ATTENUATION OF A NARROW BEAM OF BETATRON BREMSSTRAHLUNG IN THICK LAYERS OF ALKALI-HALIDE CRYSTALS

V. A. VOROB'EV, V. N. RUDENKO

Introduction

The passage of radiation quanta through matter is well studied [1—5], but there are few experimental data on the attenuation of betatron bremsstrahlung in thick layers (up to 1000 mm), especially in chemical compounds. The need for such studies is dictated by the requirements of betatron flaw detection, for calculating the sensitivity of flaw-detector sensors and biological shielding, etc. Data on radiation absorption are necessary in solid-state physics, for determining the dose—the absorbed radiation energy at any point of an irradiated specimen that is of interest to us.

This article presents the results of work on studying the absorption of 30 MeV betatron bremsstrahlung in crystals of alkali-halide salts.

Experimental Method

The source of high-energy bremsstrahlung is a 30 MeV betatron of the Tomsk Polytechnic Institute design.

Fig. 1. Diagram of the arrangement of the experimental apparatus.
1—accelerator chamber, 2—collimator, 3—transmission ionization chamber, 4—measuring chamber, 5—crystal specimens under study.

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

A narrow beam of radiation, with an angle of divergence of \(0.5^\circ\), was cut out by a lead collimator with an exit-aperture diameter of \(20\ \mathrm{mm}\), and passed through KCl, KBr, and KI crystals. The thickness of the absorbing layer of crystals was varied from 1 to \(700\ \mathrm{mm}\).

The radiation detector was a thin-walled ionization chamber of our own design. The radiation intensity was monitored by a transmission ionization chamber. In designing this chamber several contradictory requirements arose. It was necessary for the chamber to have high sensitivity and a small volume, since a narrow beam was being used. The walls of the chamber and the high-voltage and collecting electrodes were made of aluminum \(0.5\ \mathrm{mm}\) thick. The thickness of the front wall of the chamber was built up with the crystals under investigation from 1 to \(700\ \mathrm{mm}\), which made it possible to record the absorption curve of betatron bremsstrahlung in the crystals studied. With the aid of the ionization chamber, the ionization produced by secondary electrons, which were knocked out by radiation quanta from the crystal and the chamber walls, was measured. Since the number of secondary electrons is proportional to the number of radiation quanta absorbed at a given point of the absorber, the absorption of \(\gamma\)-quanta in the substance under investigation can be determined from the degree of ionization in the chamber.

Results of the investigations

Figures 2 and 3 show the dependences of the ionization in the thin-walled chamber on the thickness of the front wall made of KCl, KBr, and KI crystals at an energy of \(30\ \mathrm{MeV}\). At a depth of \(7\ \mathrm{mm}\) for KI, \(10\ \mathrm{mm}\) for KBr, and \(13\ \mathrm{mm}\) for KCl, the “equilibrium wall” effect and the ionization maximum are observed.

The magnitude of the ionization maximum increases with increasing effective atomic number of the halide \((\mathrm{Cl}—\mathrm{Br}—\mathrm{I})\), while the crystal thickness at which the maximum is observed decreases.

The increase in ionization at the maximum is due to the increase in absorbing power with increasing effective atomic number of the bremsstrahlung substance.

The decrease in the crystal thickness at which the ionization maximum is observed is a consequence of the decrease in the path length of secondary electrons on going to compounds of greater density \((\mathrm{KCl}—\mathrm{KBr}—\mathrm{KI})\).

Fig. 2. Absorption curves of bremsstrahlung with a maximum energy of \(10\ \mathrm{MeV}\).

Beyond the maximum there is a decrease in ionization, and the law of this decrease is close to exponential.

In this region the dependence of the ionization in the chamber on the thickness of the absorber in front of it can be represented in the form

\[ J = J_0 e^{-\mu x}, \tag{1} \]

where \(J_0\) is the ionization current in the chamber for that absorber thickness at which the maximum ionization is observed, \(J\) is the ionization current in the chamber after an absorber layer of thickness \(x\), and \(\mu\) is the attenuation coefficient—

...attenuation coefficient, linear or mass, depending on the units in which \(x\) is expressed—in cm or g/cm\(^2\).

Usually the effective attenuation coefficient \(\mu\) of ionizing radiation is determined experimentally, since its theoretical determination is associated with great difficulties, because the bremsstrahlung spectrum of the betatron is not monoenergetic, and also because the spectrum changes as it passes through the absorber. In our case the mass attenuation coefficient is equal to: for KCl \(\mu = 0.027\ \text{cm}^2/\text{g}\), for KBr \(\mu = 0.29\ \text{cm}^2/\text{g}\), for KJ \(\mu = 0.032\ \text{cm}^2/\text{g}\).

Fig. 3. Absorption curves of bremsstrahlung with maximum energy 30 MeV.

The mass attenuation coefficient is the most indicative, since it depends on the chemical composition of the absorber and does not depend on its density.

The increase in the attenuation coefficient with increasing atomic number of the atoms of the target substance is due to the increase in the cross sections of the photoelectric effect, pair-production effect, and Compton scattering.

In the energy range 10–30 MeV the mass attenuation coefficient in KCl, KBr, and KJ crystals changes little, since as the pair-production cross section increases with increasing radiation-quantum energy, the cross sections of the photoelectric effect and Compton scattering decrease.

Conclusion

The attenuation of 30 MeV betatron bremsstrahlung in crystals of alkali-halide salts has been studied. It was found that, as the radiation energy changes from 10 to 30 MeV, the attenuation coefficient changes only insignificantly. On going to the heavier salt (KCl—KBr—KJ), the mass attenuation coefficient increases slightly.

The data obtained make it possible to determine the attenuation of betatron bremsstrahlung as it passes through thick layers of alkali-halide salt crystals.

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