Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov

Vol. 139　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1965

TRANSFORMATION OF BREMSSTRAHLUNG SPECTRA BEHIND ABSORBERS OF DIFFERENT CHEMICAL COMPOSITION

V. A. VOROB'EV, G. P. SOKOLOV

(Presented by the scientific seminar of the Laboratory of Dielectrics and Semiconductors)

It has been proposed to divide the possible methods of radiation defectoscopy into methods based on amplitude modulation of the radiation flux, whose scientific basis is the change in the attenuation coefficient, and methods based on frequency modulation of the radiation flux as it passes through an absorber containing an inclusion [1]. The scientific basis of the methods based on frequency modulation is the relative change in the spectral composition of the radiation upon its reflection from a defect or passage through it, as compared with the change in the spectral composition of radiation that has passed through a section of the object containing no defect.

In connection with the indicated classification, a systematic investigation is being carried out of radiation spectra upon passage through homogeneous and inhomogeneous bodies.

When electromagnetic radiation with a continuous spectrum passes through an absorber, the spectral composition of the radiation changes as a result of dispersion and the appearance of secondary-radiation fluxes. Filtration of bremsstrahlung as it passes through an absorber must be taken into account in radiation physics, radiation defectoscopy, and radiation therapy, since absorption of radiation by a medium, owing to the dependence of attenuation coefficients on radiation energy [2], will be different for spectra of different composition [3, 5].

The change in spectral composition as a function of some parameter of the absorber when radiation passes through a substance is called the transformation of the radiation spectrum as a function of the given parameter.

When a narrow beam of bremsstrahlung passes through [2], the change in spectral composition is due to relatively greater absorption of quanta with lower energy [3], as a result of which the flux is relatively enriched with quanta of higher energy [4, 5].

The data available in the literature on the change in the spectral composition of bremsstrahlung pertain to metals—aluminum, copper [5]—and to water [6, 7], and are limited to the study of the shape of the spectrum as a function of absorber thickness. It is of interest to investigate

regularities in the transformation of bremsstrahlung spectra as a function of the chemical composition and thickness of absorbers.

The study of the transformation of the spectrum of electromagnetic radiation as it passes through an absorber can be carried out theoretically on the basis of the laws governing the interaction of radiation with matter for the energies comprising the given spectrum [3, 4]. The transformation of spectra can also be investigated experimentally by recording spectra with spectrometers [1, 3].

Fig. 1. Transformation of the spectrum of bremsstrahlung radiation with a maximum energy of 180 keV, filtered by copper of half-value thickness 1.29 mm, as a function of the thickness of a single crystal of potassium bromide.

In the graph: vertical axis — number of quanta $\beta$ in relative units; horizontal axis — $E_\gamma$ (keV). Curve labels: 1.29 mm Cu—filter 1; 5 mm KBr—2; 10 mm KBr—3; 15 mm KBr—4; 20 mm KBr—5.

In the present work we used both methods for studying the transformation of spectra.

Single crystals of alkali-halide salts were selected as the materials for the investigation: potassium chloride, potassium bromide, and potassium iodide. The choice of alkali-halide crystals is explained by the following reasons:

1. From alkali-halide crystals it is possible to prepare homogeneous specimens of high purity and with known anisotropy.

2. The simple crystal lattices of alkali-halide crystals are convenient for analysis, theoretical studies, and straightforward interpretation of the results.

3. The physical properties of these compounds have been studied in detail [8]. The nature of changes in the physical properties of crystals under the action of radiation has been studied [9] and can be allowed for with the necessary accuracy.

1. In passing from one composition to another, conditions can be created under which only one of the components in the compound is replaced, which makes it possible to investigate the dependence of the spectrum transformation on the chemical composition.

Fig. 2. Change in the mean energy of the bremsstrahlung spectrum with maximum energy 180 keV as a function of the thickness of single crystals of potassium chloride, potassium bromide, and potassium iodide.

In the theoretical calculation of the transformation of the bremsstrahlung spectrum, the spectrum calculated by Kramers’ formula [10] was taken as the initial one. The change in the spectral composition of the radiation as it emerged from the accelerator tube through a copper foil 1.29 mm thick was taken into account.

The bremsstrahlung spectrum thus obtained, with maximum energy 200 keV (Fig. 1), was divided into twenty intervals of 10 keV in each interval. The spectrum lines with an interval of 10 keV were attenuated according to the law

\[ I = I_0 e^{-\mu x}, \tag{1} \]

where \(x\) is the thickness of the single-crystal layer, for example, potassium bromide.

By such calculations, bremsstrahlung spectra were obtained after passage through layers of thickness 5, 10, 15, and 20 mm. In Fig. 1, the abscissa gives the energy of bremsstrahlung quanta in kiloelectronvolts; the ordinate gives the number of quanta of each energy, in relative units, calculated by formula (1).

