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Glass, J. B. Breckinridge and A. J. Marker III, eds., Proc. SPIE. 1761, 46–57 1992. 5. M. J. Liepmann, L. Boehm, and Z. Vagish, “Gamma radiation effects on some ...

Refractive-index changes caused by proton radiation in silicate optical glasses Andrei I. Gusarov, Dominic Doyle, Alex Hermanne, Francis Berghmans, Michel Fruit, Gerd Ulbrich, and Michel Blondel

We have studied experimentally, by using a differential interferometric technique, the effect of proton radiation on the refractive index of commercial 共Schott兲 silicate crown glasses, BK7 and LaK9, and their radiation-resistant counterparts. The strongest effect was observed for the radiation-hard lanthanum crown LaK9G15: At a 0.65-Mrad dose the index change was approximately 3 ⫻ 10⫺5. Radiation-hard glasses are used in optical systems operating in radiation environments because they prevent spectral transmission degradation in the visible. However, such glasses are not protected against radiationinduced refractive-index perturbations, and a diffraction-limited optical system based on such glasses may fail owing to radiation-induced aberrations. © 2002 Optical Society of America OCIS codes: 160.2750, 350.5610, 220.0200, 350.6090.

1. Introduction

Optical surfaces of spaceborne instruments are often directly exposed to space radiation. Among other factors the effect of radiation on optical instrumentation has remained a major concern since the beginning of space optics. It is well known that glasses darken on exposure to radiation owing to formation of color centers. To overcome this problem, radiationhardened glasses have been developed. Such glasses have optical properties in the working wavelength range close to standard catalog glasses but are less sensitive to radiation in terms of transmission degradation in the visible owing to their Ce doping.1 Radiation also influences the density and the refractive index 共RI兲 of glass. The problem of RI stability under simulated space radiation had already been identified in the 1960s. Changes as high as A. Gusarov 共[email protected]兲 is with the Multitel a.s.b.l., B-7000 Mons, Belgium. D. Doyle and G. Ulbrich are with the European Space Agency, European Space Research Technology Centre, 2200 AG Noordwijk, The Netherlands. A. Hermanne is with the Vrije Universiteit Brussel, B-1050 Brussels, Belgium. F. Berghmans is with the SCK CEN Belgian Nuclear Research Centre, B-2400 Mol, Belgium, and also with the Vrije Universiteit Brussel. M. Fruit is with the Astrium, 31402 Toulouse, Cedex 4, France; M. Blondel is with the Faculte´ Polytechnique de Mons, B-7000 Mons, Belgium. Received 23 March 2001; revised manuscript received 6 September 2001. 0003-6935兾02兾040678-07$15.00兾0 © 2002 Optical Society of America 678

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10⫺4 were found in commercial glasses for radiation doses comparable with an integrated electron flux accumulated in 1 year on an orbit passing through Earth’s radiation belts.2 However, attempts by other researchers have resulted in the conclusion that at an accuracy level of 10⫺4 changes are absent both for particle3,4 and for ␥ radiation.5 Recently a more accurate investigation has confirmed that the index of commercial silicate glasses is influenced by ␥ radiation.6,7 The magnitude of the effect in the visible strongly depends on the chemical composition of the glass and was smaller than 10⫺4 at dose levels below 1 Mrad. 共A dose of 1 rad corresponds to 10⫺2 J absorbed by 1 kg of material; the SI unit of an absorbed dose is 1 Gray ⫽ 100 rad.兲 It is implicitly assumed that Ce-doped glasses possess a low sensitivity to radiation not only in terms of transmission but also with respect to the RI and density stability. An optical system is supposed to be made radiation hard by simple replacement of normal glasses with their radiation-resistant counterparts. In this paper we show that proton radiation, which is the major radiation component in nearEarth space, at a dose level characteristic of modern applications can cause significant RI changes in both normal and radiation-hard glasses. We discuss the results obtained on BK7 and LaK9 共Schott兲 glasses and their radiation-hard analogs 共BK7G18, BK7G25, and LaK9G15兲. The experimental data are analyzed on the basis of the dose-coefficient approximation.7 We also compare the effects of proton and ␥ radiation for insight into the problem of ground-

