Neutron-irradiated Vitreous Silica Research Articles

Chemical shift of characteristic X-ray wavelength in silicon-containing ceramics due to neutron irradiation

The characteristic wavelength of silicon in neutron-irradiated and non-irradiated ceramics was precisely measured to detect any change in materials induced by neutron irradiation. The wavelength of characteristic silicon Kβ X-rays emitted from materials was measured by electron probe microanalysis, using a WDX-equipped scanning electron microscope. The crystal used to disperse silicon X-rays was PET (pentaerythritol). Specimens were non-irradiated metallic silicon, vitreous silica, quartz, β-silicon carbide and β-silicon nitride. Neutron-irradiated vitreous silica and quartz were also used. Measurements were conducted successively during while monitoring the temperature of PET. A chemical shift of peaks from that of metallic silicon was observed for unirradiated materials, for longer wavelength in case of β-silicon carbide, β-silicon nitride, quartz and vitreous silica in this order. The chemical shift of vitreous silica due to neutron irradiation was reduced but that of quartz increased to those of the unirradiated specimens.

Electron spin resonance measurement of irradiation defects in vitreous silica irradiated with neutrons

The electron spin resonance (ESR) measurement of irradiation defects in neutron-irradiated vitreous silica was performed to study the effects of the OH content, neutron fluence, and post-irradiation isochronal anneal to 873 K. Together with the usual paramagnetic states of E ′ centers, non-bridging oxygen hole centers (NBOHCs), and peroxy radicals (PORs), some new states were observed in the ESR spectra. The principal-axis g-values of these states lied in the range g∼2.0–2.2, which were typical of O 2 − ions in solids. Considering these new defects, the observed effects of the OH content, neutron fluence, and post-irradiation isochronal anneal were discussed to obtain some details of the reaction mechanism of the irradiation defects.

A neutron diffraction and MAS-NMR study of the structure of fast neutron irradiated vitreous silica

Neutron diffraction and magic angle spinning (MAS)-NMR measurements have been performed on vitreous silica before and after irradiation to a dose of 2.8×10 20 fast neutrons cm −2. On fast neutron irradiation the first peak in the diffraction pattern is reduced in height, broadened and shifted to a slightly higher scattering vector, reflecting the increase in density of 2.4%. This may be interpreted as an enhancement of smaller scale structures, consistent with irradiation generating a higher proportion of three-membered rings as indicated by Raman data. In real space, there is a corresponding small increase in the SiO bond length which is consistent with a reduction in the mean SiOSi bond angle of ≈9.5° as indicated by the MAS-NMR data. Fast neutron irradiation also leads to a peak in the difference correlation function at ≈3.2Å which has previously been ascribed to oxygen atom interstitials, but may be identified as the Si(2)O distance within a three-membered ring.

Saturation of the density of states of tunneling systems in defective crystalline neutron-irradiated quartz.

The variation of the longitudinal velocity of sound has been measured in a neutron-irradiated quartz sample exposed to a moderate dose of 4.7\ifmmode\times\else\texttimes\fi{}${10}^{19}$ n/${\mathrm{cm}}^{2}$ (E\ensuremath{\ge}0.3 MeV), at low temperatures for three different frequencies. A surprisingly high density of tunneling systems was found, equal to the value in neutron-irradiated vitreous silica. This result is especially interesting in view of the fact that this sample was found to be still to a large extent crystalline, whereas neutron-irradiated vitreous silica is fully amorphous. The observation of the saturation of the density of states of tunneling systems in a still largely crystalline sample is further related to studies of the damage induced by neutrons in quartz. It is suggested that the tunneling systems partly reside at the interfaces of the ${\ensuremath{\alpha}}_{1}$-${\ensuremath{\alpha}}_{2}$ domains present in neutron-irradiated quartz.

Effect of heat treatment on the low-temperature thermal conductivities of vitreous silica and neutron-irradiated vitreous silica

Heat treating neutron-irradiated vitreous silica erases the effects of irradiation on the thermal conductivity at temperatures below 1 K. This indicates that the density of low-energy, localized excitations in vitreous silica is restored to its pre-irradiation value.

