THERMOLUMINESCENCE OF X-RAY-IRRADIATED CRYSTALS OF ALKALINE-EARTH METAL FLUORIDES

E. K. ZAVADOVSKAYA, N. M. TIMOSHENKO, A. I. YAKIMENKO

In contrast to alkali-halide crystals, the thermoluminescence of alkaline-earth fluoride crystals has been studied in a very small number of works. In these works the subject of investigation was mainly either natural crystals of \(\mathrm{CaF_2}\) [1] or artificial ones activated by rare earths [2, 3].

The thermoluminescence of crystals of the fluorides of Sr and Ba has practically not been studied. Taking the above situation into account, we carried out investigations of the thermoluminescence of \(\mathrm{CaF_2}\), \(\mathrm{SrF_2}\), and \(\mathrm{BaF_2}\) crystals, which made it possible to establish certain regularities in this series of compounds.

Crystals of the fluorides of alkaline-earth metals, excited by X-rays, exhibit intense luminescence upon heating. The thermoluminescence curves of these crystals (Fig. 1) consist of peaks with maxima at temperatures: for \(\mathrm{CaF_2}\)—51, 87, 127, 231, 312; for \(\mathrm{SrF_2}\)—60, 90, 140, 220, 295°C; and for \(\mathrm{BaF_2}\)—60, 90, 120, 150, 200°C. The most intense peaks in the thermoluminescence curves are observed at relatively low temperatures and occur in \(\mathrm{CaF_2}\) at a temperature of 90°C, in \(\mathrm{SrF_2}\) at 140°C, and in \(\mathrm{BaF_2}\) at 90°C.

Fig. 1. Thermoluminescence curves of X-ray-irradiated crystals of alkaline-earth metal fluorides:
1 — \(\mathrm{SrF_2}\) (Y) \((m = 120\ \mathrm{mg},\ D_p = 5 \cdot 10^3\ \mathrm{rad})\);
2 — \(\mathrm{SrF_2}\) \((m = 120\ \mathrm{mg},\ D_p = 5 \cdot 10^3\ \mathrm{rad})\);
3 — \(\mathrm{BaF_2}\) \((m = 100\ \mathrm{mg},\ D_p = 5 \cdot 10^3\ \mathrm{rad})\);
4 — \(\mathrm{CaF_2}\) \((m = 10\ \mathrm{mg},\ D_p = 5 \cdot 10^3\ \mathrm{rad})\);
5 — \(\mathrm{CaF_2}\) (Y) \((m = 10\ \mathrm{mg},\ D_p = 5 \cdot 10^3\ \mathrm{rad})\).

The analogous character and the identical number of peaks in the thermoluminescence curves indicate an analogy of the defects created by irradiation in crystals with a fluorite-type lattice.

The maxima of the thermoluminescence peaks of the \(\mathrm{BaF_2}\) crystal, with the exception of the first two, are shifted into the region of lower temperatures than ԥ

in \( \mathrm{SrF_2} \). At the same time, the latter peak in \( \mathrm{SrF_2} \) occurs at a lower temperature than in the \( \mathrm{CaF_2} \) crystal.

The thermoluminescence spectra give grounds for assuming that greater thermal stability of radiation defects is observed in the material with the greater lattice energy.

The dose dependence of the emitted light sum (LS) (Fig. 2) of \( \mathrm{CaF_2} \) crystals has a two-stage character: an intense increase in the range of absorbed doses \((D_{\mathrm{p}})\) up to \(1 \cdot 10^3\) rad is replaced by a tendency toward saturation at higher doses. At the same time, a change in the thermoluminescence spectrum with dose is observed. Upon irradiation of a \( \mathrm{CaF_2} \) crystal for 15 minutes \((D_{\mathrm{p}} = 7.2 \cdot 10^2\) rad), an intense growth of the first peak is observed on the thermoluminescence curve; however, with increasing irradiation intensity,

Fig. 2. Dependence of the emitted light sum \((S)\) of \( \mathrm{CaF_2} \), \( \mathrm{SrF_2} \), and \( \mathrm{BaF_2} \) crystals on the absorbed dose \((m = 10\ \mathrm{mg})\).

