A partially replaced core optical
(PARCO) fiber that consists of a fiber doped with high content of OH inserted into a pure silica fiber was proposed to
realize a spatial resolution.
Dosimetry of ionizing radiation fields is playing a more and more essential role for safety and effective operation of nuclear systems. An optical dosimetry system composed of radiation resistant optical fibers and radioluminescence materials has several advantages over a conventional electric dosimetry system. Especially, in a nuclear fusion system, a real time diagnostics of intensity of radiation fields with a high time resolution is indispensable, where electromagnetic noises are surmount and it is hard to find engineering solutions to satisfy severe demands from plasma physics and engineering with conventional electric dosimetry system. Erbium oxide (Er2O3) is found to emit radioluminescence in an infrared region where radiation induced degradation of optical transmissivity of fused silica (SiO2) core optical fiber is marginal. Also, an intensity of the radioluminescence of erbium oxide in an infrared region is found not to decrease with increase of the dose of ionizing irradiation. The optical dosimetry system with erbium oxide is applicable to dosimetry of intense ionizing radiation fields.
A photo-bleaching effect on radiation-induced transmission loss of fused silica core optical fibers was examined under 60Co gamma-ray and 14 MeV fast neutron irradiation. In the visible wavelength range, radiation induced loss could be reduced by the photo-bleaching under the both irradiation. For using optical fibers in visible wavelength range such as a light guide and an image guide, the photo-bleaching is an effective technique to improve radiation resistance of optical fibers. In the meantime, a so-called microbending loss is generated by the radiations in the whole wavelength range. The microbending loss responded in a different way to the photo-bleaching between the both irradiation. The loss under the gamma-ray irradiation increased by the photo-bleaching, in contrast to its decrease under the fast neutron irradiation. It is considered that there will be multiple causes for the radiation enhanced microbending loss. Among the causes, that caused by modification of the reflection plane between the core/clad in optical fibers through the atomic displacement process can be reduced by the photo-bleaching. In general, it is concluded that the photo-bleaching technique is effective in improving radiation resistance of optical fibers under neutron associated and moderate flux radiation environments.
A possibility of applying a compact optical diagnostics to nuclear systems, such as detecting high energy neutrons in a nuclear fusion system and measuring a high-flux gamma-ray in a fission reactor with a wide dynamic range, has been studied, utilizing radiation resistant optical fibers and radioluminescent (radiation-induced luminescent) materials.
Irradiation test of pre-annealed optical fiber was carried out using gamma-ray and fast neutron source. Optical fiber has a good radiation resistance by the heat treatment and there are optimum temperatures changed by the kinds of radiation and the wavelength of transmission light. It is necessary to select the heat treatment temperature for developing the radiation resistance by the situation of the optical fiber.
Ion induced luminescence was studied for SiO2 glasses and SiO2 based optical fiber materials with different hydrogen and oxyhydrate concentration. The luminescence of the visible wavelengths was measured during the irradiation of protons and also heavier ions with low (5~10 keV) and high (0.2 ~ 2 MeV)energies, at a temperature rane between 295 and 600 K. Hydrogen concentration profiles were also examined by the ion beam analysis techniques to compare the nominal OH values. In addition to a prominent broad peak of 460 nm, characteristic peaks were detected at around 390 nm and 660 nm, depending on the OH contents. For fused silica specimens with lower OH, however, a peak at 390 nm was found at a small dose and its intensity decreased quickly with an increase of the ion dose. For synthesized silica with higher OH concentration, a small peak was found at 650 nm, corresponding to the non-bonding-oxygen-hole-center, while the 390 peak not appeared. Except for the low-OH synthesized silica, there existed a large amount of hydrogen, which does not form OH. The origin of the luminescence and the damage process will be discussed in connection with the nuclear and electronic energy loss by the penetrating energetic ions.
