Radiation-induced Absorption Bands Research Articles

Optical Absorption of Fused Silica and Sapphire Exposed to Neutron and Gamma Radiation with Simultaneous Thermal Annealing

The simultaneous effects of radiation and thermal annealing on optical absorption in fused silica and sapphire subjected to gamma irradiation up to doses of 10 Mrad, and mixed neutron/gamma irradiation up to total neutron fluence of 1.7 × 1017 n · cm−2 (211 Mrad gamma) are reported and discussed. Thermal annealing was performed post-irradiation from 200 ∘C to 800 ∘C, and concurrent-irradiation thermal annealing was performed at 800 ∘C. Optical absorption was measured over the range of 220 nm to 1100 nm. Gamma-radiation-induced attenuation peaks in sapphire at 260 nm, 300 nm, and 355 nm are observed under concurrent-irradiation thermal annealing through the transfer of electrons between precursor oxygen vacancy and divacancy sites and the increased mobility of vacancy sites. It is shown that concurrent-irradiation thermal annealing does not restore the materials to their unirradiated state. Under concurrent-irradiation thermal annealing, high-OH content fused silica retained a radiation induced absorption band in the ultraviolet due to a 213 nm absorption band from unannealed E’ centers. Low-OH content fused silica retained an absorption band from 220 nm to 900 nm with multiple peaks generated from E’ centers. Sapphire retained an absorption band from 220 nm to 350 nm consistent with the transfer of electrons between precursor oxygen vacancy and divacancy sites, the additional production of oxygen vacancy sites from neutron interactions, and increased mobility of vacancy sites. These results help to inform optical material selection and placement to support the development of optical instrumentation in applications such as fission or fusion reactors.

Influence of growth conditions on the optical spectra of gamma irradiated BaF2 and CaF2 crystals

A comparative study of the gamma irradiated BaF2 and CaF2 crystals obtained under different growth conditions, using the Bridgman technique, is presented. A special attention was attributed to the influence of trace amount of three impurities: lead, ytterbium and oxygen ions, on the γ–rays induced color centers. The small amount of Pb ions comes from the different raw materials used. The oxygen content is due to the low vacuum level during the growth process. The crystals with Yb content were intentionally doped with a small amount of YbF3. The CaF2 crystal is more sensitive to a trace amount of lead ions than the BaF2 host. The oxygen, in addition of the lead ions in the BaF2 host, is more harmful than the trace amount of PbF2. The small amount of YbF3 is useful in eliminating the color centers in visible range. The induced color centers are stable in time. The lead ions contaminated BaF2 and CaF2 crystals are not good materials for applications in which the radiation induced absorption bands are undesirable.

Effects of Ge Co-Doping on P-Related Radiation-Induced Absorption in Er/Yb-Doped Optical Fibers for Space Applications

This study demonstrates that phosphorus-related radiation-induced absorption (RIA) bands such as P-OHC and P1 in erbium/ytterbium-doped optical fibers (EYDFs) are effectively decreased by germanium (Ge) co-doping. The suppression of the generation of both P-OHC during X-ray irradiation of glass preforms and P1 during gamma-ray irradiation of optical fibers was measured for Ge concentrations between 0 wt% and 6.7 wt%. For the 10 krad X-ray irradiation of glass preforms, it was found that the RIA decreased from 5.6 dB/cm to 1.3 dB/cm when the Ge concentration was increased from 0.79 wt% to more than 4.4 wt%. Furthermore, in multimode optical fibers (MMFs) exposed to 100 krad of gamma-ray irradiation, the RIA decreased from 3.7 dB/m to 0.90 dB/m when the Ge concentration was increased from 0.0 wt% to 4.0 wt%, whereas it was 0.35 dB/m for a single-mode optical fiber (SMF) with a Ge concentration of 6.7 wt%. These results demonstrate that Ge concentrations up to 4.4 wt% are strongly correlated with improved radiation tolerance for both glass preforms and optical fibers. This effect was more prominent for SMFs than MMFs and became saturated with excess co-doping in both cases. Therefore, Ge co-doping may represent a good solution for reducing P-related RIA in EYDFs for space applications.

Radiation hardness of Ce-doped sol-gel silica fibers for high energy physics applications.

