Determination of the visible range optical absorption spectrum of peroxy radicals in...

Determination of the visible range optical absorption spectrumof peroxy radicals in gamma-irradiated fused silica

David L. Griscom a,*, Masafumi Mizuguchi b,1

a Optical Sciences Division, Naval Research Laboratory, Code 5612, Washington, DC 20375, USAb Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama 226, Japan

Abstract

Two silicas (Russian KS-4V and an F-doped silica) with <1 ppm cationic impurities, OH contents <5 ppm, and Cl

contents <20 ppm have been investigated in optical-®ber forms by di�erential-optical spectroscopy following 14-

MGy(Si) 60Co c irradiation at 27°C and isochronal anneals to 600°C. Bulk samples of the F-doped silica and of two

other high-purity silicas (Suprasil F300 and F310) were studied by electron spin resonance (ESR) and isochronal

annealing following exposure to (i) a c-ray dose of 12 MGy(Si) and (ii) the same 12 MGy c irradiation plus a sub-

sequent ®ssion-reactor irradiation. The strength of an optical band near 2 eV is shown to correlate with the intensity

of the ESR spectrum of peroxy radicals (PORs) as a function of anneal temperature. The peak energy, band width,

and oscillator strength of this band are determined to be E0� 1.97 � 0.01 eV, W� 0.17 � 0.01 eV, and

fPOR� 0.00057 � 0.00015, respectively. This POR band overlaps with, and has previously de®ed separation from,

those of the non-bridging-oxygen hole centers (NBOHCs), here ®tted by two bands (E0� 2.19 � 0.01 eV,

W� 0.51 � 0.01 eV and E0� 2.08 � 0.01 eV, W� 0.32 � 0.01 eV) with an aggregate oscillator strength

fNBOHC� 0.00054 � 0.00015. The annealing processes of the PORs and NBOHCs displayed a dose-rate dependence,

tentatively linked to the e�ect of dose rate (1 Gy/s vis-�a-vis 5.3 Gy/s) on the steady-state self-trapped-hole populations,

which in turn determine the equilibrium of radiolytic interstitial oxygen species: O0 + O0 O2. Ó 1998 Elsevier

Science B.V. All rights reserved.

1. Introduction

The most fundamental radiation-induced defectcenters in amorphous silicon dioxide are the E0

center (BSiá) [1], the non-bridging-oxygen holecenter (NBOHC: BSi±Oá) [2], the peroxy radical(POR: BSi±O±Oá) [3], and the self-trapped holes(STH: BSi± _O±SiB) [4], where the notation `B'represents three bonds with other oxygens in the

glass network and `á' denotes an unpaired electron.The atomic-scale structures of these defects areknown from electron spin resonance (ESR) studies(see, e.g. [5,6]). The E0 center has an optical ab-sorption band at 215 nm [7] and no knownabsorptions in the visible range. Both the NBOHCand the POR are believed to have absorptionbands near 260 nm [8±10], and the former exhibitsa low-oscillator-strength band (f � 6 ´ 10ÿ4) near600 nm [6,10±13]. On fundamental grounds, thePOR would be expected to have a correspondingtransition near 600 nm, and, based on data fordrawing-induced defects in optical ®bers [13], theexistence of such a band has been previously

Journal of Non-Crystalline Solids 239 (1998) 66±77

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

404 8114; e-mail: [email protected] Tel.: +81 45 924-5628; fax: +81 45 922-5169.

0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 7 2 1 - 2

inferred [6]. Nevertheless, no visible-range ab-sorption band correlated with the ESR spectrumof the POR has been spectrally resolved before.We report here unambiguous spectral resolution ofa POR absorption band near 630 nm, based onESR and di�erential-optical isochronal-annealstudies of c-irradiated high-purity silicas in bulkand optical-®ber forms, respectively.

2. Experimental details

Optical measurements in the range 400±1000nm were carried out by means of a prism-basedcharge-coupled-device(CCD)-camera spectrometerdescribed elsewhere [14]. Both ®bers for the pres-ent study had 200 lm diameter cores, 240 lm di-ameter claddings of F-doped silica, and aluminumjackets. 2 The core materials were (A) Russian-developed KS-4V silica containing <0.2 ppm OHand <20 ppm Cl and (B) a vapor-axial-deposited(VAD) silica of Japanese origin containing 0.54mass% F, �4 ppm OH, and presumably <20 ppmCl. One-meter lengths of ®bers A and B werecoiled to diameters of 62 mm and simultaneouslyexposed to 60Co c rays at a dose rate of 1.0 Gy(Si)/s at 27°C in the dark to an accumulated dose of 13MGy(Si); an additional dose of 1 MGy was ad-ministered 3 months later [15]. (Four-meter lengthsof the same two ®ber types had been previouslystudied during c irradiation at a dose rate of 5.6Gy(Si)/s to a total dose of 12 MGy(Si) but werenot subjected to thermal annealing until after asubsequent reactor irradiation which renderedthem too opaque for measurement in the range550±650 nm [14].)

