Nuclear Site Remediation - American Chemical Society

Chapter 20

The Nature of the Self-Luminescence Observed from Borosilicate Glass Doped with Es Downloaded by STANFORD UNIV GREEN LIBR on September 22, 2012 | http://pubs.acs.org Publication Date: November 29, 2000 | doi: 10.1021/bk-2001-0778.ch020

Zerihun Assefa and Richard G. Haire Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Bethel Valley Road, MS 6375, Oak Ridge, TN 37831

We have been conducting spectroscopic investigations of transuranium elements in silicate matrices. Glasses doped with Es show a blue self-luminescence (SL) band having a maximum at ~455 nm, and minor bands at 650 and 730 nm. With time, the intensity of the 455 nm band decreases, whereas the 650 and 730 nm bands increase. These S L bands are believed to arise from defect centers that are produced by alpha-particles and recoiling atoms. The band at 455 nm is assigned to originate from oxygen defect centers within the Si-O-Si network, whereas the bands at 650 and 730 nm are believed to originate from non-bonding oxygen hole centers (NBOHC), and Si-micro cluster sites, respectively. The latter two bands (650 and 730 nm) were also observed in the photo-luminescence(PL) spectra obtained with a 488 nm excitation. However, the P L profile showed a dependence on the power of the excitation flux. The 730 nm band dominates at low power, while at higher levels this band's intensity was "quenched" relative to the 650 nm emission. A new band also emerges, concomitantly, around 770 nm on

© 2001 American Chemical Society

In Nuclear Site Remediation; Eller, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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freshly prepared samples and becomes dominant at laser powers greater than 1W. Details of these investigations are discussed in this paper.

Downloaded by STANFORD UNIV GREEN LIBR on September 22, 2012 | http://pubs.acs.org Publication Date: November 29, 2000 | doi: 10.1021/bk-2001-0778.ch020

Introduction The effects of radiation in solid waste materials are complex, and a fundamental understanding is often limited. A s radioactive decay in a solid matrix can alter physical and chemical behaviors, fundamental understanding of its effects is an essential factor in evaluating long term effectiveness of the immobilization matrices, as well as radionuclides release to the biosphere (7). Borosilicate glasses are one of the primary materials being considered for long term isolation of radio nuclides (7,2 ). Studies involving radiation effects in actinide-doped glasses have been conducted to ascertain volume and micro-structural changes, as well as radiolytic decomposition effects (3 -6 ). Radiation can affect a solid matrix by several processes, and may include the transfer of energy to electrons and/or nuclei. Deposition of nuclear energy into electronic processes results primarily in ionization and/or electronic excitation, where as energy transferred to atomic nuclei, involves elastic collision and atomic displacements (7 ). A n alpha decay process normally results in the formation of a high-energy ( 4 - 6 M e V ) α-particle and a recoiling nucleus having an energy of -100 K e V ( 2 ). It is known that, while the alpha particle loses most of its energy by ionization processes, the recoiling nucleus produces mainly atomic displace ments ( 7 - 9 ) . On the other hand, beta decay produces very little atomic displace ment directly, although localized electronic excitations generated by ionization processes may produce selective atomic displacements (10). Thus, the main effect of beta radiation is ionization. The overall consequences of these and other radiation effects on a given matrix are complex and require thorough investiga tions. Many studies involving radiation effects have been conducted by using external radiation sources. For example, electron irradiation studies on thin specimens or bulk gamma irradiations using Co-60 sources have been used to simulate beta radiation effects (77 - 75 ). The long term consequences of both displacement and ionization damage can be best simulated using short-lived actinide dopants (7). In this study we have conducted spectroscopic investigations of borosilicate glasses doped with einsteinium, with one goal being to monitor the consequences of the high-energy alpha radiation on the glass matrix. Es-253, with an alpha decay of 6.6 M e V and a short half-life of 20.5 days, is convenient to simulate both displacement and ionization effects in boro-silicate matrices. We


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have applied spectroscopic techniques to follow these effects. Self-luminescence, as defined previously (16-17), refers to luminescence which is excited under the effect of alpha and/or beta decay of actinide isotope under study.