Radioluminescence from rare earth oxide materials was measured in the visible wavelength range and the range from 1000 to 3000 nm in an ion beam accelerator and a fission reactor of Japan Materials Testing Reactor (JMTR). In erbium oxide, peaks at 560, 660, and 1540 nm were observed by the proton beam irradiation, and the complicated luminescence peaks around 1800 and 2000 nm were observed by the proton beam irradiation and at the reactor full power operation of 50 MW. The peaks at 560, 660, and 1540 nm are attributed to the electron transition between the energy levels of Er3+ ions. The complicated luminescences observed in JMTR were quenched with the irradiation time elapsed, though the intensity was not changed by the irradiation of the proton beam of 2.6 x 1013 p/cm2s. It was considered that the precursors for the complicated luminescent peaks around 1800 and 2000 nm disappeared under the neutrons irradiation.
Applicability of fused silica core optical fibers to in-reactor dosimetry was demonstrated at elevated temperatures and a special irradiation rig was developed for realizing high-temperature optical dosimetry in a High Temperature Test Reactor (HTTR) in Oarai Research Establishment of JAERI (Japan Atomic Energy Research Institute). The paper will describe present status of preparation for the high temperature dosimetry in HTTR, utilizing radiation-resistant optical fibers and radioluminescent materials. Temperature measurement with a high-speed response is a main target for the present optical dosimetry, which could be applied for monitoring transient behaviors of the HTTR. This could be realized by measuring intensity of thermoluminescence and black body radiation in infrared region. For monitoring reactor powers, optical measurements in visible region are essential. At present, measurement of intensity of Cerenkov radiation will be most promising. Other possibilities with radio-luminescent materials having luminescent peaks in visible region are under survey. One of the candidates will be silica, which has a robust radioluminescent peak at 450 nm.
To promote development of radiation-resistant core optical fibers, the ITER-EDA (International Thermonuclear Experimental Reactor-Engineering Design Activity) recommended carrying out international round-robin irradiation tests of optical fibers to establish a reliable database for their applications in the ITER plasma diagnostics. Ten developed optical fibers were irradiation-tested in a Co-60 gamma cell, a Japan Materials Testing Reactor (JMTR). Also, some of them were irradiation tested in a fast neutron irradiation facility of FNS (Fast Neutron Source), especially to study temperature dependence of neutron-associated irradiation effects. Included were several Japanese fluorine doped fibers and one Japanese standard fiber (purified and undoped silica core), as well as seven Russian fibers. Some of Russian fibers were drawn by Japanese manufactures from Russian made pre-form rods to study effects of manufacturing processes to radiation resistant properties. The present paper will describe behaviors of growth of radiation-induced optical transmission loss in the wavelength range of 350-1750nm. Results indicate that role of displacement damages by fast neutrons are very important in introducing permanent optical transmission loss. Spectra of optical transmission loss in visible range will depend on irradiation temperatures and material parameters of optical fibers.
Optical transmission properties of optical fibers with a purified and undoped fused-silica core were investigated under irradiation by 14 MeV fast neutrons in the temperature range from room temperature to 573 K at a fast neutron irradiation facility of Fast Neutron Source (FNS). Growth of optical absorption in the wavelength range of 420-630 nm, which includes a well-known absorption peak called Non-Bridging-Oxygen-Hole-Center (NBOHC), took place due to the neutron irradiations. Change of the optical absorption depended strongly on the irradiation temperature. Particularly, growth rate of the radiation-induced optical absorption at the wavelength of 630 nm decreased with increase of the irradiation temperature up to 473 K, because of the dynamic recovery of the initial structural imperfections and the radiation-induced defects.
