Electron spin resonance studies of defect centers induced in a high-level nuclear waste glass...

Electron spin resonance studies of defect centers induced in ahigh-level nuclear waste glass simulant by gamma-irradiation

and ion-implantation

David L. Griscom a,*, Celia I. Merzbacher a, Robert A. Weeks b, Ray A. Zuhr c

a Optical Sciences Division, Naval Research Laboratory, Code 5675, 4555 Overlook Ave., S.W., Washington, DC 20375, USAb 331 Southshore Drive, Greenback, TN 37742, USA

c Solid State Division, Oak Ridge National Laboratory, Knoxville, TN 37831, USA

Received 24 February 1999; received in revised form 16 July 1999

Abstract

Samples of an iron-free soda-lime boro-aluminosilicate high-level nuclear waste (HLW) glass simulant were im-

plantated with 160-keV protons or He� ions or irradiated by 60Co c-rays. Electron spin resonance (ESR) spectra were

recorded before and after implantation or irradiation and after postirradiation isochronal anneals to 550°C. The most

numerous paramagnetic states in the unannealed samples following c-irradiation were boron±oxygen hole centers and

Ti3� trapped-electron centers. The most numerous paramagnetic states in the as-implanted samples were peroxy rad-

icals (PORs). Annealing of the c-irradiated samples at temperatures >300°C caused recombination of the trapped

electrons and holes, revealing an underlying POR spectrum which annealed in stages at �400°C and 550°C, similar to

the annealing of PORs in the He�-implanted sample. For equivalent depositions of ionizing energy, the initial POR

concentration in the He�-implanted sample was �100 times that in the c-irradiated sample and at least 10 times larger

than the (unmeasurable) number of PORs in the proton-implanted sample. Since TRIM calculations indicate that 160-

keV protons deliver twice the ionizing dose of He� ions of the same energy but displace only 1/10 as many atoms, it is

evident that the PORs induced by He� implantation result from displacements of oxygens in elastic collision cascades.

The absence of trapped electrons and holes in the implanted samples is ascribed to annealing during irradiation as-

sociated with the fraction of the implantation energy deposited as heat. One of the fundamental defects to be expected

in HLW glasses containing a-particle emitters is therefore the POR. Several distinct families of radiation-induced PORs

are unambiguously identi®ed in our samples by computer line-shape simulations constrained by the K�anzig±Cohen

g-value formulae. Possible reasons for the non-observation of PORs in similarly-irradiated glasses containing several

wt% Fe2O3 are proposed. Ó 1999 Elsevier Science B.V. All rights reserved.

PACS: 28.41.Kw; 61.80.-x; 61.43.Fs; 76.30.-v

1. Introduction

In the United States alone there are 100 mil-lion gallons of high-level nuclear wastes (HLWs)

in various chemical forms awaiting eventual dis-posal in geologic repositories [1]. For safety inhandling and transport from their present un-derground storage tanks to their ®nal burial sites,much of the HLWs are being immobilized byvitri®cation [1±3]. A further virtue of HLW vit-ri®cation is the fact that the glass may serve as

Journal of Non-Crystalline Solids 258 (1999) 34±47

www.elsevier.com/locate/jnoncrysol

* Corresponding author. Tel.: +1-202 404 7087; fax: +1-202

767 5792; e-mail: [email protected]

0022-3093/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 5 5 7 - 8

an additional, non-geologic barrier to the dis-persal of these radio-toxins into the environment.For this reason, one of the criteria for selectingHLW glass compositions has been chemical du-rability against attack by ground water. Whilethe e�ects of radiation on chemical durabilityhave therefore been studied extensively [1,4,5],little consideration has been given to the possi-bility that self-irradiation of HLW glasses maylead to modes of chemical decomposition whichrender them unstable even in the absence of ex-posure to ground water. The worst-case threatwould occur if the HLW glasses were to respondto irradiation in ways analogous to rock salt(NaCl). It has long been known that alkali ha-lides irradiated to su�ciently high doses decom-pose into alkali±metal colloids and interstitialhalogen molecules [6,7]. However, more recentlyit has been discovered that some rock salts alsodevelop voids under high-dose irradiation [8] andthat heating of these materials in the range�200°C can cause these voids to collapse, trig-gering shock waves capable of detonating explo-sive recombination of the radiolyticallyaccumulated elemental sodium and chlorinemolecules [9].

In the 1950±1970s, radiation-induced defectcenters in alkali halides were studied in depth byelectron spin resonance (ESR) spectrometry. Itwas found for example that the elementary halidevacancy centers (the F centers) eventually co-alesce into di-vacancies (M centers), tri-vacancies(R centers), and eventually into macro defectsincluding alkali metal colloids [10,11]. Similarly,the so-called VK centers (Xÿ2 molecularions, where X is a halogen) were recognizedas self-trapped holes, and the H centers (also ofXÿ2 character) were identi®ed as interstitial halo-gen atoms crowded onto normal Xÿ ion sites[10,12]. From the thermal anneal kinetics of thesehalogen trapped-hole centers, the eventual for-mation of (non-paramagnetic) X2 molecules wasinferred. In the present investigation, the ESRspectra of a simpli®ed HLW glass compositionwere recorded following c-irradiation or implan-tation by H� or He� ions, with the object oftracking the ®rst stages of radiolytic decomposi-tion in a manner similar to that which had been

earlier achieved in the cases of the crystallinealkali halides.

