Modification of the Radiation Resistance of Cesium Nitrate by Introducing a Carbonate Ion Additive
Abstract Generated abstract
This study examines how a 1 mole percent carbonate ion additive modifies the radiolysis and post-irradiation annealing behavior of cesium nitrate under proton irradiation. Cesium nitrate tablets with and without the additive were irradiated, then analyzed photocolorimetrically for nitrite before and after annealing at 170 °C, with optical color changes used to infer defect processes. The carbonate additive changes the dose dependence of annealing: chemical evidence of annealing appears only above about 50 Mrad, while lower doses mainly show disappearance of blue coloration attributed to F-centers. The paper proposes a defect and reaction scheme in which anion vacancies introduced by the divalent additive promote F-center formation, their destruction at higher doses, and subsequent reactions of nitrogen oxide and oxygen species that account for annealing saturation.
Full Text
Modification of the Radiation Stability of Cesium Nitrate by Introducing a Carbonate-Ion Additive
L. P. Eremin
(Presented by the scientific seminar of the Department of General and Inorganic Chemistry)
The effect of an anionic additive on the radiolysis of nitrates was discovered by us [1] during proton irradiation of $RbNO_3$ and $CsNO_3$ with an addition of $CO_3^{2-}$. The appearance of a blue coloration in the samples upon irradiation and the increase in radiation decomposition caused by the introduction of the additive were explained by the formation of $F$-centers in the irradiated nitrates. Since these effects were observed for the first time, it was expedient to continue studies of the radiation stability of nitrates with anionic additives under the action of protons.
In the present work, the influence of an anionic additive on the accumulation and post-radiation annealing of radiolysis products in nitrates was studied using $CsNO_3$ as an example. The added anion, in an amount of 1 mole %, was introduced by fusion. Samples for irradiation were prepared in the form of tablets under a pressure of $0.8\ \text{t}/\text{cm}^2$. The methods of proton irradiation and dosimetry were the same as in the preceding work [1]. Post-radiation annealing of samples irradiated at room temperature was carried out at $170^\circ C$ for 2 hours. This time is sufficient for annealing to proceed practically to completion. The analysis of radiolysis products for $NO_2^-$ before and after annealing was carried out photocolorimetrically by the method of diazotization of sulfanilic acid [2]. The results obtained are presented in the figure. The amount of annealed products was judged from the difference between the analysis results before and after annealing.
It is evident from the figure that the amount of annealed products in pure $CsNO_3$ (curves 3 and 4), as well as in alkali-metal nitrates without an additive and with an addition of divalent cations [3], tends—
Effect of dose and of an additive of 1.0 mole % \(CO_3^{2-}\) on the accumulation of radiolysis products at \(25^\circ C\) and on post-radiation 2-hour annealing at \(170^\circ C\) in cesium nitrate irradiated with protons:
1 — content of \(NO_2^-\) in specimens with the additive before annealing;
2 — the same after annealing;
3 — content of \(NO_2^-\) in specimens without the additive before annealing;
4 — the same after annealing.
tends toward saturation with the irradiation dose. This phenomenon was explained [3] by the fact that annealing in nitrates is caused by the radiolysis products: \(NO, NO_2, O_2^-(O_3^-)\), whose accumulation is associated with the presence of cation vacancies in nitrates and therefore can occur only up to a certain stationary concentration.
The introduction of a \(CO_3^{2-}\) additive into \(CsNO_3\) leads to a change in the character of annealing (curves 1 and 2) in comparison with annealing in pure \(CsNO_3\) (curves 3, 4) and in nitrates with cation additives [3]. This is expressed in the fact that annealing, detected by chemical analysis, is observed only after doses greater than 50 Mrad. Up to this dose, in the course of annealing there occurs only the disappearance of the blue coloration of the specimens, attributed to \(F\)-centers. However, the tendency of annealing toward saturation with increasing dose is also noted in the case of the anionic additive (curves 1 and 2).
At doses greater than 50 Mrad, the blue coloration of the samples weakens already during irradiation and gradually becomes yellow.
In the reflection spectra a decrease is observed in the maxima attributed to \(F\)-centers. This indicates the destruction of these centers.
To explain the results obtained, let us consider the radiation-chemical processes associated with the possible presence of anion vacancies as a consequence of the introduction of the divalent anion \(\mathrm{CO_3^{2-}}\) into \(\mathrm{CsNO_3}\).