Fig. 3. Dependence of the emitted light sum \((S)\) on the lattice energy of X-irradiated crystals of alkaline-earth fluorides \((m = 10\ \mathrm{mg},\ D_{\mathrm{p}} = 8 \cdot 10^3\ \mathrm{rad})\).

its intensity decreases, and at \(D_{\mathrm{p}} = 7.2 \cdot 10^3\) rad (irradiation time 2.5 hours) it is completely absent. Simultaneously, with the decrease in the intensity of the first peak, an increase in the intensity of the second is observed, whose maximum occurs at a temperature of \(87^\circ\mathrm{C}\). From this one may suppose that the defects responsible for the second peak of the thermoluminescence curve are the result of transformation of the defects responsible for the first peak.

The dose dependence of the LS of an X-irradiated \( \mathrm{SrF_2} \) crystal (Fig. 2) also has a two-stage character; however, in comparison with the \( \mathrm{CaF_2} \) crystal, its saturation is observed at lower absorbed doses. The LS of a \( \mathrm{BaF_2} \) crystal (Fig. 2) reaches saturation beginning with an absorbed dose equal to \(1.3 \cdot 10^3\) rad. Thus, the LS of a crystal with a lower lattice energy reaches saturation at lower absorbed doses than in a compound with a higher lattice energy.

The dependence of the LS on the lattice energy of alkaline-earth fluoride crystals is presented in Fig. 3. With increasing lattice energy of the compound, an increase in LS is observed; moreover, for the \( \mathrm{CaF_2} \) crystal—the material with the greatest lattice energy in the series—there is a sharp increase in the light sum in comparison with the \( \mathrm{BaF_2} \) crystal, the material with the lowest lattice energy. Since the LS is proportional to the concentration of electron-hole centers preserved in the crystal after pre-

of irradiation coloration, then their concentration in the series of alkaline-earth metal fluoride crystals decreases as the lattice energy of the compounds decreases.

According to Fong’s data [4], when crystals of alkaline-earth fluorides are irradiated up to absorbed doses of \(5 \cdot 10^6\) rad, no structural changes are observed. Apparently, the generation of defects responsible for thermoluminescence in the dose range studied by us occurs at defects already present in real crystals before irradiation. The initial concentration of such defects is greater in the compound with the higher lattice energy, which is also confirmed by measurements of the electrical properties of unirradiated crystals [5].

The large yield of light energy upon thermal bleaching of an X-rayed CaF\(_2\) crystal makes it possible to recommend these crystals as a material for dosimetric systems.

The thermal-bleaching curves of X-rayed CaF\(_2\) crystals satisfy the criteria established by Ch. B. Lushchik [6] for the case when the probability of recapture of electrons is small in comparison with the probability of their recombination at luminescence centers. This made it possible to calculate the value of the energy \((E)\) of the electron-trapping levels from the characteristics of the peaks \((T\) and \(\delta_2)\).

\[ E = \frac{kT_m^2}{\delta_2}, \ \text{eV}, \]

where

\[ \begin{aligned} k &\text{ is Boltzmann’s constant;}\\ T &\text{ is the temperature of the peak maximum, }{}^\circ K;\\ \delta_2 &= T_2 - T; \quad T_2 \text{ is the temperature on the descending part of the peak at which}\\ &\text{the intensity is equal to one half of the maximum.} \end{aligned} \]

Calculation of the energies of the trapping levels for a CaF\(_2\) crystal made it possible to obtain the relation

\[ \left(E = \frac{T_m}{400}\right), \]

which was used in calculating the energies of electron-trapping levels in X-rayed SrF\(_2\) and BaF\(_2\) crystals (Table 1).

The thermal-bleaching curve of a CaF\(_2\) crystal irradiated for three hours with X-rays consists of 4 peaks with maxima at temperatures of 87, 127, 231, and 312°C. During storage

Table 1

Energies of electron-trapping levels in X-rayed CaF\(_2\), SrF\(_2\), and BaF\(_2\) crystals

of the X-rayed crystals, a decrease in the intensity of the peaks on the thermal-bleaching curves is observed. An especially strong decrease in intensity is observed for the peak with a maximum at 87°C, which after 15 days of storage disappears completely. The intensity of the peak with a maximum at 127°C decreases slightly.

significantly during storage of the crystal for up to 15 days; then an intense destruction of the latter is observed. The circumstance described above can evidently be explained as follows.