2. Experimental measurements and procedures

The simpli®ed HLW glass simulant (CSG) usedin the present study had the composition (mol%)59.3SiO±7.3Al2O3±7.7B2O3±18.8Na2O±6.8CaO; itwas melted in air at 1150°C and 1225°C for a totalof 14 h and annealed near Tg (�500°C) for 15 h. Aboron-free glass sample of composition 49.1SiO2±9.0Al2O3±21.3MgO±18.8CaO±1.7(Na2O + K2O)was used for comparison purposes. Platelets ofdimensions �1� 2:5� 20 mm3 were sawed fromthe boule, polished, and subjected to 160-keVproton or He� implantation to ¯uences of6:0� 1016 and/or 1:5� 1017 cmÿ2. Replicateplatelets were exposed to 60Co c-irradiation to adose of 1:7� 106 Gy (Si).

Two small samples of 65-million-year-oldnatural glass of approximate composi-tions 68SiO2±10Al2O3±4MgO±8CaO±5FeO±4Na2O±1K2O [13,14] were available to search forpossible e�ects of a decays of naturally occurringradionuclides.

First-derivative ESR spectra were recorded ei-ther at room temperature or 77 K on a BrukerER 200 instrument operating at X-band fre-quencies (m� 9.4 GHz) with 100-kHz magnetic®eld modulation. Ten-minute isochronal annealsin 50°C increments were carried out by movingthe samples, contained in fused quartz sampletubes, to an external furnace. The samples wereexposed to the laboratory atmosphere duringannealing. Spin concentrations were determinedby double numerical integration of representativederivative spectra and comparison to the areaunder the absorption curve of a Varian `strongpitch' sample estimated to contain 3 ´ 1015 spinsper linear centimeter of sample tube (accuracy�25%). The accuracy of the numerical integra-tions was of the order of �5% for the strongersignals in the c-irradiated samples. In the case ofthe ion-implanted samples these errors were�30% due to low signal-to-noise ratios. As all ofthe ESR signals decreased in strength with in-

D.L. Griscom et al. / Journal of Non-Crystalline Solids 258 (1999) 34±47 35

creasing anneal temperature, the accuracies of thenumerical integrations also decreased. Therefore,selected spectral amplitudes became the measuredquantities in the isochronal anneal experiments;these amplitudes were then scaled to the absolutespin calibrations determined by numericalintegration for spectra recorded at lower annealtemperatures.

Measurements of g-values were accomplishedby reference to a simultaneously recorded signalof E0c centers in a chip of c-irradiated Suprasil 2fused silica for which the average principal-axisg-values are known to within an estimated accu-racy of �0.0001 (g1 � 2:0018, g2 � 2:0006, g3 �2:0003) [15]. (Here, the g-value is de®ned by therelation hm � gbH , where h is Planck's constant, mthe spectrometer frequency, b the Bohr magne-ton, and H is the magnitude of the laboratoryapplied magnetic ®eld at resonance.) Forresonances as intense and as narrow as the E0

signal (X-band linewidth �0.25 mT), the precis-ions of the g-value measurements made in thisway can approach �0.0002; otherwise, the errorlimits increase with decreasing signal-to-noise ra-tios and increasing widths of the spectra beingmeasured.

3. Experimental results

Fig. 1 illustrates the ESR spectra of boththe c-irradiated and He�-implanted (¯uence1:5� 1017 cmÿ2� samples of glass CSG (dottedand continuous curves, respectively): (a) as-irra-diated and (b) following a 10-min anneal at300°C. The ESR signals of the proton-implantedCSG samples (not shown) exhibited only asymmetric component with a g-value of 2.0026 �0.0002 and a room-temperature linewidth of0.13 mT. Isochronal anneal results are presentedin Fig. 2, where for purposes of spin concentrationcalibration the He�-implanted volume was calcu-lated as 1 lm multiplied by the area of thesample platelet. This approximation re¯ects therange of energy deposition by the 160-keV He�

ions from TRIM89 calculations [16] as shown inFig. 3.

Fig. 1. X-band (9.5 GHz) ESR spectra of HLW glass simulant

CSG following c-irradiation (dotted curves) and 160-keV He�

ion implantation (thin solid curves): (a) before and (b) after 10-

min anneals to 300°C. Spectra recorded at 77 K.

Fig. 2. Isochronal anneal data (10 min at temperature) for

defect centers induced in borosilicate glass CSG (circles,

squares, and triangles) and in the boron-free aluminosilicate

glass of this study (crosses, data shifted downward by a factor

of 3 for clarity). Filled symbols and crosses: c-irradiated sam-

ples. Open triangles: He�-implanted sample. Data recorded at

room temperature. Data points are connected by straight lines

as an aid to the eye.

