ON THE STABILITY OF ALKALINE EARTH METAL FLUORIDES UNDER THE ACTION OF RADIATION

E. K. ZAVADOVSKAYA, N. M. TIMOSHENKO

For radiation materials science, the question of the radiation stability of materials is of both scientific and practical interest. Under the action of radiation, materials change their properties. Up to the present time, the radiation stability of materials has been defined as their ability, as a result of the action of radiation, to preserve their properties unchanged [1]. A material whose properties change only slightly under the action of radiation is considered stable to the action of radiation.

However, it should be borne in mind that when radiation interacts with a substance, two types of processes occur in the latter: reversible and irreversible. As is known, a certain fraction of the radiation energy is expended in the substance on the formation of defects, which remain after the cessation of the action of radiation and are responsible for changes in the properties of the substance. By means of certain influences (storage, heating, the action of light, etc.), these defects may be destroyed; in this case, the original properties of the substance will be restored, without an accompanying change in the structure of the latter. The formation of such defects under the action of radiation is a reversible process.

Radiation stability may be evaluated from the change, produced under the action of radiation, in any one property of the material. However, such a characteristic may be insufficient, since a change in a given property may often be caused not by all defects, but by some quite definite type of defect. It is expedient to choose, as a measure of reversible processes in a substance, a characteristic that includes all the total disturbances produced by radiation. Since defect formation is an energy process, the defects that remain after the cessation of the action of radiation increase the potential energy of the substance, which is customarily called stored energy. The destruction of defects is accompanied by the release of this energy; consequently, the stored energy may serve as a measure of reversible processes.

On the other hand, when radiation interacts with a substance, the greater part of the absorbed energy is expended irreversibly (see the article in this collection), which is accompanied by heating of the material during irradiation and by the formation of radiolysis products. The result of irrever-

of such processes may be the accumulation in the substance of products that differ in their structure and composition from the parent substance. Under certain conditions, the development of such processes in a substance under the action of radiation may lead to its destruction.

We propose, in evaluating the radiation stability of materials, to distinguish between reversible and irreversible radiation-induced changes in the properties of materials. From this point of view it is of interest to determine the relationship between reversible and irreversible processes both in one and the same substance and in different substances.

As the materials to be studied we chose fluorides of alkaline-earth metals, which possess one type of lattice and differ from one another in the binding energies of the ions in the lattice. As a measure of the reversible processes we took the stored energy under irradiation, determined by differential thermal analysis. As a measure of the irreversible processes we took the amount of gas released from the irradiated material during heating.

Fig. 1. Dependence of stored energy in alkaline-earth metal fluorides on absorbed dose under irradiation with $\gamma$-rays.

From the dependence, shown in Fig. 1, of the stored energy in crystals of alkaline-earth metal fluorides on the absorbed dose of $\gamma$-radiation, it follows that, at the same absorbed dose, the stored energy in the series of fluorides is greater in $\mathrm{CaF_2}$ and smaller in $\mathrm{BaF_2}$. Over the entire range of doses investigated by us, a regular dependence of the magnitude of the stored energy on the chemical composition is observed; namely: in $\mathrm{CaF_2}$—the material with the greater lattice energy in the series of alkaline-earth metal fluorides—a larger fraction of the radiation energy is accumulated than in $\mathrm{BaF_2}$—the material with the lower lattice energy. The larger value of the stored energy in $\mathrm{CaF_2}$ corresponds to a higher concentration of defects that remain after irradiation is discontinued and that are destroyed in the process of thermal activation. The greater concentration of defects also causes a stronger change in the properties of irradiated $\mathrm{CaF_2}$. Thus, the microhardness of irradiated $\mathrm{CaF_2}$ increases by a relatively larger amount than the microhardness of $\mathrm{SrF_2}$ and, still more, of $\mathrm{BaF_2}$ (see the present collection). The stored light sum in $\mathrm{CaF_2}$ crystals irradiated with X-rays and in a reactor is greater than in $\mathrm{SrF_2}$. A stronger change in electrical properties under irradiation with X-rays is observed, in the series of fluorides, for $\mathrm{CaF_2}$ [2]. According to the previously adopted evaluation, $\mathrm{CaF_2}$ should be regarded, in the series of alkaline-earth metal fluorides, as less resistant to the action of radiation than $\mathrm{BaF_2}$.

Studies of the change in the magnitude of the stored energy as a function of the absorbed dose of $\gamma$-radiation showed that in $\mathrm{CaF_2}$ the stored energy (and, consequently, the concentration of defects) continues to increase over the dose range studied (at higher doses, it is true, less intensively). In $\mathrm{SrF_2}$ and $\mathrm{BaF_2}$ crystals, at doses of $3—4 \cdot 10^8$ rad, the stored energy reaches a maximum value (Fig. 1). Beginning with these doses, all the radiation energy absorbed by $\mathrm{SrF_2}$ and $\mathrm{BaF_2}$ is expended completely on irreversible processes, the results of which should, to a greater extent

manifest itself in SrF₂ and BaF₂ than in CaF₂. This assumption was confirmed by studies to determine the amount of gas released from irradiated powders of the materials under investigation during heating.

