FEATURES OF BETATRONS FOR ELECTRON FLAW DETECTION AND INTROSCOPY

V. V. Evstigneev, B. A. Kononyuk, V. N. Rudenko

(Presented by the joint seminar of the Research Institute of EI
and the Faculty of Physics and Technology)

Introduction

The use of betatrons as a source of high-energy electrons in radiation flaw detection imposes a number of specific requirements on the design of the accelerator.

The use of a betatron under the conditions of factory laboratories for nondestructive testing requires reducing the weight and overall dimensions of the installation to a minimum, as well as high operating reliability of the installation and ease of adjustment and repair.

At the same time, electron flaw detection has advantages in the inspection of materials and products of relatively small thickness (up to several cm) and, as a rule, of small weight; therefore it is more convenient to bring these products to the accelerator for inspection, and not vice versa, which makes it possible to manufacture the betatron in a stationary version.

At the Research Institute of Electron Introscopy several types of betatrons with an extracted electron beam have been developed, intended for electron flaw detection and introscopy, which are now being successfully operated under laboratory and factory conditions.

Electron Energy

The choice of electron energy is determined by the maximum mass thickness $t$ of the products to be inspected, according to the expression [1]:

$t = (0.6 \div 0.9) R_{\mathrm{e}} = (0.6 \div 0.9)\,0.53 - 0.1 - (2.5E + 1)(Z - 13)\,10^{-3}$,

where $t$ is in g/cm², $E$ is in MeV.

The coefficient $(0.6 \div 0.9)$ indicates that, in order to obtain maximum sensitivity, the steepest portion of the transition curve of the distribution of absorbed electron energy in the substance is used.

Usually composite products of the plastic–metal or bimetallic type are considered, and in a first approximation, to determine $t$, the total mass thickness is taken. To detect cracks located at an angle of about 45° to the surface of the product, the electron energy is chosen from the equality of their extrapolated range to the thickness of the material being radiographed, $t \leq R_{\mathrm{e}}$, i.e., less than when detecting another type of defect or density variation.

A significant limitation on the electron energy used is the background of scattered bremsstrahlung radiation from the accelerator, for protec-

from which, as the energy is increased, ever greater wall thickness is required, which complicates the practical use of the betatron.

A significant increase in the weight of the installation and in its overall dimensions also worsens the operating characteristics of the betatron at higher electron energies. However, where a betatron with an energy of up to 30–35 MeV is used also as a source of bremsstrahlung for radiographing articles of great thickness (tens of cm for steel), an important factor in expanding its capabilities is the extraction and use of the electron beam for flaw-detection purposes.

When only the electron beam is used, betatrons with energies of up to 6, 7.5, and 10 MeV have found the greatest practical application.

Electromagnet

For electron flaw detection and introscopy, a Ш-shaped electromagnet design is used, although it is known that a toroidal betatron design, as for example in the PMB-6, has considerably lower weight and dimensions.

However, as studies [2] have shown, the electron beam extracted from the PMB-6 has a comparatively low electron-beam intensity and a large angular divergence, which makes its practical use in flaw detection difficult.

The Ш-shaped design of the magnetic circuit of the 6, 7.5, and 10 MeV betatrons developed at the Scientific Research Institute of Electron Introscopy at TPI provides free access to the accelerating chamber, which is essential for betatron adjustment, makes it possible to place large objects near the exit window, and permits the focusing magnetic lenses to be located at a minimum distance from the exit window, i.e., it provides the further necessary shaping of the electron beam. The betatron poles were made with saturating pole pieces and with a groove along the height, in which the magnetizing coils are placed above the pole pieces. The pole height was chosen so that the coils protruded beyond the outer diameter of the poles by no more than 1–3 cm, which substantially reduces the magnitude of the stray magnetic field at the point where the electron beam is extracted from the accelerating chamber. Owing to this, the electron beam can be extracted at an angle of 70° relative to the longitudinal axis of the yoke, while the angular divergence of the beam at the exit does not exceed 3–5° in the horizontal plane.

The reduction of the pole-core diameter by means of grooves leads to the induction in the pole being 16 kG for E-330 steel; however, as experimental studies showed, the heating of the poles and coils was within permissible limits. The magnetizing coils were made according to the autotransformer principle, in which the primary winding constitutes part of the secondary.