36 D.L. Griscom et al. / Journal of Non-Crystalline Solids 258 (1999) 34±47

4. Discussion

4.1. The g� 2.0026 singlet

Symmetric singlet lines characterized by ap-proximately Lorentzian shapes, DHpp � 0:1±0.3mT at 300 K, and g � 2:0026� 0:0002 were re-corded in virtually all ion-implanted samples butwere absent in the c-irradiated ones. Similar singletspectra (with widths ranging up to �0.6 mT) havebeen reported previously [17±19] in a number ofmulticomponent silicate, phosphate, and ¯uoro-aluminate glasses subjected to implantation by B�,N�, O�, Ar�, Mn�, Cu�, and Pb� ions. Possibleorigins of such singlets continue to be debated inthe literature. It has been considered in Ref. [19],for example, that the responsible defects may be:(i) oxygen related, since they were enhanced in O�-implanted samples, (ii) sodium related, sincesinglet lines were recorded in three diverse high-sodium glasses but were absent in similarly-im-planted sodium-free glasses, or (iii) related tocarbon impurities, since large amounts of carbonwere detected on the surfaces of implantedsamples.

From the standpoint of HLW glass stability,the possibility of radiation-induced sodium-metalcolloid formation is a serious concern. Accord-ingly, we studied the temperature dependence ofthe linewidth of the g � 2:0026 singlet in a He�-implanted CSG glass; (the selected sample was onesubjected to a ¯uence of 6� 1016 cmÿ2, since theg � 2:0026 line intensity was the largest in thiscase). As for the case of Na-metal colloids in so-dium azide [20], the linewidth proved to be an in-creasing function of measurement temperature(varying from 0.07 to 0.17 mT in the range 100±450 K) and de®ned a curve which was slightlyconcave upwards. However, the previously ob-served [20] discontinuity at the melting point ofsodium (370.5 K) did not emerge from the statis-tical scatter in our data points. Moreover, we wereable to reduce the intensity of this line by an orderof magnitude by vigorously cleaning the implantedsurface with acetone. This observation, coupledwith the fact that the measured g-value di�erssubstantially from that of Na metal (2.0017 [20])but is similar to that of our pitch standard sample(2.0028 [21]), leads us to ascribe the g � 2:0026singlet in our implanted samples to carbon con-tamination.

4.2. Trapped-carrier centers

The spectra identi®ed in Fig. 1(a) as arisingfrom boron±oxygen hole centers (BOHCs) andTi3� are well known from the literature [22±25]. Infact, the presently measured g-values for the Ti3�

spectrum (gk � 1:938� 0:005, g? � 1:971� 0:005)are identical within experimental accuracy withthose previously reported for the so-called T2

center in Ti-doped alkali diborate glasses [23],con®rming the present signal to arise from impu-rity Ti4� ions which have trapped electrons.BOHC spectra virtually identical to that ofFig. 1(a) have recently been reported in other ir-radiated borosilicate HLW glass compositions andhave been accurately computer simulated [25].Therefore, no further spectral analyses of theaforementioned species are carried out here.However, guided by previous simulations, we inferthe presence within the dotted spectrum of Fig. 1(a)of a weaker component due to less numerous

Fig. 3. Electronic (ionization) and nuclear (collisional) energy

deposition by 160-keV He� ions as a function of implantation

depth in SiO2 from TRIM89 calculations.


silicon-related non-bridging-oxygen hole centers(HC1 type [25±27]) which underlie the BOHCresonance and thermally anneal at approximatelythe same temperature. The actual isochronal an-neal properties of this underlying component maybe similar to those shown in Fig. 2 for the HC1-type centers �g1 � 2:0029; g2 � 2:0084; g3 �2:027� in our c-irradiated boron-free aluminosili-cate glass.

As described in Section 4.1, the sharp singlet atg� 2.0026 observed in Fig. 1 is attributed to car-bon pitch deposited on the samples during theimplantation process. Thus accepting this singletas being an artifact, we were unable to observe anyESR signals unambiguously arising from 160-keVproton irradiation to a ¯uence of 1:5� 1017 cmÿ2.Taking into account the signal-to-noise ratios, thismeans that the numbers of induced centers in theproton-implanted glasses were less by factorsP 10 than was the case in samples implanted tothe same ¯uence by He� ions of the same energy.This result implicates elastic collisions as the originof most of the paramagnetic centers induced in theion-implanted samples, since the TRIM calcula-tions indicate that a 160-keV He� ion would create9.75 times more collisional displacements (in SiO2)than would a proton of the same energy, whereasthe proton would cause 1.8 times more ionizationsthan the He�.