The results of irradiation tests on Ce-doped sol-gel silica using x- and γ-rays up to 10kGy are reported in order to investigate the radiation hardness of this material for high-energy physics applications. Sol-gel silica fibers with Ce concentrations of 0.0125 and 0.05mol.% are characterized by means of optical absorption and attenuation length measurements before and after irradiation. The two different techniques give comparable results, evidencing the formation of a main broad radiation-induced absorption band, peaking at about 2.2eV, related to radiation-induced color centers. The results are compared with those obtained on bulk silica. This study reveals that an improvement of the radiation hardness of Ce-doped silica fibers can be achieved by reducing Ce content inside the fiber core, paving the way for further material development.

Optical properties and radiation hardness of Pr-doped sol-gel silica: Influence of fiber drawing process

The optical emission from the 5d – 4f allowed transition of Pr3+ ions embedded in sol-gel silica is investigated for High Energy Physics applications requiring fast scintillating materials.A complete and detailed characterization of the optical, scintillation and radiation hardness properties of Pr-doped silica is carried out employing different experimental techniques including steady-state and time-resolved photo-luminescence, radio- and thermo-luminescence, scintillation and optical absorption.Optical absorption measurements, performed after X-ray irradiation sequences up to 1kGy, evidence the formation of radiation-induced absorption bands related to point defects acting as color centers. Spontaneous partial recovery of the radiation-induced defects at room temperature, as well as after thermal treatments, is also disclosed.Particular attention is paid to the comparison between bulk silica, both before and after a melting process, and fibers. The results reveal the presence of a lower concentration of optically active defects in melted glass. Such comparison highlights a role of the fiber drawing in modifying the glass defectiveness, consisting in the occurrence of a structural reorganization of the amorphous network during the process.

Anomalies and peculiarities of radiation-induced light absorption in pure silica optical fibers at different temperatures

Undoped-silica-core F-doped-silica-cladding optical fibers (“undoped fibers”) are an important fiber type for applications requiring resistance to ionizing radiation (e.g., the nuclear industry, space, and military applications), the most important fundamental radiation-induced color centers arising in such fibers being self-trapped holes (STH). Despite the previous in-depth STH investigations, there have remained a few not-fully understood issues, such as the relationship between the radiation-induced absorption (RIA) bands due to STH in undoped fibers, on the one hand, and in bulk silica samples, on the other, the role of strain in the silica network in the STH occurrence, and possible peculiarities of short-lived STH-like radiation-induced color centers at temperatures above RT. To address these issues, we investigate the RIA spectra in undoped fibers with different frozen-in strain in their silica network immediately in the process of γ-irradiation to a dose of 1 kGy, the irradiation temperature being in the range ±60 °C or liquid nitrogen temperature (LNT). Gaussian decomposition of the RIA spectra measured at LNT has yielded STH bands at 2.6 and 2.16 eV together with the “classical” STH bands at 1.88 and 1.63 eV observed in fibers more frequently than the former bands. Based on this observation, it is proposed that all the STH bands observable in fibers fall into two classes: those inherent in silica and those strain-assisted, which can adjoin each other in the fiber silica network. The inherent STH include the well-known low-temperature infrared absorption and the bands at 2.6 and 2.16 eV; the strain assisted STH, the 1.88- and 1.63-eV bands. The 1.88-eV band is argued to be due to STH1, the 1.63-eV one, due to STH2. Anomalously high RIA at T = 0 and +60 °C is revealed and explained for the first time. The former effect is found to be caused by extreme compression of silica at T ∼ 0 °C enhancing the strain-assisted STH bands. The anomaly at T = +60 °C is found to be due to a previously unknown broad RIA band at ∼1.08 eV, which is likely to be associated with STH or self-trapped electrons and to result from either large network strain at the compression phases of enhanced thermal oscillation, or large expansion at the opposite phases. The RIA enhancement at T = +60 °C observed in this paper for the first time can influence fiber applications in the nuclear industry associated with high temperatures and high dose rates.