Experimental

Materials. The glass matrix used in this study had the following composition: S i 0 (53 %); B 0 (19%); N a p (25%); CaO (3%). A glass stock was prepared by grinding thoroughly the components and heating until dissolution was complete ( 850 °C ). The molten material was slowly cooled to room temperature to provide a transparent glass. Einsteinium was incorporated into this base matrix by remelting the glass together with the oxide (-100 μg in 10 mg glass) compound in a small platinum heater of local design. Incorporation of the Es in the matrix provided a clear glass initially but it darkened with time. 2

2

3

Spectroscopy Both self-emission and absorption spectra were collected using an Instrument SA ' s optical system, which consisted of a model 1000 M monochromator equipped with C C D , P M T and IR detectors. For the absorption studies a 400 W Xe lamp was used as the light source. The sample, doubly contained in quartz capillary tubes, was placed under a microscope objective and analyzed with the light from the X e lamp delivered via a fiber optic setup. The transmitted light, collected with a second fiber, was directed into the monochromator. The self emitted light was collected with fiber optics and directed to the monochromator. The photo-luminescence studies were conducted using argon ion lasers ( Models 306 and 90, Coherent) as the excitation sources and a double meter-spectrophotometer ( Raman Model HG.2S, Jobin-Yvon ) using the procedure described previously (75 ). Speetramax for Windows software (Instruments SA) was used for data acquisition, while Grams 32 software (Galactic Industries) was used for data analysis.


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Results and Discussion When Es is doped in borosilicate matrix, a clear glass is obtained initially, although the sample darkens within a few hours. The darkened glass is easily "bleached" and a clear glass again attained, when visible light is focused on the glass for several minutes. For example, the darkening is removed when the glass is radiated with the 514 nm argon-line at 10 mW of laser power for about 10 minutes. A l l of the spectra discussed here were collected on clear glasses following radiating with the 514 nm laser line for ~ 20 minutes. A s a secondary issue, the oxidation state of Es in the glass matrix was determined to be trivalent. Shown in Figure 1 is the absorption spectrum, where the bands at -485, 492, 555, and 598 are assignable (19) to f - f transitions in Es(III). Evidence for Es(II) has not been obtained in this study.

ι

1

500

550

Wavelength (nm)

Figure 1. Absorption spectrum of Es in a borosilicate glass. The absorption bands correspond to Es(III)

Self-luminescence spectra A feature associated with these Es doped glasses is the observance of a blue self emission visible in the dark. The self-luminescence (SL) spectra of such a sample are shown in Figures 2 and 3. Initially a broad band ( F W H M - 66 nm) that maximizes at -455 nm dominates the spectrum with minor features at 544, 650, and 730 nm. A s depicted in Figure 3, the SL spectra collected from older samples indicate a significant reduction in the intensity at 455 nm. A slight blue shift to



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\

400

1

S

1

500

600

700

1

1

800

Wavelength (nm) Figure 2. Radiation induced luminescence from a borosilicate glass doped with Es. Spectra collectedfollowing a)3; b) 15 days of sample preparation. Expansions of the low-energy regions are shown in Figures a* and b\

ι

1

I

1

1

300

400

500

600

700

r

800

Wavelength (nm)

Figure 3. Radiation induced luminescencefrom borosilicate glass doped with Es. Spectra collected after, a) 50; b) 90 days from preparation.


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-445 nm is also evident from older samples. In contrast the intensities of the lowenergy bands at 650 and 730 nm increases, concomitantly, while there was no change in the intensity of the 544 nm feature. A s shown in Figure 3, the ratio of the 730 to 650 nm band increases and the former band is dominant.


The blue-emission band. As shown in previous studies (20 - 22% one of the consequences of radiation in silica materials is the creation of defect centers. Some of the known effects include creation of oxygen vacancies, free O-atoms, and free electron and hole centers. Basic intrinsic defects found in silica matrices include the Ε ' , center; nonbridging oxygen-hole center (NBOHC), = S i - 0 ; peroxy radical, s=si0-O, and oxygen deficient centers (20). The Ε ' , center is one of the best studied oxygenvacancy-related defects in silica matrix. It has been observed in all forms of silica including crystalline and glassy materials. As a radiation induced defect, the Ε ' , center was first studied in 1956 (23 ) and it's importance for detailed analysis of ageing effects and radiation degradations have been demonstrated since then (24 -26). A characteristic feature of the Ε ' , center is that the silicon atom is bonded to just three oxygens and contains an unpaired spin (27) in a dangling tetrahedral orbital, =Siv Most commonly, E\ defects are induced in silica glasses by energetic radiations including neutron and/or γ radiation. Being one of the few optically active sites, the E'j center is characterized by absorption bands in the far U V region and emission bands in the U V and blue spectral regions. Numerous spectroscopic studies on irradiated silica glasses have indicated that the E \ center displays a U V emission band at 4.4 eV and a blue luminescence at 2.7 eV (-450 nm) when excited with a 7.6 eV (163 nm) photon (28). It is important to note that Es-doped glasses exhibit a strong self-luminescence band at 455 nm (2.7 eV) indicating that the alpha radiation is responsible for the displacement of an O" within the Si-O-Si matrix and the creation of the Ε' center. In addition to defect formation, the alpha radiation from Es is also responsible for the electronic excitation of the site and, thus, the observance of the 2.7 eV self emission of the matrix. x