Notable in Fig. 2 is the fact that the BOHCs,NBOHCs, and Ti3� are each diminished in numberby over two orders of magnitude by the cumulativee�ects of 10-min anneals to 300°C. As the BOHCand NBOHC are assumed to be trapped-hole de-fects and the Ti3� a trapped-electron center, wesuppose that this initial anneal stage is due to therelease of electrons or holes (or both) from theirtraps, allowing their di�usion and recombinationwith carriers of the opposite sign. Given that thenumbers of trapped electrons and holes must beequal, it is inferred from Fig. 2 that in the presentcase the majority of the electron traps are dia-magnetic or have otherwise escaped observation.Several di�cult-to-observe intrinsic trapped-elec-tron centers in irradiated borate and silicateglasses have been discussed [24,28,29]. Among thepossible extrinsic electron-trapping sites are im-purity Ti4� (evidently present in CSG) and Fe3�.

An Fe3� signal at g � 4:3 was also observed in theCSG glass, but its small amplitude (�100 timesweaker than the dotted spectrum of Fig. 1(a)) wasnot monitored as a function of irradiation orannealing. However, ESR evidence of electrontrapping on impurity Fe3� in 2.5-MeV-electron-irradiated HLW glass simulants has been reportedrecently [25].

A second feature noted in Figs. 1 and 2, is theabsence of any trapped-carrier centers in the un-annealed He�-implanted sample, despite the factthat the ionizing dose delivered by the helium ionswas �2.5 times larger than the c-ray dose presentlyemployed. 1 We suggest that all of the trapped-charge centers are annealed during irradiation as aconsequence of local heating (`thermal spikes'[30,31]) resulting from thermalization of a largefraction of the kinetic energy of the implantedions.

4.3. Peroxy radicals and Oÿ2 molecular ions

The reasons for identifying certain spectralfeatures in Figs. 1 and 2 as arising from peroxyradicals (PORs) will now be developed, and theimplications of their presence as the most ther-mally stable radiation-induced defects will beconsidered.

From inspection of the line shapes of Fig. 1(b)and comparison to various results in the literature(e.g., [32±35]), the defects stable above �300°C inthe present samples were suspected from the be-ginning to arise from PORs or interstitial Oÿ2ions. (The terminology `POR' is conventionallyused to denote Oÿ2 ions which are covalentlybonded into a solid.) Accordingly, computer line-shape simulations were carried out using as a

1 The ionizing doses imparted by the implanted ions were

calculated as follows: The TRIM calculations have indicated

that, for each 160-keV He�(H�) ion implanted into SiO2, 43.6

(94.8) eV is dissipated into ionizations. Assuming that similar

total energy losses and energy-loss pro®les (cf., Fig. 3 for the

He� pro®le in SiO2) pertain to the complex borosilicate glass of

this study and taking its density to be 2.3 g/cm3 (approximately

the same as that of silica), the ionizing doses were calculated

using the conversion, 1 Gy� 1 J/kg� 6:24� 1015 eV/g.


constraint the theoretical expressions derived byK�anzig and Cohen [36] for the g-values of Oÿ2ions in solids:

g1 � ge

D2

k2 � D2

� �1=2

ÿ kE

"ÿ k2

k2 � D2

� �1=2

ÿ D2

k2 � D2

� �1=2

� 1

#; �1�

g2 � ge

D2

k2 � D2

� �1=2

ÿ kE

k2

k2 � D2

� �1=2"

ÿ D2

k2 � D2

� �1=2

ÿ 1

#; �2�

g3 � ge � 2k2

k2 � D2

� �1=2

l; �3�

where ge is the free-electron g-value (2.00232), kthe spin-orbit coupling constant for the Oÿ ion, Dthe splitting of the 2p pg antibonding level (seeFig. 4), E the separation of the foregoing levelfrom the 2p rg bonding level, and l is a correctionto the orbital angular momentum (l � 1 for thefree molecular ion).

The amplitudes of the ESR signals of all de-tected paramagnetic states in the c-irradiated

glasses were substantially larger than those in theion-implanted samples in spite of the 2.5-times-greater ionizing dose delivered by the energeticions; (this di�erence is because the implantationa�ected only �1/1000 of the volume of eachsample whereas the c-rays a�ected the entire vol-ume). For this reason, most line-shape simulationswere carried out on spectra recorded for the c-ir-radiated samples. Fig. 5 shows that the line shapeof the suspected POR spectrum changes as theanneal temperature is raised from 300°C to 450°C.The g-value distributions shown in the insets toFig. 5(a) and (b) are taken to be representative oftwo di�erent POR sites, denoted `POR(Lo-T)' and`POR(Hi-T)', which dominate following annealsat temperatures of 300°C and 450°C, respectively.The line-shape simulations of Fig. 6(a) and (b)were accomplished as linear combinations ofcomponent spectra of POR(Lo-T) and POR(Hi-T)in the ratios 2:1 and 1:3, respectively. The fol-lowing ®tting procedure was used: First, distri-butions in g3 values were selected to have peakpositions and widths which, to the eye, appearedto correspond well 2 with the shapes of the low-®eld shoulders of the experimental derivativespectra. To inject an element of physical intuition,the detailed shape of every trial g3 distributionwas constrained to derive from a Gaussian dis-tribution in the splitting D (see Fig. 4) accordingto [37,38]

pg�g3� � pD�D� � jJg�D�j; �4�

where pD�D� is a Gaussian distribution of D values,pg(g3) the derived (skew-symmetric) distribution ofg3 values, and Jg�D� � �og3=oD�ÿ1

is the Jacobianof the transformation D! g3 calculated from theK�anzig±Cohen expression for g3 given above. Anotable property of this ®tting procedure is that

Fig. 4. Energy-level diagram and electronic occupation scheme

for the Oÿ2 molecular ion bound in a solid. Lifting of the p-level

degeneracies is due to crystal-®eld interactions.