Improvement of radiation resistance by introducing CeO2 in Yb-doped silicate glasses

The influences of Ce on the radiation resistance of Yb-doped aluminosilicate glasses were investigated. The Ce ions in silicate glasses can effectively trap holes and free electrons due to their multivalency, consequently suppressing the radiation-induced absorption band. This could greatly improve the radiation resistance without obvious impact on the absorption properties of Yb-doped glasses. However, the fluorescence intensity and lifetime of Yb3+ ions were significantly decreased by high concentration Ce codoping. The mechanisms involved in these effects have also been discussed. The experiment results indicate that Yb-codoped glasses with a moderate Ce-concentration can be used as active medias for radiation–resistant materials in harsh radiation environments.

Gamma ray induced modifications of quaternary silicate glasses

This paper reports the effects of gamma ray irradiation and glass composition on the optical and infrared spectra of quaternary silicate glasses. Experimental results reveal the formation of radiation-induced absorption bands in the visible region attributed to nonbridging oxygen hole centres. The band gap energy and the width of the energy tail above the mobility gap have been measured before and after irradiation. The decrease in the energy gap values has been discussed in terms of radiation-induced structural modifications and the glass composition. Changes in the infrared spectra with progressive gamma irradiation have been observed and explained with reference to the radiolysis of Si–O–Si bonds.

Formyl violet cyanide liquid dosimetry system

Aqueous solutions of formyl violet cyanide (FV-CN) have been investigated for potential use in radiation dosimetry using both absorption and fluorescence evaluation. A radiation-induced absorption band peaking at 590 nm was observed and this wavelength was chosen for fluorimetric experiments, where a radiation-induced emission band develops at 600 nm. The useful dose range of the solution was found to be 400–4000 Gy depending on the FV-CN concentration and the evaluation method. The pre- and post-irradiation stabilities of the solutions were found to be satisfactory.

New Al2O3:C,Mg crystals for radiophotoluminescent dosimetry and optical imaging

Optical and dosimetric properties of a new radiophotoluminescent material based on aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg) and having aggregate oxygen vacancy defects are presented. The Al2O3:C,Mg crystals are characterized by several new optical absorption and emission bands. It is suggested that the main optical properties of this material are due to the formation of aggregate defects composed of two oxygen vacancies and two Mg-impurity atoms. Radiation-induced optical absorption bands are centered at 335 and 620 nm and produce fluorescent emission at 750 nm with a 75 +/- 5 ns lifetime. The dose measurements are performed by illumination of the Al2O3:C,Mg crystal with 335 nm or 650 nm light and by measuring the intensity of the 750 nm fluorescence. The detector material is insensitive to room light before and after the irradiation and the traps are stable up to 600 degrees C. A dose measurement range between 5 mGy and 200 Gy, suitable for therapeutic radiology applications, was demonstrated. The short luminescent lifetime and nondestructive readout is favorable for imaging applications.

The behavior of interstitial oxygen atoms induced by F 2 laser irradiation of oxygen-rich glassy SiO 2

Interstitial oxygen atoms in glassy silicon dioxide were created by photolysis of pre-existing interstitial oxygen molecules O 2 with a fluorine excimer laser (7.9 eV). The concentration of atomic oxygen interstitials was indirectly monitored by the disappearance and subsequent recovery of interstitial molecules which were monitored by their 1272 nm photoluminescence band. Most of the oxygen interstitials (>95%) are immobile at room temperature. The onset of their mobility occurs between 200 and 400 °C where around 95% of them recombine to form O 2 molecules. The high stability of interstitial oxygen atoms is consistent with the theoretical prediction that they are incorporated into silica structure in a form of peroxy linkages (POLs) SiOOSi. The radiation-induced optical absorption band around 7.1 eV is tentatively assigned to POLs.