Bulk samples for the present ESR measure-ments comprised Suprasil F300 (0.2 ppm OH,2000 ppm Cl), Suprasil F310 (200 ppm OH, 125ppm Cl), and a piece of F-doped silica cut from thesame core rod as was used to draw ®ber B; thesehad been c irradiated four months prior to mea-surement (the irradiation had been carried out si-

multaneously with the 4 m ®ber lengths studied ata dose rate of 5.6 Gy(Si)/s [14]). Separate samplessubmitted to both the c and subsequent reactorirradiation [14] were also studied. Room-temper-ature X-band ESR spectra were recorded (Bruker300E). The partially overlapping resonances of E0

centers (recorded at 0.1 lW) and of NBOHCs andPORs (both recorded at 10 mW) were separated bysubtraction of spectra obtained during di�erentstages of the anneals. These were numerically in-tegrated and compared with a copper sulfatestandard sample (estimated accuracy �15%) todetermine absolute spin concentrations. At othertemperatures, relative changes in the three spectraltypes were determined from their amplitudes inregions of minimum spectral overlap and werenormalized to the cases where absolute calibra-tions were possible. The accuracy of such relativemeasurements can be �10% for but may be less(perhaps �25%) in situations where the measuredspectra are overlapping and the signal-to-noiseratio is <5.

Seven minute isochronal anneals were accom-plished in tube furnaces. The ®bers were investi-gated 5 months after the original 13-MGy cirradiation at 1.0 Gy(Si)/s (1 month after the ad-ditional 1 MGy irradiation). In situ measurementshad been performed during both irradiations[15,16]. Sample ®bers A and B were both held intheir original 62 mm diameter coil con®gurationsby pieces of mylar tape without use of a mandrel.These coils were loosely constrained to lie hori-zontally near the end of a 50 cm strut constructedof fused silica tubing for insertion into the furnace.Reference ®bers of the same two types were alsodeployed along the strut, just reaching to the po-sitions of the sample coils. As described previously[14], these reference ®bers had the same pigtaillengths as the sample ®bers but made only a singleloop at the radiation source. By taking the ratio ofthe light transmitted by the sample coil to lighttransmitted by the reference ®ber, induced-lossspectra were obtained which cancelled `fringing'losses in the pigtails [14]. The silica strut arrange-ment eliminated stresses in the ®bers due to ther-mal expansion of the substrate and contributednegligible thermal mass, thus avoiding certainproblems described in Ref. [14].

2 These ®bers were fabricated in the Fiber Optics Research

Center (FORC) of the General Physics Institute, Russian

Academy of Sciences, Moscow.

D.L. Griscom, M. Mizuguchi / Journal of Non-Crystalline Solids 239 (1998) 66±77 67

3. Experimental results

Fig. 1 displays the as-measured isochronal-an-neal di�erence spectra, L(k, Tann) ) L(k,TRT), forthe KS-4V-core ®ber (A), where Tann is the isoch-ronal anneal temperature and TRT connotes thatno anneal was performed. Evident deviations ofsome of these spectra from a common baseline atlonger wavelengths are interpreted as being due tonearly-wavelength-independent (`grey') micro-bending losses associated with charring of themylar tape restraining the sample coils. This con-

clusion is supported by the fact that the twosample ®bers, A and B, exhibited nearly identicalbaseline deviations for each of the anneal tem-peratures, e.g., both ®bers displayed their largestdeviations for values of Tann of 450°C, 500°C, and550°C. (We assume that the tape was reduced tomechanically compliant ash at 600°C, since nomore smoke issued from the furnace and bothspectra returned to their original baselines.)Therefore, in Fig. 2 the inferred microbending ef-fects in the spectra of F-doped-silica-core ®ber Bhave been arti®cially removed by addition or

Fig. 1. Changes in c-ray-induced optical absorption in pure-

silica-core (KS-4V) ®ber A e�ected by 7-min isochronal anneals

to the indicated temperatures. Prior to annealing, this sample

had been exposed to a c-ray dose of 14 MG(Si) at a dose rate of

1.0 Gy(Si)/s in the dark. Likely origins of the apparent baseline

¯uctuations are discussed in the text.

