METHOD FOR ECONOMICAL IRRADIATION OF THIN SAMPLES OF ALKALI-HALIDE CRYSTALS OF DIFFERENT CHEMICAL COMPOSITION WITH BETATRON BREMSSTRAHLUNG

V. A. VOROB'EV

(Presented by the scientific seminar of the Research Institute of Electron Introscopy)

In carrying out experiments in work connected with the irradiation of materials, in order to reduce the cost of irradiation one may proceed in two ways. First, the reduction in cost can be achieved by choosing the cheapest sources of radiation that most fully meet the stated requirements. Second, for existing installations, methods can be developed that make it possible to use the capabilities of radiation sources as fully as possible. The first approach requires the availability of a large number of installations whose parameters vary over wide limits; this results in many radiation sources not being used continuously in the work, and no overall economy is obtained. In addition, irradiation of individual samples requires greater expenditures of time and a larger amount of equipment than mass irradiation.

The second approach must be based on the development of methods for mass irradiation of samples.

In many investigations in the field of radiation physics and radiation technology, the question now arises of the mass irradiation of samples whose thicknesses amount to several millimeters or less. The distribution of the absorbed energy of betatron bremsstrahlung over the thickness of the object being irradiated is described by a curve with a maximum, the position of which relative to the surface of the object depends on the energy of the incident radiation flux, the effective atomic number, and the density of the absorbing material. The steepness of the front and rear slopes of this curve, describing the distribution of the absorbed dose over the thickness of the object being irradiated, depends mainly on the same parameters [1, 2, 3]. Therefore, irradiation of a composite block made up of several samples of one chemical composition leads to the result that different samples after irradiation have absorbed doses differing from one another, depending on the position of the maximum of the transition curve relative to the plane of the block facing the radiation source. The total thickness of the composite block is thus determined by the permissible difference in absorbed dose for the individual samples of the irradiated batch [3].

When samples made of materials with different effective atomic numbers are irradiated by a bremsstrahlung flux of the same intensity, the absorbed energy in each sample will be different.

and is determined by the absorption coefficient, which, other conditions being equal, depends on the atomic number of the substance [4].

We carried out an investigation of the absorption of betatron bremsstrahlung in alkali-halide crystals: potassium fluoride, potassium chloride, potassium bromide, and potassium iodide, with the aim of studying the total thicknesses of samples that can be irradiated under the condition of equality of absorbed doses in the individual samples with the required accuracy [3]. In studying the absorption of bremsstrahlung, we used as the parameter characterizing the absorption of bremsstrahlung in a substance the mass absorption coefficient [4], Table 1.

Table 1

Mass absorption coefficients of betatron bremsstrahlung in alkali-halide crystals (according to G. P. Sokolov)

As is seen from Table 1, with increasing effective atomic number there is an increase in the absorption coefficients for all the considered ranges of maximum betatron bremsstrahlung energies. Moreover, this relative increase in the absorption coefficient for the potassium series of alkali-halide crystals as a function of \(Z_{\mathrm{eff}}\) is analogous for all the considered bremsstrahlung energies. Taking the value of the absorption coefficients for potassium fluoride as unity, we obtain the relative increase in the absorption coefficients for each energy range (Table 2).

On the basis of the data obtained on the absorption coefficients of bremsstrahlung in alkali-halide crystals, we prepared composite samples from crystals of different chemical composition

Table 2

Relative increase in bremsstrahlung absorption coefficients in alkali-halide crystals with increasing effective atomic number

with the aim of equalizing the distribution of absorbed dose over the thickness of the block. In preparing the block, samples of potassium fluoride, as having the smallest absorption coefficient, were placed at the thickness corresponding to the maximum of the transition curve; plates of potassium chloride were placed adjacent to them on both sides, and then those of potassium bromide and potassium iodide, respectively. The thickness of each layer was selected on the basis of the ranges of secondary electrons formed in the crystals under the action of bremsstrahlung of the corresponding

energy [5, 6]. We have carried out a study of the distribution of the absorbed energy of bremsstrahlung in composite blocks made of the crystals indicated in Table 1, theoretically, on the basis of the absorption coefficients (Tables 1 and 2), and experimentally.

