By means of a newly constructed high pressure and low temperature optical apparatus we have measured the Raman spectra of H(2) and HD simple clathrate hydrates, synthesized in situ by the application of more than 2500 bar gas pressure on solid water. High resolution spectra of the molecular vibration have been measured at low temperature (about 20 K). In the case of HD this band is simpler than in the case of H(2), where the presence of the ortho- and para-species complicated the interpretation of the spectrum. We have determined frequency positions of the bands arising from multiple occupancy of the large cages of the sII clathrate, some of which are almost superimposed. The intensity of the bands gives information on the average and distribution of cage occupation, and of the ortho-para (o-p) ratio of H(2) molecules. Hydrogen o-p conversion rate is measured, for molecules in the small cages and in the large cages, and it is observed that these are different. A model considering both intrinsic and extrinsic conversion processes is applied to the measured data. The intrinsic conversion rate so derived is compared favorably to that measured for pure hydrogen in different situations.
This paper reports the testing results of radiation resistant fiber Bragg grating (FBG) in random air-line (RAL) fibers in comparison with FBGs in other radiation-hardened fibers. FBGs in RAL fibers were fabricated by 80 fs ultrafast laser pulse using a phase mask approach. The fiber Bragg gratings tests were carried out in the core region of a 6 MW MIT research reactor (MITR) at a steady temperature above 600°C and an average fast neutron (>1 MeV) flux >1.2 × 10 n/cm/s. Fifty five-day tests of FBG sensors showed less than 5 dB reduction in FBG peak strength after over 1 × 10 n/cm of accumulated fast neutron dose. The radiation-induced compaction of FBG sensors produced less than 5.5 nm FBG wavelength shift toward shorter wavelength. To test temporal responses of FBG sensors, a number of reactor anomaly events were artificially created to abruptly change reactor power, temperature, and neutron flux over short periods of time. The thermal sensitivity and temporal responses of FBGs were determined at different accumulated doses of neutron flux. Results presented in this paper reveal that temperature-stable Type-II FBGs fabricated in radiation-hardened fibers can survive harsh in-pile conditions. Despite large parameter drift induced by strong nuclear radiation, further engineering and innovation on both optical fibers and fiber devices could lead to useful fiber sensors for various in-pile measurements to improve safety and efficiency of existing and next generation nuclear reactors.
Brillouin optical time domain analysis is the sensing of temperature and strain changes along an optical fiber by measuring the frequency shift changes of Brillouin backscattering. Because frequency shift changes are a linear combination of temperature and strain changes, their discrimination is a challenge. Here, a multicore optical fiber that has two cores is fabricated. The differences between the cores’ temperature and strain coefficients are such that temperature (strain) changes can be discriminated with error amplification factors of 4.57 °C/MHz (69.11 sans-serifμsans-serifϵ/MHz), which is 2.63 (3.67) times lower than previously demonstrated. As proof of principle, using the multicore optical fiber and a commercial Brillouin optical time domain analyzer, the temperature (strain) changes of a thermally expanding metal cylinder are discriminated with an error of 0.24% (3.7%).
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