PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Volume 184　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1970

SETUP FOR MEASURING THE ABSORPTION SPECTRA OF FILAMENTARY CRYSTALS DURING THE ACTION OF X-RAY RADIATION

S. U. Goldenerg, S. M. Minaev

(Presented by the scientific seminar of the Physics and Technology Faculty)

The need to measure the absorption spectra of objects with small geometrical dimensions (on the order of units and tens of microns) arises in studying the kinetics of radiation coloration of filamentary ionic crystals [1] and in a whole series of investigations in biophysics, microbiology, and medicine.

The MUF-5 microspectrophotometer produced by our industry is very expensive and not readily accessible. The microphotometric attachment to the FME-1 microscopes [2], developed in recent years, is built according to a single-beam scheme and therefore has the significant drawback that, in order to obtain a reference signal before each measurement of optical density, it is necessary to remove the specimen from the field of view of the microscope. Since, when studying the kinetics of the accumulation of color centers in filamentary ionic crystals, it is necessary to measure the optical density for different wavelengths of the optical spectrum over a prolonged period of exposure to radiation (often tens of hours), the nonreproducibility of the position of the specimen relative to the light flux during its periodic displacement, as well as such factors as instability of the light source and of the photodetector, considerably reduce the reliability of the results obtained with such a microphoto-attachment.

The accuracy and reliability of measurements can be increased by using the comparison method, for which, as is known, two channels are required: a reference channel and a measuring channel. Various ways of implementing this method have been described in the literature; however, they are rather cumbersome in execution, require displacement of the object during the measurement process [3—7], or the presence of two microscopes and two photodetectors [8], the measurement of whose parameters during operation of the setup is not excluded from the measurement results.

We have developed a setup, assembled from standard units and elements, that is largely free of the shortcomings indicated above.

The optical part of the setup consists of a monochromator, a microscope, a microcell, and a microphoto-attachment. The original part of the setup is the microcell (Fig. 1). The microcell consists of two tightly closing covers 1 and 2, inside which, along grooves, knives 5 and 6 are moved by means of microscrews 3 and 4. Perpendicular to the knives a plate 7 is inserted, the width of which changes stepwise. Thus,

by setting part of the plate of the desired width; with the aid of microscrews one can adjust both the width of the slits and the distance between them. Alternating blocking of the light fluxes passing through both slits is carried out by means of a modulator—a metal

Fig. 1. Design of the microslit: 1, 2—slit covers; 3, 4—microscrews; 5, 6, 7—movable knives; 8, 9—guide tubes

Fig. 2. Photograph of the microslit

strip mounted on the armature of a polarized relay of type RP-5. The relay is fastened to the specimen holder (Fig. 2), rigidly connected with cover 1, which considerably facilitates alignment of the modulator relative to the slits.

The microslit is inserted into the microscope tube with end 8, and onto tube 9, into which the microscope eyepiece is inserted, a microphotographic attachment is fitted, in which a photomultiplier is installed in place of the photographic plate.

The microscope eyepiece is placed in tube 9 in such a way that the slits are sharply visible; this ensures coincidence of the position of the microslits with the conjugate plane of the objective when the microscope is adjusted relative to the specimen.

The specimen—a filamentary crystal—is glued, with the aid of an MM-1 micromanipulator, to a tungsten wire, which is inserted into a needle. The needle is fixed in a specimen holder mounted on the microscope stage and equipped with a special device that makes it possible to tilt the needle in the vertical plane and rotate it about its own axis. The specimen is illuminated with monochromatic light emerging from the monochromator and directed through the condenser onto the specimen by means of a mirror or a quartz prism for total internal reflection (Fig. 3). The specimen is set strictly perpendicular to the light flux and in such a way that its image falls completely or partially

1—monochromator
2—rotating mirror
3—condenser
4—microspecimen
5—objective
6—microslit
7—eyepiece
8—prism of the microphoto attachment
9—eyepiece of the microphoto attachment
10—FEU chamber

X-rays

Fig. 3. Optical schematic of the setup

onto one of the slits located closer to the radiation source—the URS-55A x-ray unit. The other slit remains free; that is, the light flux bypassing the specimen passes through it. This flux serves as the reference.

After assembly, the setup is calibrated with neutral SF-4 filters by shifting the microscope condenser and mirror. Before work on the setup begins, the width of the reference slit is chosen so that both light fluxes—the measuring and reference fluxes—are identical. The magnitude of the light flux is judged from the voltage drop produced by the photomultiplier current across a precision resistor included in the anode circuit of the FEU. The voltage drop is measured with a V 7-8 digital voltmeter, which provides an accuracy of 0.1% in measuring dc voltage. The resistor is shunted by a capacitor; the measurement time constant is 2 sec.

The optical density of the specimen at any moment during exposure to x-ray radiation is measured as follows: remotely, first one slit and then the other is opened in succession, according to the readings of

the values \(U\) and \(U_0\), proportional to \(I\) and \(I_0\), are determined from the voltmeter; the optical density \((OD)\) is calculated by the formula \(OD = \lg_{10}\frac{U_0}{U}\).

Thus, the introduction of two slits into the optical path of the microscope made it possible:

1) to measure the optical density by the comparison method without moving the specimen;
2) to maintain the same optical path for the reference and measured light fluxes;
3) to use a single photodetector;
4) to eliminate, to a considerable extent, the influence of destabilizing factors owing to the small time constant of the measurement.

Fig. 4. Accumulation curves of \(F\)-centers.
\(a\)—in a filamentary NaCl crystal 25 \(\mu\text{m}\) in diameter; \(b\)—in a 20 \(\mu\text{m}\)-wide microlayer of a NaCl single crystal

For investigations over a wide wavelength interval we use two variants of the setup: one is intended for operation in the visible part of the spectrum; it is assembled on the basis of a UM-2 monochromator, an MBI-1 microscope, and an FEU-27. The other variant is assembled on the basis of an SF-4 monochromator, an MUF-3 ultraviolet microscope, and an FEU-53, and is intended for operation in the ultraviolet. The two setups are located on different sides of the X-ray unit, which makes it possible to use both windows of the X-ray tube simultaneously.

The setup makes it possible to study microspecimens with diameters of 10 \(\mu\text{m}\) and larger, as well as microlayers of macrospecimens adjacent to their lateral face. The maximum error in measuring optical density in the range \(0.0005 \div 2\) does not exceed 5%.

Figure 4 shows the accumulation curves, obtained on the setup, of \(F\)-centers in a filamentary NaCl crystal 25 \(\mu\text{m}\) in diameter (curve \(a\)) and in a 20 \(\mu\text{m}\)-wide microlayer of a specimen cleaved from a NaCl single crystal (curve \(b\)), as a result of seven-hour irradiation with X-rays.

References

1. S. U. Gol’denberg and I. Ya. Melik-Gaikazyan. FTT 6, 3484 (1964).
2. N. V. Ivanova, I. A. Druzhinina, V. A. Ozhigova. In: Experimental Methods for the Study of Ore Minerals. Moscow, “Nauka,” 1965, p. 92.
3. L. S. Agroskin and N. V. Korolev. Optics and Spectroscopy 6, 832 (1959).
4. Wolken Y., Strather V. Appl. Optics 2, 899 (1963).
5. Yang and Clarke. Instruments for Scientific Research, No. 2, 1966.
6. Brown Paul. JOSA 5, 1000 (1961).
7. A. D. Rakcheev. Reports of the USSR Academy of Sciences 130, 758 (1960).
8. Dücker and Lippincott. Instruments for Scientific Research, No. 9, 1964.