Effects of neutron irradiation and subsequent annealing on the optical characteristics of sapphire

Mihai

et al. 2017

Sci Rep

The paper is the first comprehensive study on alpha particle irradiation effects on four mid-IR materials: CaF2, BaF2, Al2O3 (sapphire) and ZnSe. The measurements of the optical spectral transmittance, spectral diffuse reflectance, radioluminescent emission, terahertz (THz) spectral response, transmittance, absorbance, refractive index, real and imaginary parts of the dielectric constant and THz imaging are used as complementary investigations to evaluate these effects. The simulations were run to estimate: (i) the penetration depth, (ii) the scattering of alpha particle beam, (iii) the amount of material affected by this interaction, and (iv) the number of vacancies produced by the radiation exposure for each type of material. The simulation results are compared to the off-line measurement outcomes. The delay and spectral composition change of the reflected THz signal highlight the modification induced in the tested materials by the irradiation process.

mentioning

confidence: 99%

Optical and THz investigations of mid-IR materials exposed to alpha particle irradiation

Mihai

et al. 2017

Sci Rep

Section: Introductionmentioning

confidence: 93%

“…Previous work describing the optical absorption resulting from ex‐situ reactor irradiations of sapphire crystals reported growth of absorption centers at energies near 6.02, 5.35, and 4.85 eV (206, 232, and 256 nm, respectively) . Some previous work has also reported smaller absorption bands near 4.1, 3.5, 2.7, and 2.0 eV (302, 354, 459, and 620 nm) . The magnitudes of the various absorption centers observed in reactor‐irradiated sapphire have been found to increase with increased accumulation of neutron fluence, although information regarding the dynamics of the radiation‐induced absorption has been limited by the fact that previous measurements were made ex‐situ .…”

Section: Introductionmentioning

confidence: 99%

In‐Situ Reactor Radiation‐Induced Attenuation in Sapphire Optical Fibers

2014

The purpose of this work was to determine the suitability of using instrumentation utilizing sapphire optical fibers in a nuclear reactor environment. In this work, the broadband (500-2200 nm, or 0.56-2.48 eV) optical transmission in commercially available sapphire optical fibers was monitored in-situ prior to and during reactor irradiation. The sapphire fibers were irradiated at a neutron flux of 1.5 3 10 12 n/(cm 2 Ás) and a gamma dose rate of 75 kGy/h (dose in sapphire) to a total neutron fluence of 1.1 3 10 17 n/cm 2 and total gamma dose of on the order of 1.5 MGy. Consistent with previous gamma irradiation experiments, an absorption band centered below 500 nm (the minimum measurable wavelength using the measurement system described in this work) and extending as far as~1000 nm reached saturation during irradiation in the gamma shut-down field (the gamma-ray radiation field that is present in the reactor post-operation, as a consequence of the decay of radioisotopes that were produced during reactor operation) of the reactor prior to reactor irradiation. Beginning reactor irradiation and increasing the reactor power caused rapid increases in attenuation, followed by a linear increase with irradiation time at constant reactor power. Shutting down the reactor caused a decrease in the added attenuation; however, restarting the reactor caused the added attenuation to rapidly return to values almost identical to those observed at the end of the previous irradiation. The decrease in attenuation that was observed after the reactor was shut down shows the importance of the in-situ nature of the measurements made in this work (previous ex-situ attenuation measurements could not have captured this effect). A model is proposed for the experimentally obtained values of the radiation-induced attenuation that involves three previously identified color centers including a composite V-center, a F þ 2 center, and a F 2þ 2 center. The model accounts for gamma radiation-induced ionization of preexisting defects, generation of new defects via displacement damage, and conversion between defect centers via ionization and charge recombination.

“…Crystal surfaces of sapphire transform at around 473 K. 33 This is probably due to the surface diffusion. 34 With an annealing temperature of 973 K, the surfaces of the diamond-polished and H-ion irradiated crystals are flattened by the surface diffusion and transform thereby affecting light scattering and improving the optical transmittances.…”

Section: -6mentioning

confidence: 99%

Optical transmittance investigation of 1-keV ion-irradiated sapphire crystals as potential VUV to NIR window materials of fusion reactors

et al. 2016

We investigate the optical transmittances of ion-irradiated sapphire crystals as potential vacuum ultraviolet (VUV) to near-infrared (NIR) window materials of fusion reactors. Under potential conditions in fusion reactors, sapphire crystals are irradiated with hydrogen (H), deuterium (D), and helium (He) ions with 1-keV energy and ∼ 1020-m-2 s-1 flux. Ion irradiation decreases the transmittances from 140 to 260 nm but hardly affects the transmittances from 300 to 1500 nm. H-ion and D-ion irradiation causes optical absorptions near 210 and 260 nm associated with an F-center and an F+-center, respectively. These F-type centers are classified as Schottky defects that can be removed through annealing above 1000 K. In contrast, He-ion irradiation does not cause optical absorptions above 200 nm because He-ions cannot be incorporated in the crystal lattice due to the large ionic radius of He-ions. Moreover, the significant decrease in transmittance of the ion-irradiated sapphire crystals from 140 to 180 nm is related to the light scattering on the crystal surface. Similar to diamond polishing, ion irradiation modifies the crystal surface thereby affecting the optical properties especially at shorter wavelengths. Although the transmittances in the VUV wavelengths decrease after ion irradiation, the transmittances can be improved through annealing above 1000 K. With an optical transmittance in the VUV region that can recover through simple annealing and with a high transparency from the ultraviolet (UV) to the NIR region, sapphire crystals can therefore be used as good optical windows inside modern fusion power reactors in terms of light particle loadings of hydrogen isotopes and helium.