Taking our experiments [1] into account, in this case, upon irradiation, the formation of \(F\)-centers is possible.
\[ \mathrm{NO_3^-} \rightsquigarrow \mathrm{NO_3} + \bar e, \tag{1} \]
\[ \bar e + \square^- \longrightarrow \bar e/\square^- . \tag{2} \]
The radical \(\mathrm{NO_3}\) is unstable at room temperature [4] and, being located at an anion site, can form products:
\[ \mathrm{NO_3/\square^-} \longrightarrow \mathrm{NO/\square^-} + \mathrm{O_2}, \tag{3} \]
\[ \mathrm{NO_3/\square^-} \longrightarrow \mathrm{NO_2/\square^-} + \mathrm{O}, \tag{4} \]
\[ \mathrm{NO/\square^-} + \mathrm{O} \longrightarrow \mathrm{NO_2/\square^-}. \tag{5} \]
At large doses destruction of \(F\)-centers occurs, as evidenced by the conversion of the blue coloration of the specimens to yellow. Since the yellow coloration of irradiated nitrates is associated by us with the presence of \(\mathrm{O_2^-}\) (\(\mathrm{O_3^-}\)) ions [5], the radiation destruction of \(F\)-centers can be represented as follows:
\[ \bar e/\square^- + \mathrm{O} \longrightarrow \mathrm{O^-}, \tag{6} \]
\[ \mathrm{O^-} + \mathrm{O} \longrightarrow \mathrm{O_2^-}, \tag{7} \]
\[ \bar e/\square^- + \mathrm{O_2} \longrightarrow \mathrm{O_2^-}, \tag{8} \]
\[ \mathrm{O_2^-} + \mathrm{O} \longrightarrow \mathrm{O_3^-}. \tag{9} \]
Thus, at high doses in \(\mathrm{CsNO_3}\) with an addition of \(\mathrm{CO_3^{2-}}\), radiolysis products may form: \(\mathrm{NO}\), \(\mathrm{NO_2}\), \(\mathrm{O_2^-}\) (\(\mathrm{O_3^-}\)), which are responsible for the annealing in pure nitrates with cationic additives [3]. The only difference is that \(\mathrm{NO}\) and \(\mathrm{NO_2}\) in the first case are located at anion sites, whereas in the second they are at cation vacancies. Taking this into account, the following basic scheme of annealing in \(\mathrm{CsNO_3}\) with an addition of \(\mathrm{CO_3^{2-}}\), irradiated with a large dose, may be proposed:
\[ \mathrm{NO}/\square^{-} + \mathrm{O}_2^{-} \longrightarrow \mathrm{NO}_3^{-} + \square^{-}, \tag{10} \]
\[ \mathrm{NO}/\square^{-} + \mathrm{O}_3^{-} \longrightarrow \mathrm{NO}_3^{-} + \square^{-} + \mathrm{O}, \tag{11} \]
\[ \mathrm{NO}_2/\square^{-} + \mathrm{O}_2^{-} \longrightarrow \mathrm{NO}_3^{-} + \square^{-} + \mathrm{O}, \tag{12} \]
\[ \mathrm{NO}_2/\square^{-} + \mathrm{O}_3^{-} \longrightarrow \mathrm{NO}_3^{-} + \square^{-} + \mathrm{O}_2. \tag{13} \]
In addition, atomic oxygen can cause partial disappearance of the nitrate ion or recombine with the formation of molecules [3]. Since the limiting concentration of annealed products in this case is related to the initial concentration of anion vacancies (the amount of impurity), the tendency of the amount of annealed products toward saturation at large irradiation doses becomes understandable.
As for the annealing in \(\mathrm{CsNO}_3\) with an additive at small irradiation doses, it can probably occur by the mechanism of radiation destruction of \(F\)-centers, or by the reactions:
\[ \bar{e}/\square^{-} \longrightarrow \bar{e} + \square^{-}, \tag{14} \]
\[ \mathrm{NO}_2/\square^{-} + \bar{e} \longrightarrow \mathrm{NO}_2^{-}. \tag{15} \]
The absence of a difference in the chemical analysis before and after irradiation in this case can be explained by the fact that the concentration of \(F\)-centers is possibly small, or reaction (15) also takes place upon dissolution in water of samples that have not yet been subjected to annealing.
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L. P. Eremin. Candidate’s dissertation. Tomsk, 1970.