STORED ENERGY IN CaF₂ CRYSTALS UNDER IRRADIATION WITH X-RAYS AND GAMMA RAYS

E. K. ZAVADOVSKAYA, N. M. TIMOSHENKO

The study of the interaction of radiation with matter is one of the most important problems in solid-state physics. Under the action of radiation, all materials change their properties. Radiation resistance can be assessed by the resulting change in some single property of the material; however, such a characteristic may often be insufficient. As is known, part of the radiation energy interacting with a solid is spent on the formation of defects; a certain number of defects are retained after irradiation ceases, and therefore the irradiated material possesses additional free energy, which is customarily called stored energy.

Since energy is expended in the formation of any defect, stored energy represents the total disturbances arising in a solid under the action of radiation. It may therefore be assumed that investigation of stored energy will make it possible to judge most fully the radiation resistance of a material.

A number of works [1, 2, 3] have shown that radiation resistance is determined by the binding energy of ions in the crystal lattice. The authors suggest that in a crystal with a high lattice energy the process of defect formation under irradiation is hindered; on the other hand, defects induced during irradiation will be more stable, and the probability of their decay will be lower than in crystals with a lower lattice energy.

It may be assumed that in crystals with a higher lattice energy more defects are retained after irradiation; consequently, the magnitude of the stored energy will be greater than in crystals with weak ionic bonding in the lattice. These assumptions were confirmed in the works of E. K. Zavadovskaya and A. V. Kuzmina [4].

We measured the stored energy after interaction of X-radiation and gamma radiation with calcium fluoride crystals, which differ from alkali-halide crystals in structure and in their higher lattice energy.

The investigations were carried out on crystals grown at the Tomsk Polytechnic Institute by the Stockbarger method [5] in a graphite crucible. The crystals were irradiated at room temperature with X-rays on an RUP-200 apparatus with an integral incident dose of \(3.6 \cdot 10^6\) rad; the absorbed dose was determined by calculation [6] and amounted to \(6.6 \cdot 10^6\) rad. Irradiation with gamma rays was carried out using a cobalt-60 source with an integral dose of \(5 \cdot 10^8\) rad; the absorbed dose was, respectively, \(4.3 \cdot 10^8\) rad. The stored energy was measured by the method of differential thermal analysis in a high-temperature vacuum calorimeter.

As is seen from Fig. 1, in the spectra of stored-energy release in CaF₂ crystals, both under X-ray and under gamma irradiation, there are three peaks with maxima at temperatures of 135°C, 220°C, and 360°C. In the spectrum of stored-energy release in CaF₂ crystals irradiated with X-rays, intense peaks are observed at relatively low temperatures (135°C and 220°C); in the case of gamma irradiation, an intense peak is observed at a temperature of 360°C. The release of stored energy in CaF₂ crystals irradiated with X-rays ceases upon heating to 400°C, whereas in crystals irradiated with gamma rays no complete release of the stored energy is observed above 400°C.

Fig. 1. Spectra of stored-energy release in CaF₂ crystals after irradiation with X-rays (curve 2) and gamma rays (curve 1) upon heating to 400°C.

The magnitude of the stored energy in CaF₂ crystals depends on the absorbed dose. The absorbed dose under X-ray irradiation is less than under gamma irradiation; accordingly, the stored energy is also smaller under X-ray irradiation and amounts to 4.0 J/g under X-ray irradiation and 9.9 J/g under gamma irradiation.

Fig. 2. Absorption spectra of CaF₂ crystals after irradiation with X-rays (curve 2) and gamma rays (curve 1).

In the absorption spectra of CaF₂ crystals shown in Fig. 2, four peaks are observed, with maxima at 230, 335, 400, and 565 mμ. The positions of the maxima of the absorption bands coincide both for X-ray irradiation [7] and for gamma irradiation, which makes it possible to suppose that in both cases analogous defects are produced in CaF₂ crystals. It should be noted that upon irradiation with gamma rays, more defects are retained in CaF₂ crystals whose formation is associated with a greater expenditure of energy; namely, under gamma irradiation

the most intense absorption bands with maxima at 335 and 400 mµ, whereas under X-ray irradiation the intensities of the absorption bands are approximately the same. The greater intensity of the absorption bands of $\mathrm{CaF_2}$ crystals irradiated with gamma rays makes it possible to suppose that, at the larger absorbed dose in the case of gamma irradiation, more defects are produced than under X-ray irradiation (where the absorbed dose is correspondingly smaller), and this is reflected in the difference in the amount of stored energy. The amount of stored energy in $\mathrm{CaF_2}$ crystals under X-ray irradiation is determined mainly by defects that anneal at a lower temperature, whereas under gamma irradiation it is determined by defects that anneal at a higher temperature.

Consequently, it may be assumed that in $\mathrm{CaF_2}$ crystals under gamma irradiation, in the case of a larger absorbed dose, a greater number of more stable defects, with a higher energy of formation, is retained and, possibly, formed than under X-ray irradiation at a smaller absorbed dose.

REFERENCES

1. B. V. Budylin, A. A. Vorob’ev. The Effect of Radiation on Ionic Structures. Gosatomizdat, Moscow, 1962.

2. A. A. Vorob’ev. The effect of radiation on the properties of materials. Proceedings of a conference, Novosibirsk, 1963.

3. E. K. Zavadovskaya. The effect of radiation on the properties of materials. Proceedings of a conference, Novosibirsk, 1963.

4. E. K. Zavadovskaya, A. V. Kuz’mina. Abstracts of papers of the XII Conference on Luminescence. L’vov, 1964.

5. I. V. Stepanov, M. A. Vasil’eva. In the collection “Growth of Crystals.” Publishing House of the Academy of Sciences of the USSR, 1957.

6. B. P. Golubev. Dosimetry and Protection from Ionizing Radiation. Gosenergoizdat, 1963.

7. E. K. Zavadovskaya, N. M. Timoshenko, V. A. Popov, V. A. Chernyshev, L. A. Lisitsyn. DAN SSSR (in press).