Effect of Gamma Radiation on the Microhardness of Alkali and Alkaline-Earth Metal Fluoride Crystals

E. K. Zavadovskaya, N. M. Timoshenko

For alkali-halide crystals in a series with the same metal or halide, the microhardness increases systematically with lattice energy \[1\]. This dependence remains valid also for irradiated crystals \[2\]. It is of interest to determine whether there is a regularity in the change of microhardness with lattice energy for ionic crystals that differ from alkali-halide crystals in the type of crystal lattice, and how the action of radiation affects the change in microhardness of these materials. Crystals of alkaline-earth metal fluorides ($\mathrm{CaF_2}$, $\mathrm{SrF_2}$, and $\mathrm{BaF_2}$), which differ from alkali-halide crystals in the higher bond energies of the ions in the lattice and in the type of crystal lattice, were chosen as objects of investigation (Table 1).

Representatives of the alkali-halide crystals taken were $\mathrm{LiF}$ and $\mathrm{NaF}$. In addition to the stated objective, investigation of the microhardness of the above-mentioned

Table 1

* Dose $2.6 \cdot 10^8$ r

fluoride compounds, which are characterized by increased stability (chemical, thermal, etc.), is also of interest for radiation materials science. Microhardness measurements were carried out using a PMT-3 on crystals grown by the authors by the Stockbarger method \[3\].

Irradiation with gamma rays was carried out on a “cobalt-60” unit in a dose field with an intensity of 450 r/sec; the radiation energy was 1.24 MeV.

As can be seen from Table 1, the regular decrease in microhardness with decreasing lattice energy, observed for the alkali-metal fluorides (LiF and NaF), is also preserved for the alkaline-earth-metal fluorides.

Irradiation of crystals with gamma rays is accompanied by an increase in microhardness caused by the accumulation of radiation defects. When the microhardness reaches saturation after irradiation, the relative change in microhardness \((H_{\max}/H_{\mathrm{nom}})\) is greater for the alkali-metal fluorides than for the alkaline-earth fluorides (Fig. 1).

It may be assumed that radiation damage in alkali-metal fluorides is more significant than in alkaline-earth fluorides, which also accounts for the difference in the magnitude of the change in microhardness.

Fig. 1. Dependence of the relative change in the microhardness of fluorides on their lattice energy. \((H_{\max}\)—maximum value of microhardness when saturation is reached in the case of irradiation). \((H_{\mathrm{nom}}\)—value of microhardness before irradiation).

The fact that in LiF the concentration of defects induced by \(\gamma\)-radiation is significantly higher than in alkaline-earth fluorides is confirmed by measurements of optical absorption and stored energy [4, 5].

At present it is not possible to specify concretely the type of radiation defects responsible for the hardening of the crystals under consideration. According to Nad’ [6], the hardening of LiF crystals irradiated with small doses of \(\gamma\)-rays is due to \(F\)-centers, or more precisely to interstitial fluorine, which is formed as a result of the generation of Frenkel pairs by the Varley mechanism [7]. Our investigations (because of limitations on the length of the article we cannot discuss these results in detail)—of changes in microhardness, stored energy, and optical absorption during thermal annealing—give grounds for agreeing with the propositions put forward by Nad’. In another work [8], the authors attribute the increase in the microhardness of KCl crystals after irradiation with \(\gamma\)-rays

are also explained by an increase in the plastic-flow stress owing to the appearance of incorporated ions. The question of the radiation defects responsible for the increase in the microhardness of alkaline-earth fluorides remains open for the time being.

In a series of crystals having the same lattice type, under identical irradiation conditions, the microhardness of the crystal with the higher lattice energy increases more rapidly with dose and, in the saturation region, reaches a relatively greater increase; moreover, the microhardness reaches saturation at higher irradiation doses than in the case of a crystal with lower lattice energy (compare NaF and LiF, BaF₂ and CaF₂, Fig. 1 and Fig. 2).

As can be seen from Fig. 2, the increase in microhardness with irradiation dose deviates from a linear dependence already in the initial period of irradiation.

Fig. 2. Dependence of the microhardness of fluoride crystals on the gamma-radiation dose

This is evidently due to the fact that the process of accumulation of radiation defects in the crystal is accompanied by a process of radiation annealing, whose influence increases with the irradiation dose.

The saturation of the microhardness is caused by the establishment of a dynamic equilibrium between two processes—the formation of defects and radiation annealing. The relatively greater change in microhardness in crystals with higher lattice energy (for crystals with the same lattice type) should probably be explained by the fact that the reverse processes—radiation and thermal annealing—proceed more slowly than in crystals with weak ionic bonding. The residual effects of radiation hardening prove to be more stable in CaF₂ and LiF and less stable in BaF₂ and NaF. This is evidenced by the results of the study of the influence of annealing on the change in microhardness of irradiated crystals (Table 1) and by the spectra of release of stored energy [4].

The change in microhardness with dose, as well as its reaching saturation under gamma-ray irradiation, makes it possible to select, for each material, the optimal irradiation doses required in radiation technology for a purposeful increase in the hardness of the materials studied.

REFERENCES

1. A. A. Vorob’ev, A. M. Trubitsyn. Zh. Tekhn. Fiz., 26, 330, 1956.

2. B. V. Budylin, A. A. Vorob’ev. The Action of Radiation on Ionic Structures. Gosatomizdat. Moscow, p. 73, 1962.

3. I. V. Stepanov, P. P. Feofilov. In: Crystal Growth. Publishing House of the Academy of Sciences of the USSR, Moscow, p. 229, 1957.

4. E. K. Zavadovskaya, N. M. Timoshenko. Abstracts of reports of the First Conference on Radiation Physics of the Solid State. Kiev, p. 35, 1965.

5. N. M. Timoshenko. Radiation Physics of Nonmetallic Crystals, proceedings of the conference, Kiev, p. 305, 1967.

6. I. S. Nadeau. J. Appl. Phys., 33, 12, 1962.

7. J. Varley. Color Centers. Collection of articles, Moscow, Foreign Literature Publishing House, 1958.

8. T. K. Bhattacharya, H. Nonenmacher, L. J. Grossmeiner. J. Appl. Phys., 35, 1352, 1964.