Emission bands in the red-spectral region In addition to the blue emission band at 455 nm, the SL spectrum of Es-doped glasses exhibit major features in the red spectral region. Emission bands in the red region have been reported in several silica glasses exposed to x-ray, gamma,



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and/or neutron radiations (29, 30). The reported emission energies vary slightly from sample to sample, but generally lie within the 1.7 - 2.2 eV range. Although, a luminescence band around 1.9 eV (650 nm) is thought to signify intrinsic defects in silica glasses (57), the exact origin of this emission has been controversial for some time. Cumulative literature data indicate that two major defect centers could provide emission in the 1.7 - 2.2 eV region. The N B O H C is one of the defect sites believed to provide emission bands in the 1.8 - 2.0 eV range (37 - 55), although in few instances these bands have been assigned to an interstitial ozone molecule. The non-bridging center is a localized defect consisting half of the permanently broken S i - 0 bonds. Such defects are formed as a result of permanent bond breaking under ionizing radiation, such as gamma, neutron, and/or beta. Although, the recoiling nucleus accompanying alpha decay is mainly involved in atomic displacements (2,7,8,9), and hence in the creation of vacancies, the alpha particle loses most of its energy by ionization processes. Due to the high alpha activity of einsteinium, creation of non-bridging oxygen defect sites is, thus, expected. Other luminescent centers are also known to provide emission in the red spectral region. One such center involves a Si-micro cluster defect site created due to a high oxygen deficiency in the matrix. For example, a recent study (34, 35) indicated that a silica matrix exposed to a high-dose of γ-irradiation exhibits photoluminescence at -1.8 eV(~690 nm) when excited with a 496 nm radiation. The band's intensity was reported to show an increase, albeit with a slight red-shift, when the partial pressure of 0 decreases. A large dose of γ-irradiation produces a high concentration of oxygen-deficient centers (55) that ultimately lead to microclustering of the S i atoms. Thus, Si-micro cluster centers are believed to be the source for red emission in silica glasses. Support for this conclusion has been derived from a recent Si-implantation study (36, 37), where samples annealed at 1100 °C (a temperature at which the Si precipitates) exhibited photo-luminescence at 1.7 eV. The band has been assigned to a defect site at the interface between crystalline Si-nanoparticles and S i 0 . Implantation of Si ions into silica glasses provided two photo-luminescence bands, at 2.0 and 1.7 eV, when excited with a 488 nm Ar-ion laser (38). Both emissions were thought to originate from Si-rich defect sites (38, 39). It is interesting to note that silicate glasses doped with Es also display two SL bands in the red spectral region. We have undertaken photo-luminescence (PL) studies in order to facilitate the assignments of the observed bands. 2

2

Photo-luminescence study The photo-luminescence spectra of the silicate matrix doped with Es are shown in Figure 4 . The spectra were recorded using the argon laser's 488 nm line as the


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excitation source. Two major P L bands are observed at 650 and 730 nm. The bands are situated at similar positions to that found in the S L spectrum shown in Figure 3.

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Figure 4. PL spectra of a borosilicate glass doped with Es. The 488 nm laser line was usedfor excitation. The spectra are recorded at laser powers of: a)100; b) 310; c) 600; d) 800; e) 1000 mW. The spectra are separated for clarity The overall spectral profile showed a dependence on the power of the excitation flux. As shown in Figure 4a, the 730 nm band dominates at low power, while at higher levels the 730 nm emission band is "quenched" relative to the 650 nm band. The overall dependence of the 730 nm band on laser power is depicted in Figure 5. Although, an initial increase is evident between 70 and 200 mW, the intensity of the 730 nm band decreases drastically as the laser power is increased further. Also, a new band emerges, concomitantly, around 770 nm and becomes dominant at laser powers greater than 1W. In contrast, the band at 650 nm (1.9 eV) remained linear with an excitation power of 70 to 500 mW, although saturation was evident above 500 mW ( Figure 6). The P L signal exhibited by the sample is much stronger than the S L signal. Hence, at the experimental conditions under which the P L data were collected (example, significantly reduced slit width), the SL signal simply contributed to the noise level. A s discussed above, the PL and SL spectra have similar profile in the low-energy spectral region suggesting that both photon and nuclear decay processes provided similar electronic excitations of the emitting sites.