2 The g3 distributions inset into Fig. 5 are only approximately

registered with the experimental spectra, since both are

displayed with linear x axes whereas the values of H and g

bear a reciprocal interrelationship. However, the correspon-

dence is quite close enough to visually discern the relationships

between the g distributions and the experimental line shape.


once a distribution of g3 values is selected, thedistributions of g1 and g2 are automatically speci-®ed, 3 i.e., there are no further adjustable

parameters. 4 Nevertheless, the satisfactory ®ts ofFig. 6 resulted from this process, even though theline shapes in the regions of g1 and g2 of thespectra for Tanneal � 300°C and 450°C are qualita-tively di�erent!

Fig. 5. X-Band ESR spectra of c-irradiated glass CSG following 10-min anneals at: (a) 300°C and (b) 450°C. Inset g-value distri-

butions were developed to account for these spectra according to a scheme described in the text. Spectra recorded at 77 K.

3 Unlike the case for the g3 distributions plotted in Fig. 5, the

lines joining the weighting factors (circles) for g1 and g2 do not

de®ne true probability distributions, since these `data points'

are not separated by uniform increments in g. Nevertheless,

physically meaningful simulations can be accomplished by

summing the powder patterns calculated with these rigorously

correlated sets of g-values, weighted according to the g3

distributions generated in the manner described.

4 Technically, the parameters k/E and l in the K�anzig±Cohen

equations are adjustable over a small range. The present

simulations were accomplished by employing l � 1:0 and

k=E � 0:0031. Assuming the optical value of E for Oÿ2 ions in

NaCl (5.08 eV [39]) to pertain also to the present PORs, our

choice of k/E would imply that k � 0:016 eV ± an energy close

to the lesser of two literature estimates of the spin-orbit

coupling constant of Oÿ, 0.014 [32] and 0.0246 eV [40].


In Fig. 7(a) the spectrum of the unannealedHe�-implanted glass CSG has been computersimulated using the g3 distribution of Fig. 8(a) andcorresponding g1 and g2 distributions derived bymeans of the same parameterization of theK�anzig±Cohen equations as employed above. Theother parts of Fig. 7 show the spectra of (b) a c-irradiated 44K2O±66SiO2 glass following anneal-ing at 300°C and (c) a signal possibly arising fromradiation damage due to a decays of naturallyoccurring radionuclides, mostly 238U and 232Th, ina 65-million-year-old natural glass [13,14]. Simu-lations of the latter two spectra were also carriedout under the K�anzig±Cohen theory, and thecorresponding g3 distributions are shown in Fig. 8.The simulation of Fig. 7(b) essentially constitutes aproof of the presence of Oÿ2 ions in the irradiatedpotassium silicate glass, since under the highlyconstraining interrelationships among all three g-values it succeeded in ®tting not only the g1±g2

region shown in Fig. 7(b) but also a broad butwell-resolved shoulder in the g3 region (notshown). The simulation of Fig. 7(a) shows that thespectrum of the unannealled He�-implanted sam-ple may arise from POR(Lo-T) variants, since it

was accomplished with a (68%-broader) g3 distri-bution peaking close to the maximum of thePOR(Lo-T) g3 distribution of Fig. 5(a). However,the low experimental signal-to-noise ratio makesthe accuracy of this simulation less certain. 5 Thesimulation of Fig. 7(c) is even more notional, giventhe low signal-to-noise ratio and the fact that theexperimental spectrum was separated from anoverlapping ferromagnetic resonance (FMR) sig-nal [14] by numerical subtraction of a straight linesegment arbitrarily taken to approximate thesteeply sloping FMR `background' (dashed line inFig. 9(a)). Thus, the present work establishes onlythe plausibility, and not the certainty, that PORswere created in this geologic glass by �4-MeV a

Fig. 6. X-Band ESR spectra of a c-irradiated CSG glass sample following 10-min anneals at: (a) 300°C and (b) 450°C. Thin solid

curves are zoom views of the main features of the spectra of Fig. 5(a) and (b), respectively. Bold/dotted curves are computer simu-

lations based on the POR(Lo-T) and POR(Hi-T) g-value distributions of Fig. 5, with these two components added together in the

ratios: (a) 2:1 and (b) 1:3. In each case the powder patterns were convoluted by a Lorentzian line shape with peak-to-peak derivative

width 0.48 mT. Inset shows a simulated spectrum of E0 centers for the same microwave frequency as used for the POR simulations;

(note correspondence in position with weak features in the experimental spectra arising from E0 centers in the ESR sample tubes).