The distribution of the absorbed energy of bremsstrahlung over the thickness of the block was determined with the aid of X-ray films of types RT-1, RT-2, RM-1, which were placed between the corresponding layers of crystals, Fig. 1. Figure 1 shows a transition curve obtained

Fig. 1. Distribution of absorbed energy over the thickness of a sample of alkali-halide crystals under irradiation with bremsstrahlung from a betatron with a maximum energy of 30 MeV. Curve 1—distribution of absorbed energy over the thickness of a potassium chloride crystal (experimental curve 3). Curve 2—distribution of absorbed energy over the thickness of a composite sample made of crystals of potassium fluoride, potassium chloride, potassium bromide, and potassium iodide, irradiated together.

during irradiation of potassium chloride single crystals with bremsstrahlung from a betatron with a maximum energy of 30 MeV, and the distribution of absorbed energy during irradiation, with the same radiation, of a composite sample made of the crystals listed in Table 2 (solid curve) and measured with X-ray film. As is seen from Fig. 1, the total thickness of the composite block in which, by the indicated method, a distribution of absorbed energy uniform to within 10% was obtained under irradiation with bremsstrahlung from a betatron with a maximum energy of 30 MeV is 47.5 mm, as compared with 28 mm when irradiating a potassium chloride single crystal.

The discrepancy between the data on the distribution of absorbed energy over the thickness of the composite block, calculated theoretically from the absorption coefficients for the corresponding crystals (Tables 1 and 2), and the results of experimental measurements with X-ray films,

lying within 5%, is explained by the dependence of the effective energy of the secondary electrons and, consequently, of the lengths of their paths, on the effective atomic number of the crystal in which they are produced under the action of bremsstrahlung [7], as well as by the error of calculation and measurement.

An experimental verification of the results of calculations of the distribution of energy absorption in composite samples made from the crystals indicated in Tables 1 and 2, under irradiation with betatron bremsstrahlung with maximum energies of 10, 15, 20, and 25 MeV, using x-ray films of the RT-2, RT-1, and RM-1 types, gave good agreement between the experimental results and the calculated data.

Thus, the use of composite samples made of materials of different chemical composition under irradiation with electromagnetic radiation substantially increases the productivity of the irradiation process and reduces costs in the mass irradiation of samples. The application of this method, as our experience has shown, is economically advantageous even when irradiating separate batches of samples. Its economic efficiency increases in the mass irradiation of identical batches. Thus, in the joint irradiation of potassium fluoride, potassium chloride, potassium bromide, and potassium iodide crystals in the form of plates 1–2 mm thick, owing to an increase in the thickness of the simultaneously irradiated block and an increase in the absorbed energy in crystals with a lower effective atomic number, as compared with irradiation of samples composed of identical crystals, at a maximum bremsstrahlung energy in the range 20–30 MeV an increase in productivity by a factor of 1.4–1.7 was obtained. The uniformity of the distribution of absorbed energy over the thickness of the composite samples was no worse than 10%.

LITERATURE

1. Radiation Dosimetry, translated from English, ed. by I. G. Gusev and K. G. Prukhonov, IL Publishing House, Moscow, 1958.

2. S. I. Kruglov. ZhTF, XXXI (1961), 1092.

3. V. A. Vorob’ev. Candidate’s dissertation, Tomsk, TPI, 1965.

4. O. I. Leipunskii, B. V. Novozhilov, V. N. Sakharov. Distribution of Gamma Quanta in Matter. State Publishing House of Physico-Mathematical Literature, Moscow, 1960.

5. V. N. Rudenko. Candidate’s dissertation, Tomsk, TPI, 1964.

6. S. V. Starodubtsev, A. M. Romanov. Passage of Charged Particles through Matter. Publishing House of the Academy of Sciences of the Uzbek SSR, Tashkent, 1962.

7. V. A. Vorob’ev, V. N. Rudenko. Determination of the effective energy of secondary electrons in alkali-halide salt crystals under irradiation with betatron bremsstrahlung. Izv. TPI, 140, Tomsk State University Publishing House, Tomsk, 1966.