Fig. 2. Changes in c-ray-induced optical absorption in F-

doped-silica-core ®ber B e�ected by 7-min isochronal anneals to

the indicated temperatures. Prior to annealing, this sample had

been exposed to a c-ray dose of 14 MG(Si) at a dose rate of 1.0

Gy(Si)/s in the dark. Baseline ¯uctuations in the raw data nearly

identical to those of Fig. 1 have been removed by addition or

subtraction of wavelength-independent terms.

68 D.L. Griscom, M. Mizuguchi / Journal of Non-Crystalline Solids 239 (1998) 66±77

subtraction of constant terms selected to bring allspectra into correspondence at 800 nm.

The spectra of Figs. 1 and 2 reveal for the ®rsttime the presence of a band near 630 nm whichgrows in consequence of post-irradiation heattreatment. Fig. 3(a) illustrates a 2-componentGaussian decomposition of spectrum 6 of Fig. 2(a)into a relatively narrow band which grew and a

broader band which bleached, both in response toan anneal at 400°C.

Fig. 4 shows ESR spectra recorded at 10 mWmicrowave power for the F-doped-silica-core ®berat three stages of the anneal. Quantitative mea-surements of the E0 center, inverted by microwavesaturation in Fig. 4, were based on spectra sepa-rately recorded at 0.1 lW. It is quite apparent inFig. 4 that the spectral component attributable toPORs [2,3,5,6,8] has grown by a factor of �3 forTann� 300°C. Similar results were recorded for thetwo undoped silica samples, except that the E0

center yield was substantially larger in the samplewith largest Cl content (consistent with a previousstudy [17]) while the POR yield was reduced in thesample with largest OH content.

4. Discussion

4.1. Gaussian decompositions of the optical bands

The Gaussian band centered atE0� 1.97 � 0.01 eV (width W� 0.175 � 0.010

Fig. 3. Gaussian resolutions of spectrum 6 of Fig. 2 employing:

(a) single-Gaussian ®ts of the spectra of both the peroxy radical

(E0� 1.97 eV, W� 0.175 eV) and the nonbridging-oxygen-hole-

center (E0� 2.19 eV, W� 0.505 eV) and (b) a two-Gaussian ®t

of the POR band using (strong POR band) E0� 1.975 eV,

W� 0.165 eV and (weak POR band) E0� 1.93 eV, W� 0.10 eV.

Fig. 4. ESR spectra of a c-irradiated bulk sample of F-doped

silica (core material of ®ber B) at three stages of isochronal

annealing. Some spectral features and g values due to E0 cen-

ters, NBOHCs, and PORs are identi®ed. An additional feature

at g� 2.003, remaining after the 600°C anneal is not discussed

in this paper.


eV) 3 which simulates the growth feature inFig. 3(a) is attributed to the POR on the basis ofthe ESR data of Fig. 4 (and similar data in theliterature [2,6]) which show the POR to be the onlyfundamental defect to increase in numbers uponannealing above �100°C. The improved ®tachieved in Fig. 3(b) using two Gaussians suggeststhat the POR band may be slightly asymmetric.The broader ®tted band of Fig. 3 which is pro-gressively bleached as Tann ® 400°C (E0� 2.19� 0.01 eV, W� 0.51 � 0.01 eV) is ascribed toNBOHCs.

Fig. 5 displays on an energy scale the spectra ofthe induced optical absorptions in the two ®berswhich were thermally bleached by the full succes-sion of 7 min anneals through 600°C; they areshown here as positive losses, since they shouldclosely approximate the as-induced losses beforeannealing. In Fig. 5(a) it is apparent that a secondNBOHC band (E0� 2.08 � 0.01 eV, W� 0.32� 0.01 eV), in addition to the one of Fig. 3, is re-quired to ®t the as-induced spectrum of ®ber B. Bycontrast, the as-induced spectrum of ®ber A doesnot have a 2.08 eV band (see Fig. 5(b)); rather, itdisplays a 2.19 eV NBOHC band which has twicethe amplitude of that in ®ber B, a POR band with�3 times smaller amplitude, and bands centeredabove �2.5 eV (not ®tted) which have amplitudesgreater by factors of �3. Based on these results, wesuggest that the 2.08 eV band arises fromNBOHCs deriving from the removal of hydrogensfrom hydroxyl groups (present at 4 ppm in the F-doped silica of ®ber B but essentially absent in KS-

4V). The 2.19 eV band is tentatively ascribed toNBOHCs associated with broken bonds, since onemight expect more strained bonds in the pure silica(®ber A) than in the silica containing 1.7 mol%¯uorine (®ber B). Apropos, it should be noted that¯uorine doping is not expected to contribute to theradiation sensitivity of a-SiO2. Indeed, evidencehas been given that F doping levels �1.0 mass%actually reduce the radiation yields of E0 centers byabout an order of magnitude [18].