Retention and thermal release behavior of hydrogen isotopes in fused silica, synthesized silica and optical fibers were investigated by ion beam analysis technique. Initially contained H in the interior of the specimens is about 0.1~0.2 at.% at room temperature, irrespective of the nominal value of OH concentration. Besides, H atoms more than 1 x 1016H/cm2 was found at the surface. The thermal release of the H atoms from the interior was affected by re-trapping at the near surface. During 5 keV ion injection, the retained D in the implanted layer was quickly saturated with a concentration of about 1 x 1021D/cm3. Under the subsequent D injection to doses above 1 x 1018D/cm2, D atoms were trapped with a concentration about 1 at.% in the depth far beyond the projected ranges of D ions. Thermal release of D in the injected layer started at lower temperature than that from the larger depth for lower implantation dose, while the two release curves close to each other for the higher dose. Irradiation of 10 keV He ion into the fused silica caused H up-take in the He implanted depth, where no He atoms were retained.
Strong radioluminescence band and peaks were observed in fused silica core optical fibers under irradiation in fission reactors of Japan Materials Testing Reactor (JMTR) and in gamma-irradiation facility. A broad band in 400-1700 nm could be attributed to Cerenkov radiation, whose intensity was proportional to inverse of the third power of wavelength. The optical intensity of the peak at 450 nm, that was considered to be generated by oxygen vacancies, was proportional to the reactor power under the electronic excitation dose rate up to 5.0 x 103 Gy/s, and the dynamic range was at least more than five orders of magnitude. The peak intensity depends on details of silica microstructures and became twice after annealing at 773K. By using these radioluminescence peak and band, wide range dosimetry could be actualized in heavy radiation environments.
Visibility of a core region in a high temperature gas cooled reactor is poor. Realization of optical diagnostics there will improve safety and maintenancibility. Applicability optical fibers for diagnostics there was studied in the study.
An engineering and design activity in a project of developing International Thermonuclear Experimental Reactor is accumulating data on radiation effects in optical materials and components to be used for plasma diagnostics and for remote sensing. Radiation effects of neutrons, gamma-rays, and energetic-particles are quantitatively evaluated and a database is being made for designing optical components to be used in relatively heavy radiation environments.
Radiation resistant fused silica core optical fibers, 900 ppm OH doped with their hydrogen treated fibers with a plastic jacket, 18000 ppm OH doped one having very low optical transmission loss with a carbon jacket, and a fluorine doped fiber with a plastic jacket, were irradiated in a JMTR fission reactor, at 400 +/- 10 K with a fast neutron (E > 1 MeV) flux of 3.2 X 1017 n/cm2s and a gamma dose rate of 3 X 103 Gy/s for 527 hours. Their optical transmission loss at 850 nm was measured in- situ during irradiation.
Nine different fused silica core optical fibers were irradiated in a fission reactor up to an ionizing dose of 109Gy and a fast neutron fluence of 1023n/m2 at 400 K. One specimen, fluorine doped and specially sintered and annealed, showed good radiation resistance in the wavelength range of 300 - 1850 nm. Results showed that the fibers can be applied in a neutron associated heavy radiation environments.
An experimental demonstration of fiber optic temperature sensing in the in-core region of Japan Materials Testing Reactor from 250 to 750 degrees C is described. Temperature data could be obtained for two full-power weeks with neutron fluxes of approximately 1014 n/cm2/s and gamma dose rates of approximately 5 X 103 Gy/s. The measurements were based on thermally generated IR light within the optical fiber itself. The fiber thus served as both signal generator and signal transmitter to the out-of-core region. The fibers utilized in the experiments where of high OH pure-silica-core type and showed good radiation resistance. In the IR region the transmission of the fibers was only weakly affected by the incident radiation. Radiation induced luminescence and Cerenkov radiation in the optical fibers were found to have small influence on the signal in the IR window. The high OH content of the fibers used in the present experiment precluded the use of the spectral regions at 945, 1245, and 1390 nm, due to the high intrinsic and radiation induced absorption at these wavelengths. The use of silica fibers limited the maximum temperature to < 1000 degrees C. The present experiments show that optical sensors based on IR emission can be used to monitor temperature in the in-core region of nuclear reactors for extended periods of time.
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