ON A METHOD FOR OBTAINING KINETIC CURVES OF COLOR-CENTER ACCUMULATION UNDER THE ACTION OF X-RAY RADIATION IN MICROLAYERS OF ALKALI-HALIDE SINGLE CRYSTALS

L. I. Roshchina

(Presented by the scientific seminar of the Research Institute of Solid-State Physics)

In the radiation physics of solids, one of the important methods of investigation is the kinetics of accumulation of color centers formed under the action of radiation. Radiation kinetics studies the regularities in the change of the properties of solids as a function of the dose, intensity, and type of radiation, and makes it possible to obtain important information about the microprocesses underlying this radiation-induced change in properties. However, a quantitative study of the concentration of color centers arising under the action of X-ray radiation in alkali-halide crystals is made difficult by the nonuniform distribution of coloration over the depth of the crystal. This nonuniform coloration is caused by attenuation of the beam of X-rays with increasing specimen thickness. The greatest coloration occurs on the face side of the crystal, while the gradient of the concentration of color centers depends on the energy and hardness of the incident X-rays and on the absorbing capacity of the crystal.

Until now, in all investigations of the kinetics of radiation accumulation of color centers, the optical measurement of the concentration of \(F\)-centers has been carried out in the direction coinciding with the direction of irradiation, which gave an integral value of the concentration of centers over the thickness. This introduces so large an error in a quantitative study of the kinetics of accumulation of color centers that, in a number of cases, it rules out the possibility of determining important radiation parameters by means of the kinetic curve of color-center accumulation.

However, the inhomogeneity of coloration over depth could be taken into account by carrying out optical measurements in a direction perpendicular to the direction of irradiation. Such a study was first performed by Mador and others [1] for NaCl and LiF crystals. The authors of that work were interested in the distribution of coloration over the depth of the crystals and in the kinetics of accumulation of \(F\)- and \(M\)-centers in different portions of the specimen. Mador and others succeeded in measuring the absorption spectrum of an irradiated crystal only at a distance of 1–2 mm from the edge of the crystal. In the optical measurements the width of the light-beam bundle was 0.4 mm. This introduced a certain error into the measurement of the concentration of color centers, since there is considerable nonuniformity in their distribution within a segment of size 0.4 mm.

Thus, the method proposed by Mador could not be used for crystals with high absorbing power, for example, KCl, KBr, KI. On the other hand, the kinetic curve of the accumulation of \(F\)-centers in KCl has the most complex form, which ensures that the greatest amount of information can be obtained from this curve [2]. As a result, we had to substantially improve the method for measuring absorption spectra in microlayers.

The arrangement of the apparatus is shown in Fig. 1.

Fig. 1. Diagram of the apparatus:
1 — crystal, 2 — crystal holder (\(2'\) — position of the holder during photometry, \(2''\) — during irradiation), 3 — slit \(30\,\mu\) wide, 4 — SF-4 spectrophotometer, 5 — slit \(0.5\,\text{mm}\) wide, 6 — photocell, 7 — micrometer, 8 — URS-55A X-ray apparatus, 9 — chamber.

The crystal 1 is mounted in holder 2, which makes it possible to adjust the edge of the crystal so that: a) it is parallel to the \(30\,\mu\) slit 3, b) the effect of reflection from the end surface of the crystal is reduced as much as possible. During optical measurements the crystal holder is in position \(2'\). In this case the light beam from the slit of the SF-4 spectrophotometer (4) falls on slit 5, passes through crystal 1, through slit 3, and reaches photocell 6. From the light flux passing through the crystal, slit 3 cuts out a narrow beam of rays \(30\,\mu\) wide. Slit 5, \(0.5\,\text{mm}\) wide, limits the light flux incident on the crystal, thereby reducing bleaching of the color centers. During optical measurements the crystal is stationary, while slits 3 and 5 are moved by means of micrometer 7. This makes it possible to photometer the specimen in \(10\,\mu\) steps over the entire depth of the crystal. Reproducibility is guaranteed within \(\pm 5\,\mu\). During irradiation the crystal holder is in position \(2''\) under the URS-55A X-ray apparatus 8. The X-rays are directed downward. The specimen is irradiated at the end face. The distance from the tube window to the crystal is \(15\,\text{mm}\). During irradiation and photometry the crystal holder with the specimen is located in chamber 9, which completely protects the specimen from penetration of light. The time between stopping the irradiation and photometry can be reduced to several seconds.

Depending on the hardness of the X-rays, the distribution of coloration over the depth of the crystal is different. Figure 2, a shows curves of the decrease in the concentration of \(F\)-centers with the thickness of a KCl crystal irradiated with X-rays from a tube with copper (1) and molybdenum (2 anode).

(Operating conditions: \(14\,\text{ma}\), \(42\,\text{kv}\).) Softer X-rays from a tube with a Cu anode are absorbed more strongly in the first layers of the crystal.

KCl, and, consequently, the decrease in the concentration of \(F\)-centers with depth is sharper than for a crystal irradiated by rays from a tube with a Mo anode.

Fig. 2. \(a\)—curves of the dependence of the concentration of \(F\)-centers on the depth of a microlayer of a KCl crystal irradiated with X-rays from a tube with a copper (1) and molybdenum (2) anode; \(b\)—kinetic curves of the accumulation of \(F\)-centers in a KCl crystal at distances of 0.07 (1), 0.3 (2), and 0.7 (3) mm from the irradiated surface.

On this setup, accumulation curves of \(F\)-centers were obtained in a KCl crystal at depths of 0.07, 0.3, and 0.7 mm from the edge of the crystal (Fig. 2, \(b\); curves 1, 2, and 3, respectively). X-ray tube with a Mo anode, 14 mA, 42 kV. Kinetic curves for the accumulation of color centers in alkali-halide single crystals can be obtained for microlayers located at any distance from the irradiated surface.

The author expresses gratitude to I. Ya. Melik-Gaikazyan for supervising the present work.

REFERENCES

1. I. L. Mador, R. F. Wallis, W. C. Williams, R. C. Herman. Phys. Rev. 96, 617–628, 1954.

2. P. Mitchell, D. Wiegand, R. Smoluchowsky. Phys. Rev. 121, 484, 1961.

3. E. Abramson and M. E. Caspari. Phys. Rev. 129, 536, 1963.