3 All Gaussian curve ®tting was done by cut-and-try methods.

The illustrated ®ts are the ones subjectively assessed to be `best

®ts'. The error estimates given here are based on the ranges of

trial values of E0 and W yielding ®ts approaching the quality of

those shown. These estimates have no statistical meaning.

However, we believe that the illustrated 2-component ®ts of the

positive- and negative- going peaks of Fig. 3 are less ambigu-

ous, and likely to be more accurate, than any computer

optimized ®t of a spectrum comprising a linear combination of

the same two overlapping absorption bands when their ampli-

tudes have the same algebraic sign. By extension, the 3-

component ®t of Fig. 5(a) is likely to be more meaningful than

a computer-driven ®t of this same spectrum using three or more

Gaussians with all positions and widths as variables, since in the

present case the positions and widths of two of the bands were

pre-determined by the ®t of Fig. 3(a).

Fig. 5. Gaussian resolutions of the thermally bleached compo-

nent of c-ray-induced loss in: (a) F-doped-silica-core ®ber B

and (b) pure-silica-core ®ber A, following 7-min anneals at

50°C intervals to 600°C. Both resolutions employ the same two

bands used in the ®t of Fig. 3(a) plus, in (a) only, an additional

NBOHC band characterized by E0� 2.08 eV and W� 0.325 eV.


4.2. Calculation of oscillator strengths

While exhibiting qualitatively similar trends,the optical spectra of Fig. 2 and the ESR spectra

of Fig. 4 reveal di�erences when measures of theirband intensities are plotted versus Tann. In partic-ular, the thermal bleaching of NBOHCs and con-comitant increases in PORs take place at lowerTann's in the ESR experiment than they do in theoptical study (see Fig. 6). This unexpected out-come is deemed most likely an e�ect of the factor-of-six di�erence in dose rates experienced by theESR and optical samples. Given this assessment,oscillator strengths were calculated based onGaussian ®ts (not shown) of the spectrum [14] of a4-m length of F-doped-silica-core ®ber recorded 18days after c irradiation at the same dose rate and tothe same total dose as the present ESR samples(but not subjected to annealing). The componentbands used in these ®ts were constrained to be thesame as those which optimized the ®t of Fig. 5(a),except that the POR contribution was optimizedusing two Gaussians in a manner similar to that ofFig. 3(b). However, the situation was complicatedby the presence of two additional bands which, asdescribed in Ref. [14], had to be included to ac-count for the observed long-wavelength tails.

Smakula's relation for Gaussian line shapes wasemployed in the form [19]

Nf � �8:7� 1016�nRi�aiWi�=�n2 � 2�2; �1�where N is the defect concentration measured byESR in spins/cm3, n is the index of refraction of thematerial, Wi is the full width of the ith sub-band ineV, and ai is its peak absorption coe�cient in cmÿ1

(� 2.303 ´ 106 times the loss in dB/km). Thecombined oscillator strength of the two resolvedNBOHC sub-bands is found to be fNBOHC�0.00054 � 0.00015, in good agreement with earlierworks [11,13]. 4 Assuming that the POR gives rise

Fig. 6. ESR (symbols) and optical (bold curves) isochronal

anneal data for c-ray-induced defect centers in: (a) F-doped

silica and (b) Suprasil F300 silica. The ESR data pertain to bulk

samples irradiated at a dose rate of 5.6 Gy(Si)/s to a dose of 12

MGy(Si), while the optical data are for F-doped-silica-core ®-

ber B irradiated at a dose rate of 1.0 Gy(Si)/s to a dose of 14

MGy(Si). Both irradiations were accomplished in the dark. In

(a), the optical data of Fig. 2 have been converted to spin

densities using band shapes and oscillator strengths for PORs

and NBOHCs reported in this paper. The data points have been

connected with straight lines as an aid to the eye.

4 Several authors previously concerned with the oscillator

strength of the NBOHC have employed Smakula's relation in

the form pertaining to a Lorentzian optical line shape.

Multiplying these literature fNBOHC's by a factor of 0.674 to

bring them in accordance with the present Gaussian formula-

tion gives 0.00054 [11] and 0.00028 [13] as comparison values.

We consider the exact agreement of our result with that of Ref.

[11] as possibly fortuitous. For example, our value could be

arti®cially larger due to our having measured the optical

spectrum 18 days after irradiation and the ESR spectra 4

months afterward. However, we note that the optical spectra

decayed by only 2.5% in the period between 18 h and 18 days

after irradiation [14].


to the band with E0 � 1.97 eV and W � 0.17 eVwhich grows with increasing Tann, its oscillatorstrength is determined as fPOR� 0.00057� 0.00015. Sources of error are di�cult to estimatedue to the necessity of resolving overlapping sig-nals in both the ESR and optical spectra. Thequoted limits of �30% embody our sense of theerrors in both processes. Nevertheless, we notethat our spin density determination for NBOHCsin the F-doped silica is in excellent agreement withthe extrapolated dose dependencies of a largenumber of data summarized in Ref. [20], includingNBOHCs in several c-irradiated low-OH/low-Cl,F-doped materials.

