ON THE INFLUENCE OF MODIFIER IONS ON THE RADIATION-OPTICAL PROPERTIES OF ALKALI BORATE GLASSES IRRADIATED WITH ELECTRONS

E. K. ZAVADOVSKAYA, V. G. VAKHROMEEV, I. A. NIKOLAEV, S. P. ZINOV’EVA

It is known \[1–3\] that the introduction of an alkali modifier into glass-forming $\mathrm{B_2O_3}$ has a substantial effect on its radiation-optical properties. However, to date there is no unified opinion on the nature of this phenomenon.

A comprehensive study of the dependence of the optical-absorption spectra, thermoluminescence, and thermodiffusion concentration emf (TDC emf) of irradiated glasses on the amount of modifier introduced will help, to a certain extent, to answer this question.

For the study, glasses melted in a platinum crucible in air in an electric furnace at $t = 1100^\circ\mathrm{C}$ from reagent-grade boric acid and sodium carbonate of “chemically pure” grade were used. The melted glass was clarified for six hours, and then specimens were made in the form of round plates 6–7 mm in diameter and 0.7–1.1 mm thick. Irradiation was carried out with electrons of energy $E = 1.2$ MeV on an ESG-2.5 unit of the Research Institute of Nuclear Physics at TPI. The specimens were irradiated in air with forced cooling; the overheating was no more than $40^\circ\mathrm{C}$.

Before irradiation all the glasses are transparent; the irradiated ones acquire coloration, light yellow in $\mathrm{B_2O_3}$, gradually changing to intense brown in glass with a high modifier content.

Fig. 1. Optical-absorption spectra of glasses: a) $\mathrm{B_2O_3}$; b) 7 wt.% $\mathrm{Na_2O}$; c) 10 wt.% $\mathrm{Na_2O}$, irradiated with electrons to a dose of $1.8 \cdot 10^7$ rad.

The optical-absorption spectra of irradiated $\mathrm{B_2O_3}$ contain two bands, at 320 and 480 nm. The introduction of a modifier increases the optical density of the glasses and causes the appearance of an additional band at 550 nm (Fig. 1). The thermoluminescence spectra of $\mathrm{B_2O_3}$ contain one broad peak with a maximum at $130^\circ\mathrm{C}$ (Fig. 2a). The position of the peak maximum does not depend on the irradiation dose. The introduction of the modifier $\mathrm{Na^+}$ leads to an increase in the integral luminescence intensity and to the appearance of a new peak, which, with increasing $\mathrm{Na^+}$ content, shifts into the region of higher temperatures.

The TDK\(_{\mathrm{eds}}\) curves of boric anhydride contain two distinct peaks: a negative one in the range \(40\)—\(90^\circ\mathrm{C}\) and a positive one in the range \(90\)—\(190^\circ\mathrm{C}\). Introduction of a modifier causes an increase in TDK\(_{\mathrm{eds}}\) and the appearance of a second negative peak in the higher-temperature region (\(190\)—\(270^\circ\mathrm{C}\)).

Fig. 2. Thermoluminescence of glasses: a) \( \mathrm{B_2O_3} \); b) 7 wt.% \( \mathrm{Na_2O} \); c) 10 wt.% \( \mathrm{Na_2O} \), irradiated with electrons at a dose of \(1.8 \cdot 10^7\) rad

Discussion

It is known [4] that pure vitreous \( \mathrm{B_2O_3} \) is a chain-layer structure consisting of \(3\mathrm{B_2O_3}\) rings. Introduction of an alkali oxide into it causes boron to pass from the 3-coordinated environment characteristic of pure \( \mathrm{B_2O_3} \) to the mixed 3- and 4-coordinated environment characteristic of alkali borate glasses. The transition \( \mathrm{B^{III+} \to B^{IV+}} \) is apparently effected by the formation of new bridging bonds in the glass network according to the following scheme [5]:

\[ \begin{array}{ccccc} & O & & O & \\ & | & & | & \\ O{-}B & \to & O & \to & B{-}O \\ & | & \mathrm{Na} & | & \\ & O & & O & \end{array} \]

The process of structural rearrangement of the glass that occurs upon introduction of an alkali modifier into vitreous \( \mathrm{B_2O_3} \) causes a decrease in its radiation-optical stability. This is reflected in an enhancement of the thermoluminescence intensity and in an increase in the optical absorption bands (Figs. 1 and 2). Comparison of the thermoluminescence curves

Fig. 3. TDK\(_{\mathrm{eds}}\) spectra: a) \( \mathrm{B_2O_3} \); b) 3 wt.% \( \mathrm{Na_2O} \); c) 7 wt.% \( \mathrm{Na_2O} \), irradiated with electrons at a dose of \(1.8 \cdot 10^7\) rad

of its radiation-optical stability. This is reflected in an enhancement of the thermoluminescence intensity and in the growth of the optical absorption bands (Figs. 1 and 2). Comparison of the thermolumi-

of luminescence, \(TDC_{\mathrm{EDS}}\), and the optical absorption spectra gives grounds to suppose that two types of color centers—electron and hole centers—are formed in irradiated glassy \(B_2O_3\). The following mechanism of formation of color centers is possible. Under the action of irradiation, covalent bonds in the glass are broken and free electrons appear, which are captured by traps. The holes formed in this process are localized on oxygen, since calculation has shown [6] that the electron affinity of all ions entering into the composition of the glass is several times greater than the electron affinity of oxygen; therefore, in the final analysis the hole is always localized on it. It is known [7] that none of the electron centers can be associated with localization of an electron on a modifier ion, owing to the high mobility of the latter in the loose structure of the glass, but may be associated with the network former, i.e., boron. It is possible that the electron center observed by us is an electron localized on a boron atom that, before irradiation, was in threefold coordination.

The appearance of a new optical absorption band, the second thermoluminescence peak in glasses containing \(Na_2O\), indicates a new center. The negative sign of \(TDC_{\mathrm{EDS}}\) in this region makes it possible to identify it as an electron center. Taking into account the considerations given above and the fact that boron appears in tetrahedral coordination upon introduction of \(Na_2O\), it may be assumed that it is associated with an electron localized on a boron atom that, before formation of the color center, was in tetrahedral coordination. The introduction of a modifier evidently leads to the filling of deeper traps, as evidenced by the shift of the second thermoluminescence peak toward higher temperatures.

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