Bruno Boizot

Influence of external β-irradiation in oxide glasses

Abstract Radiation effects in oxide glass as external β-irradiation has been investigated by Raman spectroscopy, Cr3+ luminescence and EPR spectroscopy to analyze microstructure evolution. Comparison between simplified and complex glasses (high-level nuclear waste glass analogue) was undertaken. Migration and segregation of sodium have been confirmed on simplified irradiated glasses by a polymerization increase and dissolved oxygen presence. Densification under irradiation has also been demonstrated with the increase of three-membered silicate rings in silicate glass and the decrease of the average Si–O–Si angle in the borosilicate glass. In both glasses, Cr3+ crystal field was increased, suggesting a Cr–O length decrease. The complex borosilicate glass responds differently: densification effect and Na segregation were not detected. Under β-irradiation, electron-trapping processes on Cr6+ occurred, forming Cr4+ and Cr5+ species.

Structure of β-irradiated glasses studied by X-ray absorption and Raman spectroscopies

Abstract The structural modifications induced by external β-irradiation have been investigated to simulate the ageing processes of nuclear glasses under irradiation. Six glasses in the CaO–Al 2 O 3 –SiO 2 system, from 10 to 68 mol% of SiO 2 content, were irradiated with electrons generated by a Van de Graaf accelerator. The structure of these glasses was studied using Raman and X-ray absorption spectroscopies at the Al K-edge. We observe that a band appears at 1557 cm −1 in the Raman spectra of irradiated glasses, which is assigned to molecular O 2 , and that the average Si–O–Si bond angle decreases. An increase of the polymerization of the network is also detected in the Raman spectra of irradiated samples, which corresponds to a shift of the 1100 cm −1 band towards higher frequencies. This change in network polymerization is confirmed by an increase in the intensity of the main resonance at 1565 eV in the X-ray absorption spectra at the Al K-edge.

Dose, dose rate and irradiation temperature effects in β-irradiated simplified nuclear waste glasses by EPR spectroscopy.

The influence of dose, dose rate and irradiation or annealing temperature on the formation of paramagnetic centers has been investigated by electron paramagnetic resonance (EPR) spectroscopy on a series of β-irradiated 4-, 5- and 6-oxide glasses in order to study the use of external β-irradiation relative to simulate actual conditions during the storage of nuclear waste glasses. An unusual saturation of defect concentration occurs above 104 Gy. There is no dose rate dependence for the irradiation in a range between 2×103 and 2×104 Gy/s. The increase of irradiation temperature leads to an increase of Zr3+, a decrease of Fe3+ concentration, the formation of magnetic clusters and a strong decrease in the total concentration of paramagnetic defects. In addition, annealing and irradiation temperatures do not produce the same effects on the concentration of the various defects.

A structural approach by MAS NMR spectroscopy of mechanisms occurring under β-irradiation in mixed alkali aluminoborosilicate glasses

Two series of mixed Na/K and Na/Li aluminoborosilicate glasses have been irradiated with electrons of 1.8 MeV at doses close to 109 Gy. Si, B and Al glass former environment changes under irradiation have been probed by MAS NMR spectroscopy. It was shown that the mixed alkali effect acts on the middle range compositions by operating with a selective blockage in irradiated glasses, depending on the alkali nature and concerning either modifier or charge compensator alkalis.

Effect of Sm-, Gd-codoping on structural modifications in aluminoborosilicate glasses under β-irradiation

Two series of Sm-, Gd-codoped aluminoborosilicate glasses with different total rare earth content have been studied in order to examine the codoping effect on the structural modifications of beta-irradiated glasses. The data obtained by Electron Paramagnetic Resonance spectroscopy indicated that relative amount of Gd3+ ions located in network former position reveals non-linear dependence on Sm/Gd ratio. Besides, codoping leads to the evolution of the EPR signal attributed to defects created by irradiation: superhyperfine structure of boron oxygen hole centres EPR line becomes less noticeable and resolved with increase of Gd amount. This fact manifests that Gd3+ ions are mainly diluted in vicinity of the boron network. By Raman spectroscopy, we showed that the structural changes induced by the irradiation also reveal non-linear behaviour with Sm/Gd ratio. In fact, the shift of the Si-O-Si bending vibration modes has a clear minimum for the samples containing equal amount of Sm and Gd (50:50) in both series of the investigated glasses. In contrast, for single doped glass there is no influence of dopants content on Si-O-Si shift (in case of Gd) or its diminution (in case of Sm) occurs which is explained by the reduction process influence. At the same time, no noticeable effect of codoping on Sm3+ intensity as well as on Sm2+ emission or on Sm reduction process was observed.