337 1e + 6

-a-

i e

1e + 5 -d


Β

1e + 4 1000

100

Excitation power (m W )

Figure 5. A log-log plot showing the dependence ofthe 730 nm emission band on the laser power.

A s noted earlier, and shown in Figures 5 and 6, the dependence of the two emission bands on the laser power is complex. On one hand, the intensity of the 650 nm band increases linearly up to a 500 mW power and then reaches saturation. On the other hand, the intensity of the 730 nm band, although it slightly increases initially, is drastically "quenched" at higher powers. The spectral profile behaves as if two different species operate independently, suggesting that the two emission bands originate from different centers in the matrix. In terms of energy position, power dependence, and F W H M , the 650 nm band shows similar profile to that found in silica glasses studied previously (29 - 33). Thus, consistent with previous assignments, the 650 nm band is assigned to a N B O H C defect site. Moreover, comparison of the P L profile with time (not shown) indicated that emission from Es(III) also contributed in this spectral region. The details of this phenomenon will be published elsewhere (40). In contrast, the 730 nm band can not be assigned to the non-bridging oxygen centers, since the P L data suggested different origins for the two emissions. One center likely responsible for this emission is a Si-micro cluster unit. Emission in the region of 1.7 eV has previously been reported (36 ) on silica glasses irradiated with very high gamma radiation (-10 Gy), and the band has been attributed to highly oxygen deficient centers. Similarly, Es with its highly energetic (6.6 M e V )


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alpha particle should also facilitate the creation of oxygen deficient centers that can ultimately lead to Si-micro clustering in the silica matrix. We suggest that the 730

Figure 6. A log-log curve showing the dependence of the 650 nm ( 1.9 eV) emission band on laser power. Note that saturation occurs after -800 mWof laser power. nm band (1.7 eV) originates from centers involving Si-micro clustering. The unusual power dependence exhibited by the 770 nm emission ( Figure 3) makes assignment of this band difficult. The 770 nm emission, which is absent both in the S L and low power P L spectra, appears only under high power excitation. A s is the case in a photon avalanche phenomenon, a threshold power appears necessary to observe this emission. The appearance and increase in the intensity of the 770 nm band is also accompanied by a concomitant decrease in the 730 nm emission band. Moreover, a non-linear dependence has been noted between the emission intensity and the excitation flux. These observations are indicative of energy transfer between the Si-micro cluster units and the center responsible for the 770 nm emission. However, the exact nature of the defect center responsible for this emission is not fully established. Although numerous studies have been conducted on radiation-induced defects, to the best of our knowledge, the spectral profile exhibited by the 770 nm band is observed for the first time, and may be unique to alpha radiation effects. Further study on this system is underway in order to better understand the phenomenon.


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Summary We have conducted fundamental optical studies on defects formed by alpha radiation from within boro-silicate glasses doped with einsteinium. It is thought that these studies would help in establishing the influence of defects on the physical and chemical durability of the matrix. The self-luminescence profile of the glass shows bands at 455,650 and 730 nm, indicating that both the creation of defect centers and their electronic excitations are accomplished by the alpha radiation produced during the decay of Es-253. Based on previous studies, the 455 nm band is assigned to the well known E\ defect center. The bands at 650 and 730 nm are believed to originate from non-bonding oxygen hole centers, and Si-micro cluster sites, respectively. The photo-luminescence data showed similar spectral profile indicating that both photon and nuclear processes provided similar electronic excitations to the sites responsible for the emissions. The overall P L profile, however, showed a dependence on the power of the excitation flux. The 730 nm band dominates at low power, while at higher levels the band's intensity is "quenched"relative to the 650 nm emission. Also, a new band emerges concomitantly around 770 nm and becomes dominant at laser powers greater than 1W. Presence of excited state energy transfer between the Si-micro cluster sites and the unidentified defect centers has been inferred from the P L data.

Acknowledgment This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U S Department of Energy, under contract D E - A C 0 5 960R22464 with Lockheed Martin Energy Research Corporation. The authors are indebted for the use of Es-253 to the Office of B E S , distributed through the transplutonium element production program at the Oak Ridge National Laboratory.

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Nuclear Site Remediation - American Chemical Society

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