5 Because of the presence of some cavity background signals,

the experimental spectrum of Fig. 7(a) (solid curve) was

obtained by subtracting the spectrum of an un-implanted

sample from that of the implanted one recorded under identical

conditions. But because both spectra were noisy, some small-

scale random artifacts undoubtedly resulted. Therefore, we can

neither support nor rule out the possible presence of an

overlapping spectrum of the type denoted A1 in [19].


particles and �100-keV a-recoil nuclei of thecontained radionuclides. 6

4.4. Other oxygen-related radiation-damage centers

The established presence of radiation-inducedperoxy radicals or interstitial Oÿ2 (superoxide) ionsin several glasses of this study leads to the likeli-hood that oxygen molecules have also evolvedaccording to the disproportionation reaction

2Oÿ2 ¢ O2ÿ2 �O2: �5�

Indeed, a Raman line near 1550 cmÿ1, attributableto molecular oxygen, has been recently reported togrow in HLW glass simulants as a function of 2.5-MeV electron ¯uence [43]. Eq. (5) is known tomove to the right in the presence of water [44] andmay be presumed also to shift to the right whentrapping of radiolytic electrons and holes occurs.At su�ciently large concentrations, the inferred

6 The amounts of 238U and 232Th (half lives 4:46� 109 and

1:4� 1010 years, respectively) in the Haitian tektite glasses of

the present study are not known but are likely to be bounded by

their concentrations in whole-rock and acid-insoluble samples

of Cretaceous±Tertiary (K/T) boundary clays presumably

derived from the same source materials as the coeval [13]

tektites. The measured amounts in the Danish K/T boundary

layer are [41]: 16U6 8:6 and 1:36Th6 7:1 ppm wt, where

each smaller number pertains to the acid insoluble fraction.

Making use of a formula [42] summing over all decays of 238U

and 232Th and their daughter isotopes in secular equilibrium,

the `dose' (ionizations + collisions + heat) delivered to the Hai-

tian tektites over their 65-million-year existence is calculated to

lie in the range �4ÿ 30� 103 Gy.

Fig. 7. X-band ESR spectra recorded at 77 K for: (a) a He�-implanted CSG sample, (b) a c-irradiated 44K2O±66SiO2 glass sample,

and (c) opaque tektite glasses subjected to 65-million years of a decays of contained 238U and 232Th. Bold/dotted curves are computer

line-shape simulations constrained by the g-value theory of K�anzig and Cohen [36] for Oÿ2 ions in solids: The distributions of g3 values

used are shown in Fig. 8. The powder patterns were convoluted by Lorentzian line shapes with peak-to-peak derivative widths: (a) 0.75

mT, (b) 0.35 mT, and (c) 1.15 mT.


radiolytic oxygen molecules could be responsiblefor observations of bubble formation in glassesirradiated beyond certain threshold doses [1,45].

The inset to Fig. 7(b) shows the spectrum of Oÿ3(ozonide) ions previously recorded in a c-irradi-ated 44K2O±66SiO2 glass sample following an-nealing to 307°C [33]. The same resonance,somewhat weaker in comparison to the simulatedOÿ2 spectrum, is also present in the 44:66 potassi-um silicate glass sample studied here (principalspectrum of Fig. 7(b)). These ozonide ions arelikely formed by reaction of interstitial Oÿ2 ionswith interstitial oxygen atoms. Reasons for thedi�ering ratios of ozonide to Oÿ2 apparent in thetwo spectra of Fig. 7(b) are not known but may berelated to di�erences in the post-irradiation an-neals and/or to di�ering OH contents of the as-prepared glasses. Ozonide reacts vigorously withwater to form hydroxide and O2 [44]. Thus, theobservation of radiolytic ozonide ions is a separateindication of the likely presence of radiolytic ox-ygen molecules. We did not observe Oÿ3 in our ir-radiated HLW glass simulants, however.

4.5. Properties of the Oÿ2 -type defect centers

The Oÿ2 -type component of Fig. 7(b) is essen-tially identical with the so-called `Z resonance'observed in the amorphous e�ervescent magneticperoxyborates [34,35]. Following arguments pre-sented in Ref. [46], the Z resonance seems bestascribed to Oÿ2 molecular ions in an ionic envi-ronment, very likely an amorphous alkali peroxidephase. (Note that the diamagnetic peroxide ion,O2ÿ

2 , appears along with O2 molecules on the right-hand side of Eq. (5).) Accepting this argument, alikely radiolytic phase separation of the 44K2O±66SiO2 glass is inferred.

The type and quantity of alkali oxide present inthe base glass should a�ect both the properties andthe quantities of any Oÿ2 ions induced by irradia-tion. Since the Z resonance was also observed in ac-irradiated K2O±5SiO2 glass [33], we infer that17 mol% K2O is su�cient to promote radiolyticphase separation (and presumably O2 evolution) ina binary silicate glass. (Ozonide ions were not de-tected in the 1:5 glass composition, however [33].)The present complex CSG composition contains18.8 mol% Na2O.