4.3. Interpretation of the isochronal anneal data

The oscillator strengths determined above wereused to convert the optical intensities of Fig. 2 todefect concentrations (bold curves in Fig. 6(a)). InFig. 6, we perceive three distinct temperatureranges where di�ering annealing e�ects are mani-fested: (I) 0±250°C, where the E0 centers andNBOHCs bleach abruptly as Tann ® 250°C andthe PORs exhibit concomitant increases, (II)�250±400°C, where the annealing rates of the surviving E0

centers and NBOHCs are less and the rate of in-crease of PORs is correspondingly less, and (III)above�400°C, where all induced defects, includingPORs, decrease. Most action in the present ESRdata (symbols) takes place in range I, whereas thelargest non-range-III changes in the optical data(bold curves) occur in range II. Since in the pastseveral excellent ESR-optical correlations havebeen achieved for silica samples irradiated atcommon dose rates [2,6±13], the `discrepancy' be-tween the ESR and optical data of Fig. 6 is pre-sumed to arise from some previously unrecognizedfacets of the radiation damage process in silicawhich are dose-rate dependent. Indeed, the presentoptical data may represent the ®rst spectroscopicstudy of defects induced in a silica of this purity byirradiation to a rather high total dose (14 MGy(Si))accomplished at a dose rate as low as 1 Gy(Si)/s(requiring 5 months in source).

Speci®cally, we suggest here that the generallyaccepted mechanism by which E0 centers are con-verted to PORs upon annealing [21],

BSi � �O2 ! BSi±O±O�; �2�may be e�ective principally in temperature rangeII, rather than in range I as generally assumed.Based on the known di�usion coe�cient, D�D0

exp()E/kT), of O2 molecules in silica (D0� 2.9 ´10ÿ4, E� 1.17 eV [22]), the rms di�usion length,p

pDt, is calculated to be only 4 nm forTann� 200°C and tann� 7 min, i.e., eight timessmaller than the mean distance between defects (32nm) at a concentration of 3 ´ 1016 cmÿ3. By con-trast, for a range-II temperature of 400°C, thedi�usion length for O2 becomes 290 nm, i.e., ninetimes larger than the distance between defects.Thus, there is a priori reason to believe thatmechanism (2) might operate in range II but not inrange I.

It has recently been shown [23] that neutronirradiation progressively destroys those interstitialoxygen molecules initially present in Suprasil W1,presumably according to O2 ® 2O0. It may turnout that c irradiation also breaks up O2 molecules.We are therefore in accord with the proposition[23] that (range-I) thermal bleaching results fromdi�usion of interstitial oxygen atoms according to

BSi � � O0 ! BSi±O� �3�and/or

BSi±O � �O0 ! BSi±O±O � : �4�Detailed reconciliation of this model with the

ESR data of Fig. 6(a) would require that thethermally activated increase in POR concentrationequal the sum of the concomitant decreases in E0

center and NBOHC concentrations. In fact, theincreases in POR concentrations in both sets ofdata of Fig. 6 are too small by a factor of �2/3.Other workers have reported POR `de®cits' of afactor of �1/2 in both oxygen-rich [24] and(chloride-rich) oxygen-de®cient [25] silicas, con-cluding ``that there must be e�ective annealingmechanisms at 225°C other than that of oxygendi�usion-limited reactions.''

We agree that there could be other mechanismsof annealing in silicas containing larger amountsof OH or chloride. For example, in high-OH silicasinterstitial oxygens are likely to become decoratedwith radiolytic H0s to form H2O molecules, which


in turn have been argued to react with E0 centers intemperature range II [26]. In high-Cl materials,ClO2 and ClO3 radicals are formed [27] whichwould subtract from the supply of oxygen inter-stitials available for reactions (2)±(4). At the sametime, interstitial Cl0s and Cl2 molecules may back-react with the numerous E0 centers deriving from®ssion of Si±Cl bonds [17]. However, we do notbelieve that a combination of, say, oxygen- andchlorine-di�usion-limited reactions are likely toanneal both E0 centers and NBOHCs according tothe virtually identical isochronal anneal pro®lesseen in Fig. 6(a) (since, e.g., Cl0 should react withE0 centers but less readily with NBOHCs). In thecase of our F-doped and KS-4V silicas, the Cl andOH contents are both very small and neither ma-terial is either oxygen rich or oxygen de®cient.Therefore, we suggest that all range-I annealing insuch stoichiometric, impurity-free samples is dueto O0 di�usion limited kinetics and that the POR`de®cits' must be explainable within this frame-work. As an example, the following model mightbe considered. Assuming that broken bond defects(E0±NBOHC close pairs) are as common as oftensupposed (e.g., [20,28]), one E0 center and oneNBOHC could be simultaneously eliminated byreaction with a single oxygen atom without cre-ating a POR (or other paramagnetic center):