The degassed powder, calcined in vacuum, was placed in a test tube, which was sealed to an LT-2 manometric lamp; this assembly was then placed in a γ-radiation field. After irradiation the test tube with the powder was heated, and the change in pressure in the system was recorded with the aid of the LT-2 lamp. From the change in pressure, the increase in the amount of gas in the system was calculated.

When materials irradiated with an equal dose are heated, the strongest gas evolution is observed for the weaker material—BaF₂ (Fig. 2). Consequently, the results of irreversible processes are manifested to a greater extent in the weaker compound; weakening of the structure of the parent substance by radiolysis products (gaseous ones) occurs more strongly in BaF₂.

Fig. 2. Increase in the amount of gas released from an irradiated material during heating in comparison with the amount of gas before irradiation for fluorides of alkaline-earth metals.

The foregoing becomes understandable if one takes into account the lattice energy and the relative free volume of the compounds under consideration. BaF₂ has a lower ion binding energy; therefore the breaking of bonds between them by radiation will evidently occur more intensively than in SrF₂ and CaF₂.

On the other hand, the larger relative free volume in BaF₂ apparently promotes more vigorous coagulation of radiolysis products (gas) and facilitates their release not only during heating but also during irradiation. We observed the evolution of gaseous products from BaF₂ powder during irradiation with γ-rays, whereas for SrF₂ and CaF₂ no gas evolution was observed (see in this collection). However, radiolysis products accumulate under the action of γ-radiation in CaF₂ and SrF₂, as evidenced by gas evolution from powders of these materials upon heating.

It should be noted that in a number of works attempts have been made to relate the stability of materials toward the action of radiation to the magnitude of the free volume, defined as the difference between the volume of the crystal and the sum of the volumes of the ions composing it. Thus, Canning and Hill [3, 4] developed this point of view and showed its applicability to a whole series of nitrates they studied. Baberkin [5] also came to the conclusion that free volume plays a role in radiolysis; he drew attention to the fact that the yield of radiolysis for nitrates of alkaline-earth metals decreases in parallel with a decrease in free volume. He associated the smaller radiolysis yield of zinc nitrate compared with cadmium nitrate with the smaller value of the free volume for the former salt.

Heating of the material under the action of radiation may play a major role in the formation of by-products. Chemical reactions initiated by radiation will evidently proceed more strongly on the surface of the more highly heated material. Measurement of the temperature difference between the irradiated specimen and the surrounding medium in the channel of a “cobalt-60” installation showed that, of the compounds considered, BaF₂ is heated most strongly by γ-radiation, CaF₂ less strongly. Appar-

Apparently, chemical reactions that promote the formation of by-products will proceed more vigorously on the surface of more strongly heated \(\mathrm{BaF_2}\). If, moreover, one takes into account that \(\mathrm{BaF_2}\) has lower chemical and thermal stability, the formation of by-products and the consequent weakening of the \(\mathrm{BaF_2}\) structure as a result of chemical reactions will, evidently, occur to a greater extent than for \(\mathrm{CaF_2}\).

Conclusions

1. The stored energy in a substance is a characteristic of the total radiation-induced disturbances that remain in the material after exposure to radiation and that do not cause an irreversible change in its structure.

2. In the series of alkaline-earth metal fluorides, a greater amount of energy is stored in the more stable compound—\(\mathrm{CaF_2}\); i.e., reversible radiation-induced disturbances are stronger in \(\mathrm{CaF_2}\) than in \(\mathrm{BaF_2}\).

3. Irreversible processes that cause a stable change in the structure of a substance and, under certain conditions, lead to its destruction are more strongly developed under the action of \(\gamma\)-radiation in the less stable compound—\(\mathrm{BaF_2}\).

4. Stored energy may characterize the stability of a material under the action of radiation. The more stable material should evidently be considered to be the one in which irreversible processes proceed less intensely.

Literature

1. A. A. Vorob'ev. The Action of Radiation on the Properties of Materials. Conference Proceedings, p. 5, Novosibirsk, 1963.

2. E. K. Zavadovskaya, N. M. Timoshenko, V. A. Popov, V. A. Chernyshev, L. A. Liksina. Izv. TPI, vol. 139, p. 309, 1966.

3. I. Cunningham, H. G. Heal. Nature, 179, 1021, 1957.

4. I. Cunningham, H. G. Heal. Trans Farad Soc., 54, 1355, 1958.

5. A. S. Baberkin. ZhFKh, 35, p. 373, 1961.