The optical constants of quartz, vitreous silica and neutron-irradiated vitreous silica: (II) Analysis of the infrared spectrum of vitreous silica

Optical constant data for vitreous silica and neutron-irradiated vitreous silica, given in part I are examined in an attempt to gain further knowledge of the structure of the glass. Strong features of the spectrum can be described by band broadening parameters, which are calculated using an extension of the Wilson GF matrix method, and are shown to be quantitatively related to the distribution of SiO distances and oxygen bond angles obtained from X-ray scattering data. The approximation commonly used to generate the optically active vibrational spectra of glasses, namely to form the product of the vibrational density of states function and a weakly frequency-dependent intensity factor gives a relatively poor representation of the experimental spectrum. The magnitude of ‘disorder-induced’ absorption in regions well away from the major bands is semi-quantitatively estimated by subtracting the contributions of the major bands. Interpretation of some of the features of this difference spectrum is possible in terms of vibrations of non-bridging oxygen atoms, but if this interpretation is correct, it is necessary to postulate clustering of ‘dangling’ oxygen atoms, which would not be consistent with a random network model for the structure. An alternative explanation, that the vibrations are framework modes, leads to the conclusion that the preferred configuration in vitreous silica resembles the arrangement of silicon-oxygen tetrahedra in cristobalite.

The optical constants of quartz, vitreous silica and neutron-irradiated vitreous silica (I)

The infrared reflectivity of three forms of silica, α-quartz, vitreous silica and neutron-irradiated vitreous silica (≈ 2.7 × 10 20 neutrons cm −2) has beenmeasured from 400–2000 cm −1. These data have been analysed by a Kramers-Kronig transform to give the real and imaginary parts of the complex dielectric constant. Considerable care has been taken to identify and minimize errors arising in the measurements of the reflectivity spectra and in the subsequent analysis. Data are presented for the optical constants, oscillator frequencies, band strengths and halfwidths of each band. The spectra for vitreous silica and neutron-irradiated silica show two regions of absorption which are not present in the crystalline form — a strong band is observed near 950 cm −1 and a broad band from 600–800 cm −1. A difference spectrum obtained by subtracting the spectrum of the imaginary part of the dielectric contrast for vitreous silica from the corresponding data for neutron-irradiated silica reveals more detailed structure in the form of a weak but sharp band at 620 cm −1. Interpretation of these results is contained in a companion paper.

X-Ray Diffraction from Neutron Irradiated Vitreous Silica

Both large-angle and small-angle diffraction studies have been made on pile irradiated silica glass. The radial distribution curves obtained from the large-angle scattering were in essential agreement with the earlier work of Simon. The small-angle scattering from samples irradiated with a relatively low fast neutron dose showed a noticeable rise in the scattering at the smallest angles. Two possible causes of this scattering are suggested. In addition, all irradiated samples showed a general increase in the scattering over the entire small-angle region. This scattering which showed little angular dependence was attributed to increased density fluctuations within the sample after irradiation. The irradiation induced scattering was almost completely removed by annealing the glass at 1300°C.

Silicon valence in SiO films studied by X-ray emission

The characteristic wavelength of silicon in neutron-irradiated and non-irradiated ceramics was precisely measured to detect any change in materials induced by neutron irradiation. The wavelength of characteristic silicon Kβ X-rays emitted from materials was measured by electron probe microanalysis, using a WDX-equipped scanning electron microscope. The crystal used to disperse silicon X-rays was PET (pentaerythritol). Specimens were non-irradiated metallic silicon, vitreous silica, quartz, β-silicon carbide and β-silicon nitride. Neutron-irradiated vitreous silica and quartz were also used. Measurements were conducted successively during while monitoring the temperature of PET. A chemical shift of peaks from that of metallic silicon was observed for unirradiated materials, for longer wavelength in case of β-silicon carbide, β-silicon nitride, quartz and vitreous silica in this order. The chemical shift of vitreous silica due to neutron irradiation was reduced but that of quartz increased to those of the unirradiated specimens.

Fast Neutron Induced Luminescence in Vitreous Silica and Quartz

A reddish luminescence mouch more intense than the scattered light was observed in excited fast neutron irradiated vitreous silica and quartz. The cause of this luminescence was investigated. It is probably necessary to observe the detailed behavior of the absorption bands during lumoinescence in order to identify the luminescence centers; however, this work does not identify the excitation centers as being those with which the tail of the ultraviolet absorption is associated. (J.R.D.)

Low-Temperature Thermal Conductivity in Neutron Irradiated Vitreous Silica

Measurements of the thermal conductivity, K, of vitreous silica have been made at low temperatures before and after fast neutron bombardment. The observed increase of K upon bombardment is in agreement with the prediction of Klemens. Over the exposure range used, K increases approximately linearly with dose to the level used (6×1019 n/cm2) whereas the density appears to have nearly saturated, suggesting that the two properties are not directly related. A possible explanation for the K increase upon bombardment is that the neutron bombardment relieves macroscopic strains. Alternatively, it is possible that bombardment causes an increase in local order in the vitreous material.