Micro-Raman and EPR studies of β-radiation damages in aluminosilicate glass

Abstract Structural changes under β-irradiation have been investigated by means of micro-Raman and electron paramagnetic resonance (EPR) spectroscopies on a calcium aluminosilicate glass with the composition 48.22 SiO 2 , 12.64 Al 2 O 3 , 2.48 Na 2 O, 3.45 Cs 2 O, 0.46 Rb 2 O, 32.75 CaO. The Raman spectra of this sample show no significant structural changes up to 4×10 9 Gy. This result could be correlated to the fact that alkaline and alkaline earth migration under β-irradiation are reduced for this glass composition because these ions are trapped close to aluminium ions for charge compensating reasons. EPR spectra of these irradiated glasses indicate the creation of three paramagnetic centres (Al–OHC, Oxy and E ′ defects) which increase with dose.

Weak hyperfine interaction of E′ centers in gamma and beta irradiated silica

We report on the effects of photon (γ) and electron (β) irradiation in a dose range extending from 100 to 5×109 Gy in a variety of silica samples studied by electron paramagnetic resonance. The E′ centers and a weak intensity satellite signal of their resonance line were generated both in γ- and in β-irradiated samples. We investigated the dependence of their intensity on the irradiation dose. Evidence of the existence of a common generation mechanism for the related paramagnetic point defects is found. These defects are induced mainly through the conversion of precursors except at very high doses, where the direct activation from the unperturbed matrix is concurrent. Our data support the model attributing the satellite signal to the weak hyperfine structure of the E′ center arising from interaction with a second nearest neighbor nuclear spin.

Tunable luminescence from Ce-doped aluminoborosilicate glasses

A series of aluminoborosilicate glasses were prepared using the melt-quenching technique for mixture of stoichiometric amounts of SiO2, Al2O3, H3BO3, Na2CO3, and ZrO2 with adding of different amounts of CeO2. The samples were investigated by means of luminescence spectroscopy. Tunable luminescence from violet to blue/green was observed from these glasses with different Xe-lamp excitation wavelengths ranging from 370 to 480 nm as well as with laser excitation of 266 and 355 nm. Moreover it was found that the possibility of tuning the light by changing of excitation wavelength was not unique. The same effect was observed by adjusting conditions for luminescence measurements as well as under exposure to β-irradiation. The obtained phenomena could be explained taking into account structural characteristics of this glass and it could be concluded that tunable luminescence results from the presence of different Ce-sites the glass matrix. Thus the results suggest that Ce-doped glasses could be considered as conversion materials for blue light-emitting diode chips to generate white light-emitting diodes.

Transient visible-UV absorption in beta irradiated silica

Abstract We performed optical absorption measurements in the range 1.24–6.2 eV on commercial silica samples during and after room temperature β-irradiation. Absorption is induced from 1.3 up to 5.5 eV and can be resolved with different bands. Two band structures peaked at 2.3 and 4.2 eV are found in natural silica but not in synthetic silica and are attributed to impurity related defects. Absorption at energy >4.2 eV shows an ubiquitous band at 5.7 eV attributed to E′ centers, and a band at 4.7 eV in synthetic samples attributed to non-bridging oxygen hole centers. Transient absorption is observed after irradiation in the spectral range above 3.0 eV with a reduction greater than 30% in a time scale of the order of few hours. A transient band peaked at 4.1 eV is detected both in natural and synthetic samples and is attributed to intrinsic centers. These transient effects are ascribed to relaxation of optically active defects by reaction with diffusing atoms or molecules released during irradiation.

Ultra stable oscillators dedicated for space applications: oscillator and quartz material behaviors vs radiation

This paper presents the first results obtained in the R&D study initiated by the CNES at the end of 2004. Numerous French experts have been gathered to determine and to tentatively understand the mechanisms responsible for the radiation sensitivity of quartz resonators and to correlate the results of various analyses in order to reduce or to anneal their susceptibilities