Ion diffusion-controlled processes in fluoride crystals

Ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF 2 , BaF 2 and LiBaF 3 crystals (X-ray irradiated at 290 K) have been investigated by means of ionic conductivity and the correlated ionic thermally stimulated depolarisation current (TSDC), and radiation-induced optical absorption band thermal bleaching techniques at 290-650 K. It was found that under a DC field fluorides store large ionic space-charge. In CaF 2 , BaF 2 and LiBaF 3 , by using the ionic TSDC technique we were able to detect a series of the interstitial anion and/or anion vacancy delocalisation stages in the extrinsic ionic conductivity region. At least 4-6 wide and overlapping ionic TSDC peaks were observed. The activation energy values of the ionic conductivity and ionic TSDC coincide, or are very close: 1.313 ± 0.002 and 1.27 ± 0.02eV for CaF 2 , 1.040 ± 0.002 and 1.00 ± 0.02 eV for BaF 2 and 0.91 ± 0.04 and 0.780 ± 0.013 eV for LiBaF 3 , respectively. The correlated data of the ionic TSDC and thermal bleaching (of the F band and other radiation-induced absorption bands) suggest that above RT ion diffusion-controlled TSR processes take place in CaF 2 , BaF 2 and LiBaF 3 crystals. These processes are initiated and controlled by the anion defect thermal detrapping, migration and interaction with the trapped charges.

Optical transient resonance of copper nanoparticle composites synthesized by negative ion implantation

Steady-state optical absorption and nonlinear transient resonance have been studied for Cu nanoparticle composites, fabricated in magnesium aluminate spinel by high-current negative ion implantation. The implantation was conducted at high ion fluxes of 60 keV up to a total dose of 3×10 16 ions/cm 2. Optical absorption strongly depended on dose rate, ranging from 1 to 100 μA/cm 2. The dose rate dependence of the absorption spectra around the surface plasmon resonance behaved analogously to that of a-SiO 2, though the radiation-induced absorption band depended on stoichiometry of the spinel. The spectral shape of transient absorption at various dose rates is similar but the bleached plasmon band shifts is in parallel with the steady-state surface plasmon resonance. The dose rate chiefly influences dielectric properties of a spinel matrix but not that of particles.

Modeling of excimer laser radiation induced defect generation in fluoride phosphate glasses

Fluoride phosphate (FP) glasses with low phosphate content are high-transparent in the deep ultraviolet (UV) range and attractive candidates for UV-optics. Their optical properties are complementary to fluoride crystals. The anomalous partial dispersion makes them desirable for optical lens designs to reduce the secondary spectrum. Their UV transmission is limited by trace impurities introduced by raw materials and decreases when exposed to UV-radiation (lamps, lasers). The experiments of the paper published previously in this journal were used in order to separate radiation induced absorption bands in the fluoride phosphate glass FP10. In this paper the generation mechanism of the phosphorus–oxygen related hole center POHC 2 is investigated in detail in glasses of various compositions (various phosphate and impurity contents) in order to predict the transmission loss in case of long-time irradiation. Experiments were carried out using ArF- and KrF-excimer lasers (ns-pulses). POHC 2 generation strongly depends on the phosphate content and on the content of Pb 2+. A model was developed on these terms. Rate equations are formulated, incorporating the influence of the Pb 2+-content on the defect generation, a two-step creation term including an energy transfer process and a one-photon bleaching term. This results in a set of coupled nonlinear differential equations. Absorption coefficients and lifetimes of the excited states were calculated as well. Experimental results compared well with the numerical analysis of the theoretical rate equations.

Generation and healing behavior of radiation-induced optical absorption in fluoride phosphate glasses: The dependence on UV radiation sources and temperature

High purity fluoride phosphate (FP) glasses have a large transmission range from the vacuum ultraviolet to the infrared. They are attractive candidates for lens systems in microlithography equipment and excimer laser optics. Fluoride single crystals and vitreous silica are well-known traditional materials for ultraviolet optics. Crystal sizes are limited and glass is better for fabricating optics. For lens systems, a variety of glasses with different refractive indices and dispersion are required. The UV resonance wavelengths of FP glasses with a low content of phosphate using a two-term Sellmeier dispersion formula are comparable with those of silica and fluoride single crystals. It is known, that UV radiation induces the generation of several defect centers leading to additional absorption bands. The investigation of the kinetics of defect generation is very important for the prediction of transmission losses in case of long-time irradiation. To predict the kinetics of defect generation, first the separation of absorption bands is necessary. Experiments were carried out using UV-lamps, the KrF excimer laser (ns- and fs-pulses) and the ArF excimer laser (ns-pulses). The healing behavior of radiation-induced absorption bands was investigated using thermal treatment (temperature-dependence) and bleaching experiments (radiation-dependence). The results of these experiments enable the separation of absorption bands and the prediction of the defect generation depending on the used radiation source. The healing of strong defects at room temperature is postulated to be a diffusion-controlled process.