Fig. 1. Schematic representation of the proton irradiation geometry. The hatched area represents the irradiated part of the sample, and the left-hand graph indicates the diammetrical dose distribution averaged over the sample thickness. The effect of surface dilatation uz is strongly exaggerated.

based simulation of space radiation when ␥ radiation only is used. 2. Experimental Procedure

For modern diffraction-limited spaceborne optical systems, uniform RI changes of ⬃10⫺5 can result in significant degradation of system specifications.8,9 In the case of radial-index gradients, effects such as focal length and Gaussian image scale perturbations are enhanced by an order of magnitude compared with corresponding uniform changes. Therefore one should quantify induced RI changes of the order of 10⫺6 to assess the radiation-induced aberrations. Absolute RI measurements at this accuracy level are not straightforward. We have applied a differential interferometric method6,10 based on the use of samples with a radial stepwise dose distribution profile. The proton irradiation was performed in a cyclotron in ambient conditions at two different energies: 26.75 and 38 MeV. For 27-MeV protons the range is less than 4 mm 共3.48 mm for BK7兲 and the particles are stopped in the samples, while for 38 MeV the range is more than 6 mm 共6.88 mm for BK7兲 and the nonuniformity of the longitudinal dose distribution in a 5-mm-thick sample is less than 50%. The proton beam has a low divergence, mainly defined by Coulomb repulsion. In the cross section the beam intensity is Gaussian shaped with a variation of less than 15% over a 25-mm diameter of the exit beam. We used a collimator with a 10-mm-diameter opening in front of the sample to screen its periphery 共Fig. 1兲. This irradiation geometry gives a nearly 100% radial dose-step distribution. For ␥ irradiation a Co60 source with a dose rate of 6 krad兾h 共H2O兲 was used. To estimate the doses absorbed in glass a correction factor of 1.2 was taken into account. The irradiation was interrupted several times for intermediate measurements to be performed 共see Ref. 11 for more details兲. The geometry of the irradiation was similar to that in Fig. 1. The samples were mounted in a holder that also included a lead shield. The shield 共⬃11 mm thick兲 was designed so that the peripheral annular area received half of the total dose received by the central part with a diameter of 10 mm.

Fig. 2. Transmission wave-front map obtained on a pristine sample.

Measurement of the induced optical path difference 共OPD兲 between parts of a wave front passed through a stepwise irradiated sample allows retrieval of the relative RI change corresponding to different radiation levels. Measurements were always taken in the same controlled laboratory conditions at 20 °C. However, temperature gradients 共⬎0.1 °C across the sample diameter兲 can influence measurements. We have observed such gradients, induced by sample handling, on live interferograms and always allowed them to decay before taking measurements. The wave-front measurements were made at ␭ ⫽ 0.63 ␮m with a commercial Fizeau interferometer in a double-pass configuration. Samples were identically manufactured as 30-mm-diameter planeparallel disks of 5-mm thickness with an rms polishing error of less than ␭兾120. A wave-front map 共WFM兲 measured on a pristine sample 共Fig. 2兲, shows typical fabrication errors. Such aberrations complicate the quantification of the induced effect, especially for ␥-irradiated samples with an extended transition between 100% and 50% dose areas. We therefore subtracted the WFM taken before irradiation from that taken after irradiation. Fiducial marks on the sample surfaces were used to ensure accurate realignment during wavefront measurements. To check the accuracy of the interferometeric method, we have analyzed the WFMs obtained on the control 共nonirradiated兲 samples. With a careful sample 共re兲positioning in the interferometer the subtraction procedure gives a peak-to-valley wave-front error difference of ⬃2 nm, which means that we can safely quantify an induced effect corresponding to an index 共or volume兲 change greater than 4 ⫻ 10⫺7. Such a high level of sensitivity is achievable owing to the good mechanical and thermal stability of our interferometer and the high optical quality of the samples and also because we are performing relative rather that absolute index measurements. 3. Experimental Results

