On the Phosphorescence by Visible Light of X-rayed Minerals, Crystals, and Powders

A. A. Vorob'ev, A. F. Kalganov, and V. A. Moskalev

After preliminary X-raying, the emission of visible light is observed in many substances, some of which are presented in the table. The specimens were exposed to X-rays for 60–90 min at a tube current of 10 ma, a voltage of 100–150 kv, and at a distance of 15 cm from the tungsten anticathode. The total radiation dose falling on the specimen reached up to 30,000 r at an irradiation intensity of up to 330 r/min. A sufficiently intense and prolonged afterglow of minerals was also observed after their irradiation with X-rays for several minutes. The radiation from the specimens is clearly visible in the dark. Some crystals—for example, Iceland spar, crystals of solid solutions of the systems NaCl—NaBr and NaCl—KCl, fluorite, and others—glow so strongly that fine scratches are visible on their surface, and near the specimen one can distinguish the letters of printed text. Damaged regions of a crystal, for example its unevenly broken edges, glow more intensely. A scratch on the surface of a NaCl or KCl crystal flashes brightly at the moment it is formed.

Crystals of alkali-halide salts, NaCl, KCl, KBr, and others, grown from chemically pure starting materials, glow considerably more weakly than crystals of solid solutions of these same salts. The indicated regularities are in agreement with the theory of luminescence [1; 7]. According to the theory of luminescence, radiation from crystals is possible only in the presence of disturbances of the crystal lattice. The phenomenon we observed—emission by X-rayed crystals and powders in the visible part of the spectrum—can be explained by transitions of electrons from one energy level to another. For crystals of alkali-halide salts, the glow is associated with the transition of electrons from the conduction band to levels.

The radiation of the specimens after their X-raying weakens with time. When the specimens are kept in the dark, they phosphoresce longer. After some time of storage in the dark, the specimen retains its coloration, but no radiation from it is noticeable. For example, KBr or NaCl crystals, after being stored in the dark for three days, showed no noticeable radiation, although their coloration was preserved. When the specimens are stored in the light, they become decolorized and lose the ability to radiate (are bleached) after several hours or tens of hours.

With increasing temperature, the intensity of radiation from the specimen increases. The duration of the glow decreases with increasing temperature. At high temperature the crystal may lose its coloration and cease to radiate within several minutes. Powders, with increasing temperature, also considerably increase the intensity of radiation. An X-rayed specimen that has lost the ability to radiate, after being stored for some time, upon heating to a temperature of 200–300°C again gives

bright, but short-lived, emission. Short-lived phosphorescence is also observed after irradiation with ultraviolet light. The color and intensity of the luminescence emission depend on the composition of the chemical compound, its crystal structure, and the degree to which it has been disturbed by impurities and mechanical treatment. The degree of comminution of the crystals—the total size of their surface—has a significant influence on the intensity of the emission.

An X-rayed specimen placed on photographic film causes it to blacken. A crystal wrapped in black paper and placed on photographic film does not give radiation that penetrates the wrapping and exposes the photographic layer.

The coloration of melts of salts of alkali-halide compounds by X-rays, and their subsequent emission, was also observed.

During deformation and crushing of X-rayed and non-X-rayed crystals, a flash of emission was observed.

Transparent crystals of alkali-halide salts become colored under the action of X-rays and harder rays, and electron bombardment. Coloration of the crystals was also observed when they absorbed radiation corresponding to intrinsic absorption. It is known that the edge of the intrinsic-absorption band lies in the ultraviolet part of the spectrum. Other methods of coloring alkali-halide and other ionic crystals are also known, for example by heating the crystals in vapors of metallic sodium or potassium, and others [2].

