One method of hardening optical glasses against radiation-induced darkening has been to add CeO2 to the batch composition. In the present investigation we prepared a series of lanthanum crown glasses with varying degrees of CeO2 additions and melted them at 1,400°C with and without bubbling oxygen gas. We examined the influence of added oxygen on the optical transmissions of these glasses in the spectral range 460 to 760 nm following gamma irradiations ranging from 10 to 250 krad. The results showed that dose-for-dose the radiation-induced optical attenuations of the oxidized glasses were greater than for the glasses without added O2.
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Optical glasses applied to space-borne optical systems are exposed to high energy radiation like gamma-, electron, proton and neutron radiation. The accumulation of this high doses radiation can result in the transmission loss of optical glasses [1–4], especially near the UV-Visible edge of the spectral range, which is not very beneficial to the performance of optical systems and must be avoided or reduced to a manageable level in order to keep the required performance of optical systems during the prolonged mission times. Therefore, some special glasses capable to withstand high energy radiation, namely radiation resistant glasses (RRG), have been developed [5,6].
High energy radiation can produce free electrons and holes in optical glasses, which can be trapped and form defect centers [7,8]. These defect centers increase optical absorption of glasses in the visible spectrum, which leads to discoloration of glasses. Some studies [9,10] have shown that the cerium as a polyvalent ion can trap free electrons and holes in glasses exposed high energy radiations, which holds up the forming color centers. Consequently, cerium-doped glasses can improve the radiation resistance of the glasses. However, the cerium as the stabilizer can change the intrinsic color of RRG and decrease the intrinsic transmittance of RRG in the UV-visible spectral range. In general the higher the cerium content the more the glass is stabilized against higher total doses, but the more the intrinsic transmittance is reduced. Thus, it is very difficult that an attempt is made to enhance not only the sensitivity to high energy radiation but also the intrinsic transmittance of optical glasses.
In earlier work, the glasses exposed γ irradiation, which had the high OH contents  or high-water-content  were found that the induced absorption decreased in the visible region. Hydrogen-impregnated glasses also were found to resist radiation induced absorption in the visible and UV ranges . Just exceptions were some glasses containing large concentration of species such as Pb2+ and Ti4+. In this paper, we try to increase the radiation resistance by adding oxygen to the lanthanum crown glasses (nd = 1.74693, νd = 50.95) melt. The Experiment indicates added O2 don’t improve the sensitivity to high energy radiation of the lanthanum crown glasses, nor their intrinsic transmittance.
2. Experimental procedure
2.1 Preparation of glass samples
In the experiment, the lanthanum crown glasses with different CeO2 concentrations were prepared by high temperature melting. For the glass containing 0.25Wt% CeO2, marked G1, the prescribed compositions (Wt%) were 11.5SiO2/14.7B2O3/13.6Al2O3/55La2O3/3.55Y2O3 /2.4ZrO2/0.25CeO2. For the glasses containing 0.5Wt% CeO2 and 0.65Wt% CeO2, marked G2 and G3, respectively, the compositions were appropriately adjusted on the base of G1 for the purpose of objecting the same refractive index and dispersive as the glass matrixes. G1 for instance, the mixed batches 700g were added step by step into a 0.5L platinum crucible placed in an electrical resistance furnace and melted for 3 hours at 1410°C. After 3 hours mechanical stirring with platinum stirrer in air, leading to high homogeneity, bubble free and transparent glasses, the melt was poured onto a preheated brass mold (580°C) and then was annealed in a muffle furnace at near Tg (Transformation temperature), followed by a slow cooling from near Tg to room temperature at 3°C /min. Then the samples were subject to a annealing circle of 348 h in the furnace at 0.5°C /h in order to relieve stress. Lastly the glasses were cut and polished, getting the samples with 20mm length, 20mm width and 10mm thickness.
During the high temperature melting, high pure O2 were added to the melt of the lanthanum crown glasses for ten minutes by quartz tube with the diameter of 4mm. The quartz tube was located a place lower 2cm than the surface of the melt. Added O2 is beneficial to prepare RRG. On the one hand, some micro-bubble in the melt can be eliminated by O2 transporting them to the surface. On the other hand, some Ce3+ ions are oxidized Ce4+ ions, which may be helpful for increasing radiation resistant of RRG.
2.2 Measurement of properties
The irradiation experiments were performed at room temperature and the step-dose accumulation approach was followed. In the present case glass samples were irradiated at Co60 sources for which the dose-rates were 10.2rad/s under 10krad~20krad lower dose radiation and 50.4rad/s under 250krad higher dose radiation, respectively. First, the lanthanum crown glasses were irradiated up to a fraction of the total dose following that the transmission spectra were measured using Varian Cary 3 UV-Visible Spectrophotometer with a one-hour delay after the irradiation experiment came to end. Then the cycle of irradiation-measurement was repeated several times until a total dose (20 krad and 250 krad for the low and the high dose-rates, respectively) was accumulated.