The WFM obtained on proton-irradiated samples are shown in Fig. 3. The maps measured before irradiation are subtracted in all cases. A clear 1 February 2002 兾 Vol. 41, No. 4 兾 APPLIED OPTICS

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Fig. 3. Transmission wave-front difference maps of BK7-series glass samples irradiated with 共a兲–共c兲 27-MeV and 共d兲–共f 兲 38-MeV protons. The measurements were performed 13 days after irradiation. The proton fluences were 共a兲 ⫺1.85 ⫻ 1012, 共b兲 ⫺2.35 ⫻ 1012, 共c兲 ⫺3.1 ⫻ 1012, 共d兲 2.2 ⫻ 1012, 共e兲 ⫺2.25 ⫻ 1012, 共f 兲 ⫺2.9 ⫻ 1012 p⫹兾cm2, corresponding to an average ionizing dose of 0.5– 0.8 Mrad.

aperture-imprint effect6 corresponds to the nonuniform irradiation geometry. Its sign is reversed for radiation-hard glasses compared with their normal counterpart. For BK7 the RI decreases, while for two other glasses in the series it increases. We attribute the inversion of the sign to the presence of the cerium dopant. In general, the OPD between two areas ⌬⌳共r兲 that received different doses is due to a volume RI change ⌬n共r, z兲 and a surface dilatation effect uz resulting from the radiation-induced density change,11 Fig. 1: ⌬⌳共r兲 ⫽ 共n 0 ⫺ 1兲u z共r, z ⫽ 0兲 ⫹



L

dz⌬n共r, z兲 ⫹ 共n 0 ⫺ 1兲u z共r, z ⫽ L兲,

0

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(1)

where L is the sample thickness and n0 is the glass RI at the probing wavelength 共633 nm兲. To characterize the density effect, we measured the reflected wave fronts from each side. For dose levels of 1 Mrad or below the surface dilatation was sometimes detectable but always too small to be accurately quantified. We observed significant dilatations at doses only above 20 Mrad.12 That allowed us to neglect the density effects throughout the present study. Therefore Eq. 共1兲 can be rewritten in simplified form: ⌬⌳共r兲 ⫽ L⌬n⌰共R ⫺ r兲,

(2)

where ⌬n is a RI change averaged over the sample thickness, R is the radius of the nonscreened part, and ⌰共r兲 is the step function, which assumes an ideal dose distribution. Equation 共2兲 shows that the

Fig. 4. Histogram representation of the wave-front map 关Fig. 3共a兲兴 for BK7. The peaks of the bimodal distribution correspond to the central 共irradiated兲 and the peripheral 共shielded兲 parts of the sample.

glasses except BK7 the RI decreases with time after irradiation. We thus found that radiation-induced index changes in Ce-doped glasses are comparable with or even larger than those in their normal counterparts. This result does not contradict the term radiationresistant because the present concept of radiation hardness refers exclusively to preservation of the spectral transmission characteristics of the glass in the visible part of the spectrum. Our results show that changes in the UV part of the spectrum, which are neglected when only visible transmission is of concern, can significantly contribute to RI instability under radiation.

4. Dose Coefficients

transmission wave-front measurements alone allow characterization of the induced RI changes. The OPD was found from both the height histogram representation of the data and the radial profile method discussed in detail elsewhere.7,11 Both approaches gave the same result. Figure 4 shows such a histogram representation for a BK7 glass sample. The non-Gaussian shape of the peaks is a consequence of the deviation of the dose distribution from the ideal step function. It is known that radiation-induced effects are not stable and exhibit 共temperature-dependent兲 postradiation annealing. Annealing rates generally increase with elevated temperature. In our case all annealing was at room temperature only. Quantitative results for the BK7 and Lak9 series are summarized in Fig. 5. The amplitude of the induced OPD decreases slowly with time for BK7G18 and BK7G25, while an unambiguous growth is observed for BK7 glass. For LaK9 the effect of irradiation is very small and seems to change its sign, while LaK9G15 shows the highest sensitivity. For all

Fig. 5. Postproton irradiation behavior of the OPD: diamonds, BK7 共dose 0.54 Mrad兲; squares, BK7G18 共0.53 Mrad兲; open triangles, BK7G25 共0.68 Mrad兲; asterisks, LaK9; solid triangles, LaK9G15 共0.65 Mrad兲; solid circles, LaK9G15 共0.18 Mrad兲.