The pulsed power-supply system of the betatron [3] makes it possible to further reduce the dimensions of the magnetizing coils and the magnetic circuit, which was used in the 7.5 MeV betatron.

Accelerating Chamber

If a sealed-off vacuum chamber has advantages over a chamber with continuous pumping or a semi-sealed-off chamber in terms of the simplicity of installing the control system and the number of devices ensuring its operation, then a sealed-off or semi-sealed-off chamber is convenient for installing in it

of the extraction capacitor, a thin window for extracting electrons (of the order of \(13 \text{ mg}/\text{cm}^2\)), and replacement of injectors that have failed.

In the 6 and 7.5 MeV betatrons with extraction of the electron beam, a ceramic semisealed chamber with a titanium pump is used, with an internal aperture of \(50 \times 85\) mm (cross-sectional area \(40 \text{ cm}^2\)), which makes it possible to accelerate \(4 \cdot 10^9\) particles per pulse. With an extraction efficiency of not less than 50%, the average current of the electron beam at the outlet of the accelerating chamber is \(10^{10}\) particles/sec, with a spot size of \(5 \times 10\) mm.

The extraction capacitor has a cut-off plate length of 8 cm and is placed at an azimuth of \(130^\circ\) with respect to the exit window. Owing to the nonsymmetric displacement of the electrons from the equilibrium orbit by the action of sector and central windings, a straight Kerst-type injector can be installed in the accelerating chamber, which improves the conditions for capture of electrons into acceleration in comparison with the matched injectors previously used on betatrons with electron-beam extraction.

On the 10 MeV betatron, the use of a 150 kV high-voltage injector, developed at the Scientific Research Institute of Nuclear Physics, Electronics, and Automation under B. V. Okulov at the TPI, made it possible to increase the intensity of electron radiation by an order of magnitude in comparison with the use of a 50 kV Kerst injector.

Formation of the Electron Beam

For electron flaw detection and introscopy it is necessary that the beam be formed according to specified spatial, temporal, and energy distributions. Studies of the dynamics of electron motion in both the radial and axial planes have shown [4] that, when electrons are extracted from a betatron with perturbing sector magnetic fields imposed, the electrons are grouped according to the phases and amplitudes of oscillations into several beams and leave the accelerator at definite azimuths. By choosing the parameters of the perturbing field, two or three electron beams with uniform density are obtained from the betatron. The spatial distribution of the electrons also determines their temporal distribution. Experimentally, this is confirmed by obtaining various shapes of the radiation pulse. By changing the parameters of the perturbing field, it is possible to obtain from one to several maxima in the pattern of the temporal distribution of the electrons, which is due to separation of the electron beam and their grouping by phases and orbits.

By adjusting the operating modes of the betatron, the energy spectrum of the electrons extracted from the betatron varies within the limits from 1 to 5% [5].

Main Characteristics of Betatrons

for Electron Flaw Detection and Introscopy

In all installations, the betatron control panel is combined with the flaw-detector panel.

Fig. 1. External view of a 6 MeV betatron with electron-beam extraction

The external view of a 6 MeV betatron, which serves as the radiation source in an electron flaw detector for inspecting thicknesses up to \(3\ \mathrm{g/cm^2}\), is shown in Fig. 1.

REFERENCES

1. B. A. Kononov. Defektoskopiya. 1968, No. 4, 77.

2. B. A. Kononov, V. I. Solomatin. In the collection “Electron Accelerators.” Proceedings of the VII Interuniversity Conference on Electron Accelerators. Issue 2. Moscow, Atomizdat, 1970, p. 36.

3. V. V. Ivashin. In the collection “Electron Accelerators.” Proceedings of the VI Interuniversity Conference on Electron Accelerators. Issue 3. Moscow, Atomizdat, 1970, p. 27.

4. B. A. Kononov, V. V. Evstigneev. Proceedings of the International Symposium on Betatrons. Bucharest, 1972.

5. B. A. Kononov, V. V. Evstigneev. Proceedings of the Interuniversity Conference on Radiation Methods of Nondestructive Testing. Tomsk, 1968.