The experimental study of the spectrum transformation during the passage of bremsstrahlung through a substance was carried out using a scintillation spectrometer of photoabsorption [11, 12].

To characterize the spectrum transformation, we proposed using the mean energy of the spectrum \(\overline{E}_x\), determined by the formula:

\[ \overline{E}_x = \frac{ \displaystyle \int_{0}^{E_{\max}} N(E,x)\,E\,dE }{ \displaystyle \int_{0}^{E_{\max}} N(E,x)\,dE }, \tag{2} \]

where \(N(E,x)\) is the number of quanta with energy \(E\) in the spectrum after passage through an absorber of thickness \(x\).

The mean energy is the quotient obtained by dividing the integral energy at the exit from the absorber by the number of emerging quanta.

The results of the calculations are presented in Fig. 2, where the absorber thickness in mm is plotted along the abscissa, and the mean energy per quantum in the spectrum after passage through an absorber of the given thickness is plotted along the ordinate.

In Fig. 3, the absorber thickness in mm is plotted along the abscissa, and the energy at the spectrum maximum along the ordinate.

It follows from Figs. 2 and 3 that filtration of a bremsstrahlung flux in a narrow beam, as the absorber thickness increases, proceeds in the direction of enriching the flux with quanta of relatively higher energy, as was also observed for aluminum [5].

Fig. 3. Shift of the peak in the bremsstrahlung spectrum with maximum energy 180 keV as a function of the thickness of single crystals of potassium chloride, potassium bromide, and potassium iodide.

We have shown that, with increasing thickness of the filtering layer, the mean energy increases and the position of the maximum of the spectrum of the transmitted radiation shifts toward higher energies. The greatest rate of increase of the mean energy with increasing crystal thickness is found for potassium iodide, whose effective atomic number is greater than that of potassium chloride and potassium bromide.

With increasing absorber thickness, the mass attenuation coefficient of a narrow bremsstrahlung beam in these compounds decreases, both as measured experimentally and as calculated for the mean energy from the experimentally recorded spectra.

The rate of decrease of the attenuation coefficients of bremsstrahlung with maximum energy from 10 keV to 30 MeV, measured by us with absorber thickness, is greatest for potassium iodide. As the effective atomic number of the substance of the absorber increases, filtration of the radiation from the soft component increases. The rate of change of the mean energy, the attenuation coefficient, and the shift in the position of the maximum of the spectrum with absorber thickness decreases as the thickness increases, and the curves (Figs. 2 and 3) tend toward saturation. The steepness of the curve and the absorber thickness beginning from which the attenuation coefficient and the mean energy to which it corresponds remain unchanged are determined by the radiation energy, the chemical composition of the absorber, and the cross-sectional area of the beam.

References

1. N. A. Vorob'ev. Report at the scientific seminar of the Problem Laboratory, EdIIP, TPI, 25/IX 1963.

2. O. I. Leipunskii, B. V. Novozhilov, V. N. Sakharov. Propagation of gamma quanta in matter. State Publishing House of Physico-Mathematical Literature, Moscow, 1960.

3. O. V. Bogdankevich, F. L. Nikolaev. Work with a bremsstrahlung beam. Atomizdat, Moscow, 1964.

4. A. V. Bibergal', U. Ya. Margulis, E. I. Vorob'ev. Protection from X-rays and gamma rays. Medgiz, 1955.

5. F. N. Kharadzha. General course of X-ray engineering, Gosenergoizdat, 393, 1956.

6. Gettinger. Spectra of primary and secondary radiation. Experimental studies using a scintillation spectrometer (translated from English), 1960.

7. D. V. Cormack. Spectral distributions in a medium irradiated with X-or gamma-rays. Report, IX Congress Radiology, Tbilisi, 1960.

8. A. A. Vorob'ev. Physical properties of ionic crystalline dielectrics, book 1—2, Tomsk University Press, Tomsk, 1961.

9. B. V. Budylin, A. A. Vorob'ev. Effect of radiation on ionic structures, Gosatomizdat, Moscow, 1962.

10. V. I. Gorbunov, V. B. Kuznetsov. Selection of the optimum thickness of intensifying foil in radiography of large thicknesses. Report at the International Conference on Materials Testing, Czechoslovakia, Žilina, 1964 (in press).

11. N. Starfelt. Improved NaJ Sintillation spectrometer for the study of continious X-rays spectra. Nucl. Instr. 3, 25, 1958.

12. V. I. Gorbunov, V. B. Kuznetsov, Yu. B. Yankilevich. A scintillation spectrometer for the study of X-radiation, Proceedings of TPI (in press).