The data summarized in Fig. 5 were obtained for different proton beam energies and fluences. To quantitatively compare such results, we use the Dose-coefficient approximation.7 With this approach a change in RI 共⌬n兲 is described with a dose coefficient ␤共␭兲: ⌬n共␭兲 ⫽ ␤共␭兲 D,

(3)

where D is the absorbed dose. For ␥ radiation the dose-coefficient approximation was proved experimentally applicable for doses as great as 800 krad. Figure 6 shows the postirradiation behavior of the dose coefficients at ␭ ⫽ 633 nm. For BK7 the dose coefficient for 38-MeV protons is ⬃1.5 times bigger than that for 27-MeV protons 关Fig. 6共a兲兴, while for BK7G18 and BK7G25 关Figs. 6共b兲 and 6共c兲兴 the difference is within the measurement error. The result for BK7 can be explained by the saturation effect. For normal silicate glasses, saturation of radiation-induced changes appears in a dose range of 0.5–1.0 Mrad. In BK7 samples irradiated with 27-MeV protons the deposited dose near the Bragg peak is well above the saturation threshold and this diminishes the dose coefficient. For irradiation with 38 MeV the dose nonuniformity is less than 50% and the saturation effect is not significant. For radiation-hardened glasses BK7G18 and BK7G25 the saturation threshold is higher and the effect does not play a role at dose levels used here. Note that implanted protons usually induce an increase in the RI, which stems from glass compaction.13 Implanted proton concentration above 1017 cm⫺3 is necessary to produce an appreciable effect. In our experiment the proton fluence is rather small, 1011–1012 cm⫺2, and that effect is therefore not significant. Figure 6 shows that postradiation relaxation may result in an increase in the amplitude of the RI change at 633 nm. This means that prolonged lowdose-rate irradiation can have a stronger effect than that at high-dose-rate testing—a conclusion usually not anticipated. 1 February 2002 兾 Vol. 41, No. 4 兾 APPLIED OPTICS

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Fig. 7. Spectra of 800-krad ␥-radiation-dose-induced absorption coefficient for BK7 glass: 1, 2 h after irradiation; 2, 24 days after irradiation; 3, difference between 2 and 1.

similar: The OPD is positive for BK7 and is negative for BK7G18兾BK7G25. However, for BK7G18 and BK7G25 the dose coefficients differ. For both proton and ␥ radiation the effect on LaK9 is too small to be quantified accurately. For LaK9G15 the dose coefficients almost coincide. On the phenomenological level, radiation-induced RI modifications stem from transmission and density changes. In our experiments, surface dilatations, i.e., density changes, are virtually absent. Therefore index changes are explained by the color-center model by the Kramers–Kro¨ nig relation 共the principal value integral兲: ⌬n共␭兲 ⫽

Fig. 6. Postirradiation behavior of the dose coefficients DC: 共a兲 BK7, 共b兲 BK7G18, 共c兲 BK7G25. The curves correspond to the following: diamonds, Ep ⫽ 27 MeV; 䊐, Ep ⫽ 38 MeV; crosses, Co60 source, D ⫽ 400兾800 krad.