As in the case of the binary potassium silicateglasses [33], c-irradiation of a sample of CSG

Fig. 9. X-band ESR spectra of 65-million-year-old tektite

glasses from Haiti [13,14,53]: (a) 43 mg of mm-sized grains

appearing opaque and (b) 36 mg of mm-sized grains appearing

dark yellow in transmitted light. Short dashed curves are no-

tional extrapolations of the `background' signals (due mostly to

Fe3� and ferrites) expected in the absence of putative Oÿ2 res-

onances near g � 2. Six vertical tick marks in (a) are the cal-

culated positions of Mn2� hyper®ne lines. The di�erence in

sample masses is included in the overall gain factors shown.

Fig. 8. Distributions of g3 values respectively used for the line-

shape simulations of Fig. 7(a)±(c).


initially induces trapped-carrier centers which,upon annealing, recombine to reveal the weaker,underlying spectrum of PORs. It is important tonote, however, that de®nitive identi®cation ofcertain Oÿ2 -type signals as PORs in the potassiumsilicates was achieved by observation of 29Si hy-per®ne structure (hfs) [33]; only in this way was itpossible to con®rm the Oÿ2 ion to be covalentlybonded to a single silicon in the glass structure. Inthe present case, isotopically enriched sampleswere not available, but on the basis of its averageg-values (Table 1) POR(Lo-T) is reasonably as-cribed to a true POR, as opposed to an Oÿ2 inter-stitial. In fact, the g-values of POR(Lo-T) aresomewhat similar to those tentatively imputed [47]to the `small peroxy radical' (SPOR) in pure silica(Table 1), a species theoretically predicted [48] butnever de®nitively con®rmed in glasses. (The SPORmay be viewed as an Oÿ2 ion in a bridging position,which is formally the same as the result of at-taching an O� ion to a normal bridging oxygen.) IfPOR(Lo-T) should indeed turn out to be a SPORvariant, then Table 1 suggests that POR(Hi-T)could be a non-bridging POR variant analogous tothe non-bridging POR in pure silica [49]. The non-bridging nature of the POR in pure silica with theg-values determined in Refs. [27,49] has been un-ambiguously demonstrated by recording its 17O[50] and 29Si [51] hfs. Still, the g-value data of

Table 1 do not presently rule out the possibilitythat POR(Hi-T) is an interstitial Oÿ2 ion analogousto the superoxide species in 44K2O±66SiO2 glass.

4.6. Possible e�ects of iron

Glass CSG and the several glasses of Refs.[25,43] were batched without iron oxides. How-ever, a weak g � 4:3 spectrum of impurity Fe3�

present at levels of perhaps �10 ppm has beenobserved in all of these glasses, and the trapping ofelectrons on this Fe3� has been documented as adecrease in ESR intensity with increasing radiationdose [25]. On the other hand, following a c-raydose of 30 MGy (Si), no damage at all was re-ported in an ESR study [46] of a Savannah RiverHLW glass simulant (DWPF) containing 4.8 mol%Fe2O3. While trapping of electrons by Fe3� un-doubtedly took place in the case of the DWPFglass, attempts in Ref. [46] to measure the radia-tion-induced intensity decrease were unsuccessfuldue to the tiny fraction of the total iron likely tohave changed its valence state; for example, 1018

valence changes per gram would a�ect only oneiron in �800. However, the total absence of anyother ESR evidence of damage, such as the ap-pearance of NBOHCs or PORs, presents a puzzle.Even superimposed on the strong spectra of Fe3�

and precipitated ferrite phases, the oxygen-hole-

Table 1

Average g-values of radiation-induced peroxy radicals and Oÿ2 molecular ions in pure silica and some silicate glassesa

Material hg1i hg2i hg3i hD=ki Defect nature (evidence)

Pure silica 2.0020 2.0085 2.027 81 SPOR(?) (g-values: [47])

Glass CSG 2.0017 2.0078 2.053 39.5 `POR(Lo-T)' (g-values: this work)

K2O±5SiO2 2.0017b 2.0075 2.061 32.5 POR (29Si hfs: [33])

Pure silica 2.0014 2.0074 2.067 30.9 POR (g-values: [27,49]; 17O hfs: [50], 29Si hfs: [51])

Glass CSG 2.0001 2.0060 2.095 21.6 `POR(Hi-T)' (g-values: this work)

44K2O±66SiO2 1.9916 1.9971 2.204 9.9 Oÿ2 ions in ionic environment (17O hfs: [33];

g-values: this work)

Tektite glass 1.9908 1.9963 2.205c 9.0c Oÿ2 (?) (g-values: this work)

a Statistical averages of all three principal-axis g-values were determined (this work only) using simulation-tested g3 distribution

functions illustrated in Figs. 5 and 8; the corresponding values of hD=ki are mean values of Gaussian energy distributions used in Eqs.