The appearance of additional absorption bands in ionic crystals is associated with the formation in them of electron-trapping centers. According to data on the absorption spectra of X-rayed crystals of alkali-halide compounds in the ultraviolet region, the color centers in these crystals are divided into two types: hole centers ($D$-centers) and electron centers ($E$-centers). Experimental data lead to the conclusion that, at low temperatures, the emission of X-rayed alkali-halide crystals is due mainly to the recombination of $E$-centers and positive holes thermally released from $D$-centers. When the crystal is heated, the absorption band of $D_1$-centers that occurs at low temperatures completely disappears, as do about 60% of the $F$-centers [3; 8]. At present, stable electron-trapping centers $F$, $R$, $M$, and $N$, and unstable trapping centers $F'$ and $R'$, are distinguished. The $F$ trapping center is a vacant site in the crystal lattice formed after the removal of a negative halide ion. The other centers ($M$, $N$) are combinations of several neighboring $F$-centers. In the presence of trapping centers in an ionic crystal, additional energy levels appear in the energy spectrum for electrons, situated between the ground state and the conduction band. As a result, electronic transitions from one level to another, with emission and absorption of a quantum of visible-light energy, become possible.

When such a crystal is irradiated with radiant energy of wavelength shorter than that corresponding to the edge of intrinsic absorption, i.e., 160 millimicrons, fluorescence should be observed in accordance with possible transitions from the conduction band to lower-lying levels and to the ground state. The transition of electrons from the conduction band to the $F$ level will be accompanied by the emission of light in the short-wavelength part of the visible spectrum.

K. D. Sinelnikov, A. K. Walter, I. V. Kurchatov, and S. Litvinenko [4] observed an intense blue glow during the coloration of NaCl by X-rays at low temperatures.

Table 1

Emission of visible light by X-rayed minerals, crystals, and powders

Continuation of Table No. 1

V. M. Kudryavtseva [5] observed ultraviolet phosphorescence of colored rock-salt crystals when illuminated with blue light. Irradiation with blue light caused transitions of electrons from the \(F\)-band into the conduction band. The transition of electrons from the conduction band into the main band was accompanied by fluorescence in the ultraviolet region. N. N. Fedenev [6] observed radiation accompanying transitions of electrons from the conduction band to the level \(U\) and from the level \(F\) to the level \(U\) when a crystal was irradiated with white light.

In crystals with a disturbed structure, it is probably also possible for them to glow as a result of the following double transitions. The transition of an electron from some level, for example \(F\), into the main band will be accompanied by the emission of an energy quantum of the order of five electron volts. Under the action of such a quantum, the transition of electrons from some level into the conduction band is possible, as is its reverse transition to the level \(F\). Such a transi-

the transition will likewise be accompanied by the emission of a light quantum in the short-wavelength part of the spectrum (the blue-violet part).

G. P. Barsanov and V. A. Sheveleva [7], who recently published summary materials on the study of the luminescence of minerals (4073 specimens), indicate that prolonged phosphorescence perceptible to the eye is observed comparatively rarely. Among minerals this property has been noted only in certain calcites.

The described phenomenon of luminescence of minerals and chemical compounds after their irradiation with X-rays may find application in luminescence analysis. The use of an X-ray installation instead of the widespread method of cathodoluminescence has its advantages. The prolonged afterglow of minerals that occurs after their irradiation with X-rays also has advantages for luminescence analysis in comparison with fluorescence under the action of cathode or ultraviolet rays.

The phosphorescence of materials after the action of a dose of X-rays may be used for indication and dosimetry of hard radiation, especially at elevated temperature. The sensitivity of the phenomenon to mechanical actions during treatment of a material may find application in indicating deformation of the material and its destruction, etc.

Assoc. Prof. E. K. Zavadovskaya took part in developing the initial data for planning the investigations. Assoc. Prof. E. K. Zavadovskaya, postgraduate student A. M. Trubitsyn, and Assoc. Prof. A. I. Aleksandrov provided us with many specimens for the investigations. The authors take this opportunity to express their gratitude to them.

LITERATURE

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4. Sipelnikow C., Walfher A., Kurtschatow I., Litwinenko S., Einfluss der Temperatur auf die Röntgenisierung von Steinlzsalz, Phys. Zeit. Sowjetunion, B. 3 H. 3, 262—267, 1933.
5. Kudryavtseva V. M. ZhETF, 4, 709—716, 1934.
6. Fedenev N. N. Trudy SFTI, 4, 208—212, 1937.
7. Barsanov G. P. and Sheveleva V. A. Trudy Mineralogical Museum of the Academy of Sciences of the USSR, issue 4, 1—35, 1952.
8. Adirovich E. I. Some Questions in the Theory of Luminescence of Crystals, GIZ, TTL, 1951.