5. Comparison of Proton and ␥-Radiation Effects

In near-Earth space, protons are the dominating component while primary ␥ radiation is virtually absent. Nevertheless Co60 ␥ radiation is routinely used to simulate the space-radiation environment and little information is found in the literature on the effects of proton radiation. Therefore it is important to compare the effects on the RI produced by those two types of radiation. The effects are qualitatively 682

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␭2 ␲2



d␭⬘

⌬a共␭⬘兲 , ␭ 2 ⫺ ␭⬘ 2

(4)

where ⌬a is the naturally induced absorption coefficient. According to Eq. 共4兲 an increase in the absorption in the visible and the near UV 共␭ ⬎ 400兲 experimentally observed in BK7 共Fig. 7兲 should result in an RI increase, opposite the observed response. It is necessary to assume therefore that there exists a strong induced absorption decrease at lower UV wavelengths. We measured transmission spectra down to 240 nm in ␥-irradiated BK7 glass samples. A strong initial absorption prevents measurements at shorter wavelengths. The induced absorption spectra in proton- and ␥-irradiated samples are very similar. The radiation-induced transmission increase does exist and the maximum lies below 240 nm 共Fig. 7兲. The genesis of the UV-absorption bands in multicomponent glasses is quite involved. It is known that in ␥-irradiated glasses, E⬘-type centers form a band centered near 213 nm,14 and Fe3⫹ ions, which are usually present as a technological impurity, are responsible for a number of very strong absorption bands in the UV.15,16 The charge transfer between ferrous, ferric, and other ions can result in induced clearing. Negative induced absorption in silicate glasses has been discussed.17 Its origin was attributed to nonbridging oxygens, which are believed to contribute significantly to the intrinsic absorption in the UV. Electron paramagnetic resonance spectroscopy of space-irradiated samples

has confirmed that both oxygen hole centers and ferric ions are generated by space radiation.18 For BK7 the concentration of Fe3⫹ ions was estimated as 1017 cm⫺3 after a 100-krad dose, which agrees reasonably with our data. After irradiation the induced absorption in the visible gradually decreases 共Fig. 7兲. Such behavior makes a negative contribution to the induced RI, corresponding to the observed index postirradiation evolution. However, absorption at 240 nm increases and this seems to overcompensate the contribution from the former effect. This overcompensation makes it necessary to accept that additional UV bands are necessary to explain the temporal RI behavior. For example, oxygen-hole centers, which are not stable at room temperature, may contribute to further the RI decrease. We have observed that in terms of dose coefficients, ␥ photons produce a stronger effect than protons. Similar results from spectral transmission measurements have been reported.4 It was found that x-ray radiation produced a similar radiation effect as protons in terms of the absorption band shape but with a considerably larger amplitude. Protons are much more efficient in the production of atomic displacement by elastic collisions. However, for our energy range, less than 0.1% of the energy of the primary proton is transferred through this mechanism while the remaining energy is dissipated through the ionization channel, creating stable defects. Therefore nonionizing energy losses are not important in our case, and the radiation damage in glasses is defined by ionization for both ␥ and protons. If that were not true, the difference between protonand ␥-photon-induced damage should be the same for both normal and radiation-hardened glasses. We believe that the reason for the difference between the effects of ␥ photons and protons is related to microscopic features of the ionization mechanisms for the two types of radiation. Charged particles ionize atoms by excitation of electrons by Coulomb interaction. The cross section for Coulomb ionization is proportional to the factor 共Ep兾I兲兾ln共Ep兾I兲, where I is the ionization potential and Ep is the energy of the charged particle. Therefore protons and secondary electrons preferentially ionize defect precursors in the forbidden gap, creating shallow defects. Such defects can effectively anneal thermally. In contrast, Co60 ␥ photons interact through the photoelectric effect. Excited atoms relax through the Auger mechanism. In this way thermally stable deep defects are created. This is shown very clearly for BK7 in Fig. 6共a兲. It would be interesting to extend the dosecoefficient parameterization to transmission 共spectral absorption dose coefficients兲. We have found that for a number of glasses the normalization of the absorption coefficient to the dose reduces the data to a single curve 共Fig. 8兲. The scaling works for proton and ␥ radiation separately, but the difference between the two types of radiation is clearly observed.

Fig. 8. Comparison of induced absorption produced by ␥ and proton radiation: curves 1, 2, 38-MeV protons for doses 0.4 and 0.95 Mrad, respectively; curves 3– 6, Co60 ␥ radiation with doses of 50, 100, 200, and 400 krad. Measurements were performed 4 months after irradiation.