BSi � �O±SiB�O0 ! BSi±O±O±SiB: �5�

4.4. A proposed model accounting for the observeddose-rate e�ects

Returning to the `discrepancy' between the ESRand optical data of Fig. 6(a), we advance the no-tion that high dose rates may not only break up O2

molecules but might also impede re-dimerizationof the separated O0 atoms. We propose the fol-lowing mechanism: Based on the number ofmetastable self-trapped holes (STHs) inferred tobe present in low-OH/low-Cl silicas during irradi-ation [15,16], it seems possible that radiolyticallydisplaced oxygen atoms may be metastablly trap-ped by STHs according to

The species on the right-hand side of Eq. (6) isknown as the small peroxy radical (SPOR). TheSPOR has been predicted in silica on the basis oftheoretical calculations [21], and an ESR spectrumtentatively attributed to the SPOR has been re-corded in a low-OH/high-O2 silica subjected toeither X irradiation or 6.4 eV laser light at cryo-genic temperatures [29]. For su�ciently largesteady-state populations of STHs (i.e., for su�-ciently large dose rates) and su�ciently small in-terstitial O0 concentrations (i.e., for su�cientlysmall total doses), it seems likely that virtually alldisplaced O0s would be immobilized by reaction(6). Moreover, even assuming some hopping mo-bility, the associated positive charges would causethe metastable SPOR defects to repel one another,thereby preventing oxygen dimerization as long asthe radiation remains in progress.

The question then is, how large are the steady-state STH concentrations and are they dose-ratedependent? The only evidence to date comes fromoptical data obtained by one of the authors [15,16]and his interpretation that the intense 660 and 760nm bands (which arise only in very-low-OH/very-low-Cl silicas [14]) are due to STHs. The numbersof these putative STHs under steady-state condi-tions at a dose rate �1 Gy/s have been determinedto be NSTH fSTH � 1.7 ´ 1014 cmÿ3 [16], where theoscillator strength fSTH(6 1) is not yet known. Weargue that the absence of these bands in high-OHor high-Cl silicas results from the STHs beingquenched by reactions with larger numbers of ra-diolytic H0 and Cl0 atoms [17], forming metastablebridging OHÿ and OClÿ structures, respectively.Reaction (6) is a process fully analogous to thoseinvolving hydrogen or chlorine, except that thenumber of available O0 atoms may be smaller (andthe binding energy of the Oÿ2 species is undoubt-edly larger [21]). For example, the numbers ofdisplaced oxygens in stoichiometric low-OH/low-Cl silicas induced by an ionizing dose of 50 kGyshould be P 3 ´ 1015 cmÿ3 (greater than or aboutequal to the measured numbers of E0 centers [20]),whereas a number of Cl0 or H0 atoms of the orderof 5 or 10 ´ 1015 cmÿ3 have been generated inhigh-Cl and high-OH silicas, respectively, by thesame dose [17]. If we assume for the sake of ar-gument that fSTH� 0.02, then under steady-state


conditions NSTH would just equal the number ofH0 or Cl0 atoms in the high-OH or high-Cl silicas,respectively, but could exceed the number of dis-placed O0 atoms in a Cl-free/OH-free silica.

4.5. In¯uence of dose rate on defect-formation andradiation-hardening kinetics

Fig. 7 portrays the kinetics of defect formationin F-doped-silica-core ®ber B as a function of c-raydose from �30 Gy(Si) to �100 MGy(Si) as mir-rored in the induced optical losses measured in situat two ®xed wavelengths. Data for k� 700 nm(Fig. 7, triangles) measure essentially only STHs atall doses below �2 MGy(Si) [15,16]. However, asthe dose increases above �20 MGy(Si), the STHsare rapidly decreasing and the absorptions due toNBOHCs + PORs emerge as a peak near k� 600nm which grows approximately as the 0.6 power of