X-ray-induced thermally stimulated luminescence and electronic processes in a segmented poly(ester urethane)

Thermally stimulated luminescence (TSL) glow curves of x-irradiated Estane® 5703 exhibit considerable variability as the sample thickness is reduced from 1.25 mm to 20 μm. The dependence is presumably associated with the diffusion of oxygen into the sample, which alters electronic trapping sites. Specimens of 1.25-mm thickness exhibit reproducible glow curves, are relatively impervious to oxygen exposure, and are characterized by glow peaks at 145, 405 and 440 K. Radiation-induced optical absorption bands occur at 320, 360, 566 and 375 nm, the latter identified with the diphenylmethyl radical. Temperature dependence of these bands, coupled with electron-spin-resonance data, show that TSL is not associated with destruction of the diphenylmethyl radical, but likely results from the onset of polymer molecular motion ( γ relaxation), which releases trapped charges that recombine with other radicals. Radiative emission characteristic of phenyl deexcitation, but in the absence of diphenylmethyl radical destruction, implies that recombination occurs at presently unidentified radical sites, and part of the electronic energy is transferred along the polymer chain to the phenyl group.

Radiation resistance and optical properties of lead fluoride Cherenkov crystals

Optical properties of large size lead fluoride (PbF 2) crystals of three different manufacturers and their degradation caused by 60Co γ-radiation have been investigated. Transmission losses have been systematically studied at absorbed energy doses between 0.1 and 7 kGy. Several radiation induced absorption bands have been observed. Optical bleaching with light of wavelengths ≳365 nm has been found very effective to restore the original characteristics even after repeated irradiations. This observation together with the high density and the ultraviolet extended transmission make PbF 2 an excellent choice for high rate and high resolution e.m. calorimetry.

660 and 760 nm bands evidenced in a low-OH low-Cl fibre by Gaussian deconvolution of radiation-induced loss spectra

Gaussian deconvolution of the radiation induced absorption band, previously observed around 660 nm, in a low OH low Cl pure silica core aluminium coated fibre, is used to show that the band is due to the superposition of two absorption bands at 660 and 760 nm. These bands are endemic to the new type of silica used for the fibre fabrication.

Colour Centres in LiF for Measurement of Absorbed Doses up to 100 MGy

Radiation damage studies on electronic components, polymers, composites, ceramics, metals, and insulating materials exposed to very large doses of ionising photons and particles (i.e. electrons, protons, and neutrons) require reproducible and accurate passive dosimetry. Absorbed doses in the range 10 2 -2 x 10 6 Gy have previously been measured with optical quality, undoped LiF by spectrophotometry of radiation-induced absorption bands of stable colour centres. These include the relatively stable F centre (247 nm), M centre (443 nm), R 1 centre (315 nm), R 2 centre (374 nm), and N 2 centre (547 nm). It is now possible to extend this range to 10 8 Gy by spectrophotometry of the N 1 centre (517 nm) and an 'X' centre (785 nm). These dosemeters can be used from cryogenic (4 K) to relatively high radiation temperatures (up to 500 K). The radiation-induced coloration can be bleached for re-use of the dosemeters by a two hour thermal annealing treatment at 830 K.

Long-Term Changes in the Radiation-Induced Optical Absorption Bands of LiF:Mg,Ti

Optical absorption spectroscopy plays an exceptional role in the identification of charge traps responsible for the different TL peaks of the TL phosphors, Experiments carried out under different conditions, e.g. with different types of ionising radiation and/or different dose rates, applying different annealing procedures and/or different storage times after the irradiation, may lead to contradictory results. Therefore, a systematic investigation was conducted of the build-up and decay characteristics of the optical absorption bands of different LiF:Mg,Ti single crystal samples. Important changes were found in the long (hours and days) time scale. For example, the 350 nm optical absorption increases continuously in the Mg free LiF:Ti sample, while the intensities of the 310nm and 380nm absorption bands decrease in the sample containing Mg even several hours after irradiation with 5μs, 4 MeV linear accelerator electron pulses. These experimental results may have serious consequences for the interpretation of optical absorption data and for the understanding of defect processes and energy storage mechanisms of thermoluminescence in LiF:Mg,Ti.