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Expanding Optical Instrumentation Development in Fusion and Fission Applications

In a pre-proof article posted in the Journal of Nuclear Materials, researchers analyzed the effects of concurrent radiation and thermal annealing on optical absorption in integrated silica and sapphire when exposed to gamma irradiation.

Study: Optical Absorption of Fused Silica and Sapphire Exposed to Neutron and Gamma Radiation with Simultaneous Thermal Annealing. Image Credit: RHJPhtotos/Shutterstock.com

Radiation-induced attenuation (RIA) was studied for integrated silica and sapphire through simultaneous and post-irradiation thermal annealing. The researchers found vital differences between simultaneous-irradiation thermal annealing and post-irradiation thermal annealing in the optical behavior of silica and sapphire.

The study has promising potential in selecting and placing optical materials for developing optical instrumentation utilizations such as fusion or fission reactors. It has also helped the researchers to understand the radiation effects on such optical materials.

Radiation-Induced Attenuation in Optical Materials Like Fused Silica and Sapphire

Radiation safety and economic performance of nuclear reactors can be considerably increased by reducing the frequency of inspection shutdowns with nuclear reactor instrumentation, allowing online monitoring of crucial reactor components.

Laser-induced breakdown spectroscopy (LIBS) could identify the degradation of nuclear reactor components by performing a spectroscopic investigation of the chemical makeup of the reactor coolant while the reactor is operating.

Understanding the radiation effects in optical materials such as fibers and lenses under appropriate operational settings is crucial since LIBS-based instrumentation necessitates the transmission of plasma emission and energetic laser pulses through such optical materials.

Ordinary optical materials such as silica and sapphire have optical properties, including attenuation and refractive indices, which change when exposed to ionic radiation effects in nuclear reactors.

Several studies have been conducted on the radiation-induced attenuation (RIA) and radiation effects in integrated silica and sapphire when irradiated by neutrons and gamma rays followed by thermal annealing. However, no data exists regarding the in situ behavior of the optical materials under simultaneous high temperature and radiation effects due to a considerable delay in time between irradiation, inspection, and thermal annealing.

The researchers in the current study conducted RIA measurements between 220 nm to 1100 nm using high-OH content Heraeus Spectrosil 2000 integrated silica (S2000), low-OH content Heraeus Infrasil 302 integrated silica (I302), and optical category sapphire. These optical materials were exposed to neutron and gamma irradiation under post-irradiation and simultaneous-irradiation thermal annealing up to 800⁰ C to observe their radiation effects. 

Experimental Setup for Optical Absorption of Silica and Sapphire

The first experimental setup for measuring absorption consists of an Ocean Insight HR4000 Spectrometer covering the 220-1100 nm spectral range and an Ocean Insight Halogen/Deuterium light source.

The second experimental setup includes an annealing furnace installed above the 60Co pool dry tube and is used for concurrent and post-thermal annealing of optical materials.

The current study conducted the irradiations in the nuclear reactor and 60Co irradiation pool in the Ohio State University Nuclear Reactor Laboratory. With the aid of a cylindrical fixture encompassing 60Co gamma sources, one I302 sample was exposed to irradiation of 10Mrad in the Pennsylvania State University Radiation Science and Engineering Center.

A specially made silicon carbide coil furnace with silica-alumina insulation was used for annealing the samples dry and in the air.

The furnaces were built to fit inside the 60Co pool and the nuclear reactor dry tubes to conduct simultaneous thermal annealing and irradiation on the samples.

In the post-irradiation annealing experiments, the samples were heated to the specified temperatures following each irradiation dose.

In contrast, in concurrent annealing cases, the samples were continuously heated during irradiation to the specified temperatures until the listed dose was reached.

Promising Potential of Optical Instrumentation Development in Fission and Fusion Applications

The study demonstrates the consequences of concurrent irradiation and thermal annealing and the radiation effects on optical graded sapphire, I302, and S2000.

The team observed critical distinctions in the behavior of these optical materials under simultaneous and post-irradiation thermal annealing conditions.

In the case of S2000, thermal annealing at 600 ⁰C post-irradiation to n-Doses 1 and 2 restored the material to its unirradiated form. At 800 ⁰C, the simultaneous-irradiation thermally annealed samples with the same doses preserved the radiation-induced attenuation in the UV range.

Under simultaneous-irradiation thermal annealing for n-Dose 1 and n-Dose 2, I302 also displayed an equilibrium radiation-induced attenuation spectrum between 220 nm to 900 nm, as opposed to the post-irradiation thermal annealing scenario, where the I302 mainly reverted to the unirradiated state after annealing to 800 ⁰C.

Compared to the equivalent dose post-irradiation thermal annealing case, which almost annealed the sample to its unirradiated state after heating to 800 ⁰C, sapphire showed a possible equilibrium radiation-induced attenuation range for n-Dose 1 and 2 for the concurrent-irradiation thermal annealing conditions. For this spectrum, a residual absorption peak was achieved at 260 nm while an increased absorption peak was achieved at 300 nm.

The current study was conducted with the original goal of supporting LIBS-based instrumentation in a highly radioactive and thermal environment withstanding significant radiation effects.

A comparison of the absorption spectra of the optical materials taken as the samples suggests that S2000 is the most desirable material to achieve LIBS-based instrumentation, up to an annealing temperature of 800 ⁰C and neutron fluence of 1.7 x 1017 n. cm-2.

At the relevant LIBS wavelengths of 532 nm and 1064 nm, S2000 showed only marginal radiation-induced attenuation. Under simultaneous-irradiation thermal annealing, I302 generated considerable radiation-induced attenuation up to 900 nm, which may restrict LIBS lasers at 532 nm.

Compared to the reported lack of significant radiation-induced attenuation in S2000, sapphire did not exhibit radiation-induced attenuation at 532 nm or 1064 nm for simultaneous-irradiation thermal annealing. Residual radiation-induced attenuation peaks in the UV range may interfere with LIBS plasma spectroscopy.

Reference

B.W. Morgan, M.P. Van Zile, C.M. Petrie, P. Sabharwall, M. Burger, I. Jovanovic, Optical Absorption of Fused Silica and Sapphire Exposed to Neutron and Gamma Radiation with Simultaneous Thermal Annealing. 2022. Journal of Nuclear Materials. https://www.sciencedirect.com/science/article/pii/S0022311522004317

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