(3) and (4), together with k=E � 0:0031 and l � 1:0. Nominal values of g1, g2, and g3 taken from Refs. [27,47,49] were found in fair

agreement with the K�anzig±Cohen formalism for Oÿ2 ions in solids when using the same parameterization of Eqs. (1)±(3) and the values

of hD=ki tabulated here.b This value, scaled from a spectrum shown in Ref. [33] is almost certainly too large due to the e�ect of an overlapping HC1 component.c Not measurable, but these values were used for the spectral simulation of Fig. 7(c).


center resonances (possessing widths much smallerthan those of most Fe3� and ferrite componentsnear g � 2) should be observable if the induceddefect concentrations were as large as 100 ppb. Wespeculate, therefore, that amounts of iron P 1mol% in a glass might act as a radiation protectionagent.

It is easy to imagine that NBOHCs and BOHCscould be suppressed by competitive trapping ofholes on divalent iron ions. But the case of dis-placed-oxygen defects (e.g., E0 centers, PORs, andOÿ2 interstitials) might seem to be qualitativelydi�erent from a radiation hardening point of view.However, it has been argued recently [52] thatSPORs are created in irradiated pure SiO2 by re-action of interstitial neutral oxygen atoms withself-trapped holes (STHs) and that in the absenceof STHs the displaced oxygen atoms would morereadily dimerize to form O2. Extending this line ofreasoning to borosilicate HLW glasses, we maysuppose that if there were no STHs (as a conse-quence of all holes trapping on Fe2� ions) and if allelectrons were trapped on Fe3� ions, then all ra-diolytically and collisionally displaced neutral ox-ygens might dimerize directly without theintermediate steps of O2ÿ

2 and Oÿ2 formation im-plicit in Eq. (5).

4.7. Geological glasses

The spectrum of Fig. 7(c), tentatively ascribedto radiolytic POR or Oÿ2 formation, is only ob-served for the opaque mm-sized Haitian tektiteglasses of this study ± which were shown by ESRto contain �4.5 times less Fe3� than a small sam-pling of dark-yellow Haitian tektites [14]. Al-though the latter glasses may have calcium oxidecontents as much as triple those of the opaqueglasses [53], statistically their total iron oxidecontents are likely to be about the same [53]. Thus,we presume that the Fe3�/Fetotal ratios of thestudied opaque tektites are lower than those of thedark-yellow ones by a factor of �4.5.

Increasing the spectrometer gain by a factor of21 reveals a possible Oÿ2 signal in the central, g � 2region of the dark-yellow tektite glass spectrum ofFig. 9(b). Though clearly broader than the spec-trum of Fig. 9(a), the increased gain factor

required to reveal it suggests that, even if due toOÿ2 ions, it may represent a substantially smallernumber of spins. This is tantalizing, but so-farunsupported, evidence for a form of radiationprotection by Fe3� ions.

5. Conclusions

In studies of several boron-containing HLWglass simulants, the immediate e�ect of c-irradia-tion (this work), X irradiation [25], or 2.5-MeV birradiation [25] is the production of boron±oxygenhole centers and complementary trapping of elec-trons on inadvertent Ti4� or Fe3� [25] impurities oron Zr4� ions deliberately added to the batches [25].Upon annealing in the vicinity of 300°C, all ofthese trapped carriers are caused to recombine,revealing the underlying signals of `oxygen holecenters' [25] ± which are speci®cally identi®ed inthe present study as PORs or interstitial Oÿ2 ionsby means of computer line-shape simulationsconstrained by the K�anzig±Cohen g-value for-malism [36]. By contrast, 160-keV He� ion im-plantation, though imparting an ionizing dosewhich is �3 times larger than the present c-raydose, is found to induce about 200 times morePORs and no measurable trapped-carrier centers.We conclude therefore: (i) that trapped-carriercenters, unseen but likely to have been producedby ion implantation, are immediately annealed bythe thermal energy co-deposited by the ions and(ii) that the increased POR yields relative to thoseconsequent to purely ionizing radiations resultfrom collisional processes.

There is evidence in the recent literature [46]that the presence of Fe3� in amounts approaching5 mol% may suppress not only trapped-carrierdefects but perhaps also PORs and Oÿ2 ions underc-irradiation. Results obtained for the geologicalglasses of the present study (both containing �5mol% iron oxide) are suggestive that PORs or Oÿ2ions are induced by a decays of contained actinidesand lend anecdotal support to the notion that theFe3�/Fetotal ratio may in¯uence defect productionrates. In any event, the HLW glass simulant of thepresent study CSG, like those of Refs. [25,43], wasessentially free of iron. That is, the amounts of


iron impurities were less than, or comparable tothe total numbers of trapped carriers induced byionizing radiation doses of 2� 106 (this work) to109 Gy [25]. Clearly then, there is a need for ad-ditional ESR studies of irradiated HLW glass si-mulants containing at least the amounts of ironoxide dictated by the waste streams slated forvitri®cation.

Acknowledgements

Work at the Naval Research Laboratory wassupported by the US Department of Energy underInteragency Agreement No. DE-AI07-96ER45619. Oak Ridge National Laboratory ismanaged by Lockheed Martin Energy ResearchCorporation for the US Department of Energyunder contract number DE-AC05-96OR22464.

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