6. Conclusions

We have experimentally investigated the effect of proton radiation on the RI of commercial 共Schott兲 silicate crown glasses BK7 and LaK9 and their radiation-hard analogs. We applied the dosecoefficient approach to describe the induced changes. The magnitude of the effect is rather small, but for radiation-hard lanthanum crown LaK9G15 at 1 Mrad, a dose that can be accumulated during prolonged space missions, RI changes can reach a level of 3 ⫻ 10⫺5. For diffraction-limited systems such changes can cause significant degradation of the optical specifications. It cannot be excluded that some glasses show even higher sensitivity to radiation. Radiation-hard glasses were developed for optical systems operating in radiation environments. Under radiation loads they maintain their spectral transmission characteristics in the visible. However, such glasses are not protected against RI perturbations and may show a sensitivity higher than that of standard glasses. An optical system based on radiation-hard glasses will not suffer from transmission degradation in the visible, but it may fail due to radiation-induced aberrations, and our results demonstrate the necessity to investigate this problem further. At a given wavelength, radiation can both decrease and increase the RI, depending on the chemical composition of the glass. For, example, the ␥-radiation dose coefficient is ⬃10⫺9 Gray⫺1 for BK7 and ⫺0.7 ⫻ 10⫺9 Gray⫺1 for BK7G18. The change in the sign is attributed to the effect of the Ce ions. It is not unreasonable to suppose that glass with 1 mol.% of Ce 共BK7G10兲 would be radiation-hard in terms of both spectral transmission and RI stability under radiation. Otherwise a pair of BK7 and BK7G18 glasses could be used to build a radiation-tolerant doublet. In most cases postradiation annealing leads to a decrease in radiation damage. However, for BK7 the initial effect is negative and its amplitude appears to grow with time. This situation is potentially dangerous because it is usually assumed that accelerated testing has a more severe effect on the 1 February 2002 兾 Vol. 41, No. 4 兾 APPLIED OPTICS

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material parameters compared with long-term lowdose-rate irradiation in space. This may also be of concern to optical designers who are forced to use nonradiation-hard glasses such as BK7 because of limited availability or nonavailability of Ce-doped analogs. We have found that proton and ␥ radiation produce qualitatively similar effects in terms of both RI and transmission variations in the visible. We use the dose-coefficient approach based on the absorbed dose to make quantitative comparisons. It shows that for the BK7 series ␥ radiation produces a stronger effect than protons do, while for LaK9G15 the dose coefficients are similar. We cannot rule out that for some glasses proton radiation has a stronger effect. These conclusions confirm that there needs to be concern about the accuracy of results obtained with ␥ sources traditionally used to simulate the spaceradiation environment. References 1. J. C. Stroud, “Color centers in a cerium-containing silicate glass,” J. Chem. Phys. 37, 836 – 841 共1962兲. 2. I. H. Malitson and M. L. Dodge, “Radiation-induced instability in refractive properties of some optical glasses,” J. Opt. Soc. Am. 55, 1583 共1965兲. 3. J. Bourrieau and M. Rome´ ro, “Effect of space charged particle environment on optical components and materials,” in Proceedings of the ESA Symposium on Spacecraft Material, ESA SP-145 共European Space Agency, Munich, 1979兲, pp. 275–285. 4. P. R. Silverglate, E. F. Zalewski, and P. Petrone, “Protoninduced radiation effects on optical glasses,” in Damage to Space Optics and Properties and Characteristics of Optical Glass, J. B. Breckinridge and A. J. Marker III, eds., Proc. SPIE 1761, 46 –57 共1992兲. 5. M. J. Liepmann, L. Boehm, and Z. Vagish, “Gamma radiation effects on some optical glasses,” in Damage to Space Optics and Properties and Characteristics of Optical Glass, J. B. Breckinridge and A. J. Marker III, eds., Proc. SPIE 1761, 284 –295 共1992兲. 6. D. B. Doyle and R. H. Czichy, “Influence of simulated space radiation on optical glasses,” in Space Optics 1994: Space Instrumentation and Spacecraft Optics, T. M. Dewandre, J. J.

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