THERMAL ANNEALING OF RADIATION DAMAGE IN ALKALI METAL NITRATES

Yu. A. ZAKHAROV, E. P. ABAKUMOV

(Presented by the scientific seminar of the Department of Radiation Chemistry)

The principal regularities of the radiolysis of alkali metal nitrates have now been studied in sufficient detail [1].

The final products of radiolysis are nitrite, intermediate forms of oxidized nitrogen, and oxygen. Radiolytic oxygen, upon irradiation at room temperature, is retained in the crystal lattice up to 10–13% decomposition of the nitrate. When irradiated crystals are heated, the retained oxygen is released; at the same time, in parallel with this, conditions are improved for the diffusion and recombination of the nitrite ion and radiolytic oxygen.

Earlier studies carried out on annealing phenomena [1] in irradiated nitrates, chromates, and chlorates were, for the most part, qualitative and preliminary in character. The influence of various factors—such as pressing, grinding, irradiation with a softer type of radiation, and the introduction of various additives—on the thermal annealing of radiation products was observed.

In order to clarify in greater detail the role of various factors, we carried out a comparative study of thermal annealing using a group of alkali metal nitrates as an example. The change in the rate of radiolysis of alkali metal nitrates in the series of salts from LiNO₃ to CsNO₃ is associated with the change, along the series, in the magnitude of the free volume of the crystal. Therefore it is of interest, in particular, to establish whether there is a correlation between the magnitude of the free volume and the degree of annealing of radiation damage in nitrates.

The chemically pure nitrates used in the work were recrystallized twice from distilled water. The resulting crystals were ground, and the 150–200 micron fraction was taken for investigation. Irradiation was carried out with protons of energy 4.8 MeV on the cyclotron of the Scientific Research Institute of Nuclear Physics at Tomsk Polytechnic Institute. Accumulation curves of nitrogen-oxygen products during irradiation of the nitrates were constructed. The initial radiation yields calculated from these data were 0.6; 1.1; 1.70; and 1.95, respectively, for NaNO₃, RbNO₃, CsNO₃, and KNO₃. These values are close to those reported in the literature, which relate mainly to γ-irradiation [1].

The absorbed dose of protons was determined by calculation [2] and ranged from 150 Mrad for CsNO₃ to 400 Mrad in the case of NaNO₃.

The content of nitrite ion and of the sum of NO and NO₂ was determined colorimetrically from the coloration obtained with the reagent dimethyl-α-naphthylaminesulfanilic acid.

The degree of annealing was expressed as

\[ \alpha=\frac{C_{\text{initial}}-C_t}{C_{\text{initial}}}\%, \]

where:

\(C_{\text{initial}}\) — initial concentration of nitrite ion;
\(C_t\) — concentration of nitrite ion at an annealing time equal to \(t\).

Annealing was carried out at temperatures of 80°C and \(185 \pm 0.5^\circ\text{C}\) and lasted up to 4 hours. The curves of thermal annealing of radiation-produced nitrogen-oxide products in nitrates are shown in Fig. 1.

Fig. 1. Thermal annealing of radiation damage in alkali-metal nitrates.
Curves 1, 2, 3 — annealing at 185°C; curves 1′, 2′, 3′ — annealing at 80°C; curves 1, 2, 3 correspond respectively to CsNO₃, RbNO₃, NaNO₃.

The data obtained indicate a regular increase in the rate and completeness of annealing in the series of nitrates NaNO₃—RbNO₃, CsNO₃, parallel to an increase along the series in the free volume of the crystal. The existence of the established correlation between the tendency toward annealing and the free volume of the crystal may naturally be connected with the facilitation of diffusion conditions for radiolysis products as the free volume of the crystal increases. Thermal annealing in irradiated nitrates is most likely composed of processes of oxidation by radiolytic oxygen of the nitrogen-oxide products (\(\mathrm{NO_2^-}\), NO) to the nitrate anion. Facilitation of the diffusion conditions for radicals and molecular radiolysis products should in this case, naturally, lead to an acceleration of the annealing process.

Thus, in compounds with a relatively large free volume, both the accumulation processes and the annealing processes of radiolysis products are facilitated. Since the latter develop at elevated temperatures and have a large activation energy, the sequence of radiation stability of nitrates under these conditions may be less

distinctly expressed, or even reversed in comparison with the low-temperature stability series. This same reason may be one of the probable explanations for the fact that the series of radiation stability of nitrates under irradiation with α-particles (high temperatures) is less distinctly expressed than in the cases of radiolysis by γ-rays or by fast electrons [1].

References

1. Yu. A. Zakharov, V. A. Nevostruev, present collection, p. 60.
2. A. P. Oblivantsev, V. M. Lykhin et al., present collection, p. 116.