dose (Fig. 7, diamonds, for dose >20 MGy). Thesolid data points were recorded in a separate study(the experiment of Ref. [16] extended to higherdoses) for a dose rate of 5.3 Gy(Si)/s, whereas theopen symbols represent data obtained for thepresently discussed ®bers at a dose rate of 1.0Gy(Si)/s. The `radiation hardening' of the STHbands at doses >1 MGy is clearly evident; (thise�ect has been suggested to be due to radiationmodi®cation of the mobility edge for valence-bandholes [15]). Signi®cantly, the threshold for thishardening occurs at an accumulated dose �5 timesless for the sample exposed at the smaller dose rate(open symbols). With regard to the presently pro-posed model, this phenomenon means that reac-tion (6) would have been more e�ective during thelarger dose rate irradiation, thereby impeding theformation of O2 molecules relative to the case forthe smaller dose rate. This argument then com-pletes our proposed explanation of why the prin-cipal defects induced in ®bers c irradiated to a doseof 14 MGy(Si) at a rate of 1 Gy(Si)/s do not beginto anneal until temperature range II (via reaction(2)), whereas the E0 centers, NBOHCs and PORsinduced in a bulk sample of the ®ber core materialirradiated to the same total dose at a 5.6 timesgreater dose rate, undergo substantial annealing inrange I (via reactions (3) and (4) ± and possibly (5)).

4.6. Samples exposed to both c and reactor irradi-ation

Ref. [14] describes the results of sequential ex-posures of a suite of optical ®bers to 60Co c irra-diation (5.6 Gy(Si)/s to a dose 12 MGy(Si)) and®ssion-reactor irradiation (c dose rate 70 Gy(Si)/sto a dose of 4 MGy(Si)). The incremental (reactoronly) induced losses in the F-doped-silica-core ®-ber sample (same as present ®ber B) measured at670 nm are shown as the bold curve in Fig. 7.Although losses at wavelengths in the range �550±650 nm (where the NBOHCs and PORs actuallypeak) were too large to be measured in the 4 msample lengths [14], we infer from the 670 nm dataof Fig. 7 that the production rates of NBOHCsand PORs in the reactor must have been greaterthan in the case of 60Co c irradiation by at least afactor of 5.

Fig. 7. Radiation induced optical losses in samples of F-doped-

silica-core ®ber B recorded in situ during c irradiation at 1.0

Gy(Si)/s in the dark (,}), c irradiation at 5.3 Gy(Si)/s in the

dark �.r�, and ®ssion-reactor irradiation at 70 Gy(Si)/s with

25±50 lW propagating white light (bold curve). The irradiation

temperatures were 27 and 40°C for the c and reactor irradia-

tions, respectively. The wavelength of 700 nm (,.) falls near

the peak of the STH bands [15,16], while the wavelength of 600

nm �}r� corresponds to the peaks of the NBOHC and POR

bands. The reactor irradiation data (k� 670 nm) represent ad-

ditional damage induced in a ®ber previously exposed to a c-ray

dose of 12 MGy(Si); (the dashed part of the bold curve spans a

factor-of-six increase in reactor power).


Bulk samples of the F-doped-silica core mate-rial and of Suprasil F300 (the core material of theAnhydroguideTM ®ber of Ref. [14]) had been ex-

posed simultaneously with the ®bers to the full``c + reactor'' total dose. ESR isochronal annealdata for these two samples are shown in Fig. 8(a)and (b), respectively. These data, when comparedwith those of Fig. 6 for Tann� 0, con®rm that thereactor indeed induced �10 times more NBOHCsand PORs than did the 60Co c irradiation aloneeven though the reactor dose was 3 times less!Shown as bold curves in Fig. 8 are the (arbitrarilynormalized) optical losses at two ®xed wavelengthsderived from the isochronal anneal spectra re-ported in Ref. [14]. Note that, unlike the boldcurves in Fig. 6(a) which represent speci®c defectconcentrations estimated from Gaussian ®ts to theoptical spectra, the ®xed wavelength data of Fig. 8correspond to unknown anneal temperature de-pendent linear combinations of the contributionsof NBOHCs, PORs, and perhaps other defects.Nevertheless, the induced optical losses at 670 nmshown in Fig. 8(b) appear to coincide with the sumof the concentrations of NBOHCs and PORs (´'s).This observation may explain many previous fail-ures to correlate the `2-eV absorption' with aparticular ESR-active defect by isochronal an-nealing studies (see Ref. [6] for a critical review).That is, the annealing dependence of the `2-eVband' can now be understood in terms of aweighted average of the complementary annealingdependencies of NBOHCs and PORs (the formerdecreasing while the latter increases). This cir-cumstance results in a relatively invariant tem-perature dependence of the composite band heightwhich does not match those of either of the com-ponent bands taken individually.