3. Results and Discussion
From the point of view of an optical engineer, optical glass is characterized by its refractive index and dispersion at the working wavelengths. Schott, one of the famous optical glass manufacturers in the world, offers a variety of radiation resistant glasses covering main parts of the Abbe diagram . These glasses are suitable for earth orbit based applications with lifetimes of up to 10 years. It can be seen from the Abbe diagram  that in the refractive index area such as 1.7~1.8 or more than 1.8, only a few of radiation resistant glasses are investigated, it still has very large space not to be developed.
With the rapid development of space optics, we think, some radiation resistant glasses with higher refractive index such as 1.7~1.8 or more than 1.8 should be developed in order to give more selections to an optical designer. In fact, it is the intent that we research the lanthanum crown glasses (nd = 1.74693, νd = 50.95). Glasses with higher refractive index can reduce the amount of used glasses, shorten the length and lessen the weight of the whole optical system, which are greatly favorable to a complicated optical system, sometimes based on tens of different glasses. However, glasses with higher refractive index often show decreased transmission in the short-wave visible spectrum due to the introduction of high polarizability constituents such as Pb, Ba and La, thus it is a very critical issue if these glasses are used in the UV-Visible range.
The cerium presents the two valence states of Ce4+ and Ce3+ in glasses. Friebele  described the effect of the cerium’s valence state on radiation-induced optical absorption. He thought Cerium is normally found as either Ce3+ in strongly reduced glasses or in both Ce3+ and Ce4+ in glasses melted under normal atmosphere or only slightly reduced. The increasing of the Ce3+ concentration increased the suppressed of the visible coloration in reduced glasses, but the radiation protection can be greatly enhanced if the glasses were prepared in less reducing conditions so that both Ce3+ and Ce4+ were present. In our work, added O2 is expected to be helpful for the conversion from Ce3+ to Ce4+. When RRG are exposed to high energy radiation, the cerium will firstly interact with motional electrons at high speed and change Ce3+ ions, namely Ce4+ + e→Ce3+, thus the forming color centers is avoided, which indicate that Ce4+ ions are in favor of increasing resistance to radiation fields. After radiation the Ce3+ ions concentration are increasing and the Ce4+ ions concentration are decreasing. Therefore, the assumption is put forward that changing the concentration balance of Ce4+/Ce3+ in RRG and causing the concentration of the former higher than that of the latter before Co60 radiation, may improve radiation resistance and the transmission of glasses. In this paper some experiments were carried out to prove the assumption.
It is can be seen from Fig. 1(a) that for the RRG with added O2, the transmittance (denoted T%) dependence of the wavelength is consistent with one without O2 before radiation, the quicker drop at short wavelength and no changing at long wavelength, and the T% difference between no O2 and added O2 is smaller at short wavelength, up to 1% at 460nm, which indicate added O2 has no obvious effect on the transmittance of G1 at 450nm~800nm before radiation. But in Fig. 1(b), the distinct change that added O2 induce a loss of over 1% than no O2 at 450nm~560nm is observed after 10krad radiation. With the growth of dose-radiations in (c) and (d), added O2 reduces by 0.8% and over 1% than no O2 from 450nm to 560nm, respectively. These show added O2 does not improve radiation resistance of G1.
For G2 containing 0.5Wt% CeO2 in Fig. 2 , the T% of added O2 dependence of the wavelength is accordant with that of no O2 before radiation. For added O2 and no O2, the big change of transmittance that T% has a reduction of over 10% at short wavelength from 450nm to 560nm, up to 30% at 460nm, is present when exposed to 250krad dose-radiations. The difference of T% between added O2 and no O2 is smaller, but the former is lower than the latter at the whole wavelength.
With the increasing of CeO2 concentration, up to 0.65Wt% in G3 from seen in Fig. 3 , added O2 still does not improve the T% before 250krad radiation, in other words that added O2 does not enhance the sensitivity to 250krad radiation and the T% reduces 5% than no O2 at 490nm~560nm, which indicating added O2 is not conducive to improve radiation resistance. Why such experimental results are obtained? One reason is thought that added O2 may change the lanthanum crown glasses structure. To be more precise, although Ce4+ ions go up, oxygen holes in the lanthanum crown glasses also increase under the condition of added O2, and the increasing of the latter is quicker than of the former and the latter has more important influence on radiation resistance than the former. Therefore, the reduction of oxygen holes is greatly taken into account in order to improve radiation resistance of RRG. It is the work that we do next.
Space radiation induces a loss in optical transmission in optical glasses, especially near the UV-Visible spectral range. The loss in transmission has much important influence on the performance of optical systems, so the problem must be overcome. Luckily, RRG have been developed to improve the radiation resistance of the glasses. However, the higher cerium content stabilizes against higher total doses at the price of a loss of transmission of RRG. In this work, high pure O2 is added into the lanthanum crown glasses in order to improve the sensitivity to high energy radiation and transmittance of glasses. The experimental results show that added O2 cannot realize above objects.
This research was financially supported by the National Natural Science Foundation of China (NSFC, No. 51002181).
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