Isothermal destruction, at room temperature, of the defects responsible for the peak with a maximum at a temperature of 87°C is accompanied

Fig. 4. Dependence of the emitted light sum \((S)\) on the storage time of X-rayed crystals of CaF\(_2\), SrF\(_2\), and BaF\(_2\) \((D_{\text{p}} = 5 \cdot 10^4\ \text{rad})\).

by the release of electrons from a level with \(E = 0.9\) eV and their subsequent capture by a level with energy \(E = 1.03\) eV. The latter leads to the fact that the rate of decrease of the intensity of the peak with a maximum at 127°C is smaller in the presence of the preceding peak than in its absence. After destruction of the defects responsible for the peak at 87°C, the pumping of the level with \(E = 1.03\) eV by electrons ceases and, consequently, the rate of destruction of the defects responsible for the peak with a maximum at 127°C increases, while the intensity of the peak with a maximum at a temperature of 231°C changes only slightly. Analogous phenomena are observed for SrF\(_2\) and BaF\(_2\) crystals as well and thus indicate the complex process of defect transformation in X-rayed crystals during storage. The decrease in the intensity of the thermoluminescence peaks of crystals during storage is accompanied by a decrease in the magnitude of the light sum (Fig. 4), the dependence of which has a two-stage character. An intense decrease in the light sum during storage of X-rayed crystals of the fluorides of Ca, Sr, and Ba for 10 days is accompanied, at longer storage times, by a slight decrease in it. It should be noted here that the rate of decrease of the light sum with storage time increases as

Fig. 5. Dependence of the emitted light sum \((S)\) of crystals \((1\)—CaF\(_2\) (Y), \(2\)—SrF\(_2\) (Y)) on the concentration \((C)\) of YF\(_3\). CaF\(_2\) (Y) \((m = 10\ \text{mg},\ D_{\text{p}} = 1 \cdot 10^3\ \text{rad})\), SrF\(_2\) (Y) \((m = 120\ \text{mg},\ D_{\text{p}} = 1 \cdot 10^3\ \text{rad})\).

the decrease in the lattice energy of the compound, thereby confirming the assumption of the lower thermal stability of radiation defects in BaF$_2$ [18].

The thermoluminescence curves of X-rayed CaF$_2$ and SrF$_2$ crystals (Fig. 1) activated with YF$_3$ are similar to the curves for pure crystals: the peak maxima of these curves occur, for both pure and impurity-containing crystals, at the same temperatures.

It should be noted that in CaF$_2$ (Y) crystals an increase is observed in the intensity of the peak with a maximum at 312°C, and for SrF$_2$(Y) of the peak with a maximum at 220°C. The specific growth of these peaks, on the basis of literature data [7], makes it possible to regard interstitial fluorine ions as responsible for their occurrence.

With increasing concentration of the YF$_3$ impurity in CaF$_2$ and SrF$_2$ crystals, the light sum increases (Fig. 5); however, beginning with a concentration of 0.25 mol% YF$_3$, it decreases, which apparently should be attributed to the presence of concentration quenching of thermoluminescence.

Conclusions

1. The thermoluminescence curves of CaF$_2$, SrF$_2$, and BaF$_2$ crystals have a similar character, which makes it possible to judge the analogy of the defects produced by irradiation in crystals with one type of lattice.

2. In the material with the higher lattice energy, CaF$_2$, the concentration of radiation defects is greater and their thermal stability is higher than for BaF$_2$—the crystal with the lower lattice energy in the fluoride series.

3. The dependence of the light sum of alkaline-earth-metal fluoride crystals on the absorbed dose has a two-stage character.

4. With increasing irradiation dose in the CaF$_2$ crystal, a transformation of low-temperature defects into higher-temperature ones is observed.

5. The rate of decrease of the light sum with storage time increases as the lattice energy of the compound decreases.

6. The introduction of the YF$_3$ impurity increases the luminescence of the crystals.

Literature

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3. V. R. Arkhangel’skaya. Optics and Spectroscopy, XIV, 1, 336 (1961).

4. Fong F. K. J. Chem. Phys., 41, 5, 1383 (1964).

5. E. K. Zavadovskaya, N. M. Timoshenko, V. A. Popov, V. A. Chernyaev, L. A. Lisitsyna. Izv. TPI, vol. 139, 309 (1965).

6. Ch. B. Lushchik. DAN SSSR, 101, 641 (1955).

7. Scouler W. I. and Smakula A. Phys. Rev., 120, 15, 1154 (1960).

8. N. M. Timoshenko. Dissertation, Tomsk, 1966.