All four ESR data sets of Figs. 6 and 8 appearto show NBOHCs converting approximately one-for-one into PORs in the temperature range �100±300°C. These results therefore support reaction(4), rather than reaction (2), as the dominantrange-I anneal mechanism. Participation of E0

centers in this process via the combination of re-actions (3) and (4) is not ruled out. In fact, thecusp in the concentration of NBOHCs atTann� 150°C apparent in Fig. 8(b), if repeatable,would be the ®rst explicit evidence for reaction (3)in irradiated silica. In any event, the decay of E0

centers via reaction (2) can account for no morethan 20% of the range-I increase in PORs in

Fig. 8. ESR (symbols) and optical (bold curves) isochronal

anneal data for c-ray-plus-®ssion-reactor-induced defect centers

in: (a) F-doped silica and (b) Suprasil F300 silica. The ESR and

optical-®ber samples were irradiated simultaneously, ®rst at a c-

ray dose rate of 5.6 Gy(Si)/s to 12 MGy(Si) and then at a re-

actor dose rate of 70 Gy(Si)/s to an additional dose of 4

MGy(Si). The ESR samples were irradiated in the dark,

whereas white light was propagated in the ®bers during both

irradiations [14]. Fixed-wavelength optical data taken from [14]

are arbitrarily normalized. The ´'s in (b) represent the sum

[NBOHC]+[POR]. The data points have been connected with

straight lines as an aid to the eye. (Note di�erence in y-axis scale

units between this ®gure and Fig. 6).


Fig. 8(b). By contrast, the range-II increases inPORs and decreases in E0 centers in Fig. 8(b) canbe fully explained on the basis of reaction (2)alone.

The ESR and optical samples of Fig. 8 hadidentical radiation histories with one exception,namely, the ESR samples were irradiated in thedark whereas white-light powers P 20 lW werecontinuously propagated in the ®bers [14]. Thedemonstrated susceptibility of the 660 and 760 nmSTH bands to optical bleaching under steady-stateirradiation conditions [15,16] may therefore ac-count for the smaller initial absorptions and morerapid `radiation hardening' of the absorption at670 nm during reactor irradiation (Fig. 7, boldcurve). (Note that both c irradiation experimentsof Fig. 7 were conducted in the dark.) Thus, wesuppose that the reactor might have induced 10times more STHs during the in-the-dark irradia-tion of the ESR samples of Fig. 8 than are ap-parent in the illuminated ®bers of Fig. 7. If thispremise is accepted, the model proposed in Sec-tion 4.4 may still explain the dominant range-Iannealing e�ects manifested in the ESR data of thereactor-irradiated samples (Fig. 8). Clearly, how-ever, this suggestion requires further testing, whichshould involve experiments wherein the ESR andoptical samples are identically irradiated in thedark. Moreover, in the future, means must be de-veloped to separate the e�ects of the fast neutron¯ux (3.3 ´ 1016/cm2 s for E > 2.8 MeV, in thepresent case) from those of the intense c ®eld (70Gy(Si)/s) before the reactor irradiation processescan be fully understood.

5. Conclusions

We have demonstrated that the POR has anoptical band partially overlapping that of theNBOHC and possessing a similar oscillatorstrength. This demonstration appears to resolve along-standing mystery relating to the historicallack of correlation between the `2-eV optical ab-sorption band' and the ESR spectrum of either theNBOHC or the POR taken individually. Amongother things, this discovery opens up the possibil-ity that some part of the `R-band' photolumines-

cence bands near 1.9±2.0 eV (reviewed in Ref. [10])may arise from PORs rather than NBOHCs.

The results reported here add to the body offundamental knowledge that underpins our un-derstanding of holographically-written Bragggratings in silica-based ®bers. We note, for exam-ple, that in a previous ESR-isochronal-annealstudy of defects induced in a (Ge-free, high-OH)silica glass by focused 6.4 eV excimer laser light[30] the POR concentrations reached their maximafor Tann � 375°C, implying in the context of thepresent data that the photolytic interstitial oxygenspecies were mainly O2 molecules. We further callto mind earlier circumstantial evidence that inter-stitial O2 molecules might be directly induced inGe-doped silica under c irradiation [31]. Finally,the c-ray dose-rate e�ects we report here may ul-timately prove instructive in the Bragg gratingcontext, given the similarities which have beenestablished between the types and numbers of de-fects induced by excimer-laser photolysis and c-rayradiolysis of amorphous silicas [29].

Acknowledgements

D.L.G. expresses his gratitude to the JapanSociety for the Promotion of Science for the grantof a `Fellowship for Priority-Area Research inJapan' at the Research Laboratory of EngineeringMaterials, Tokyo Institute of Technology (TIT).Professors H. Hosono and H. Kawazoe arethanked for their kind hospitality, material sup-port, and advice. The presently reported ESRstudies were performed at TIT, whereas the optical®ber investigations were carried out at the NavalResearch Laboratory.

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