BETATRONS OF NII EI FOR INDUSTRIAL FLAW DETECTION AND MEDICINE

B. I. Gorbunov, G. A. Kunitsyn, O. V. Sokolov

At present, ionizing radiation has found wide application in nondestructive methods for inspecting various materials and products. Inspection of products of great thickness requires the use of ionizing radiation with energies in the tens of MeV. The use of isotope sources of γ-radiation and X-ray apparatus in this case becomes difficult because of the long exposure time and poor detectability.

Sources of high-energy bremsstrahlung are electron accelerators (microtrons, synchrotrons, linear accelerators, betatrons, direct-action accelerators). Betatrons have become most widely used in radiation inspection where the inspected specimens are of great thickness. It is enough to note that in our country alone there are dozens of betatrons of various energies which, over a long period, have been operated at enterprises as radiation sources for the radiographic inspection of thick-walled products made of various materials.

The prevalence of betatrons is explained by the following qualities:

1. High energy of bremsstrahlung. The possibility of smooth energy control, which permits inspection of materials of different density over a broad range of thicknesses.

2. The relative simplicity of the design and operation of a betatron in comparison with such accelerators as the microtron, linear accelerator, and synchrotron.

3. Small size of the focal spot, which makes it possible to obtain high sensitivity to the defects being detected.

4. The possibility of obtaining a radiation dose rate sufficient in magnitude.

5. Reliability of operation.

At the Research Institute of Electron Introscopy at TPI, betatrons of medium energies and intensities are being developed and manufactured, intended mainly for flaw detection of materials and products. In addition, betatrons are used for gamma therapy, activation analysis, and biological and physical experiments. With the aid of betatrons developed at NII EI, high-productivity inspection of materials and products up to 500 mm thick is possible (by

steel), and the inspection of nonuniform and multicomponent materials (such as concrete and reinforced concrete)—up to thicknesses on the order of 4–4.5 t/m². The installations are being developed both as stationary versions and on a moving mechanism, making it possible to move the betatron within a laboratory, shop, or construction site. Transportable versions are also being developed. In this case the betatron can be transported by truck, electric cart, or another means of transport.

In the development of betatrons, much attention has been devoted to studying the process of electron injection into the betatron. The capture of electrons into acceleration was considered as a process of collective interaction. Research into the mechanism of electron capture into acceleration is being carried out both theoretically and experimentally. The results obtained make it possible to move toward a practical increase in the efficiency of electron capture into acceleration.

Fig. 1. Dependence of the weight of the capacitor bank \(G_b\) on the energy \(A\)

Fig. 2. Dependence of the weight of the electromagnet \(G_{\mathrm{em}}\) on the energy \(A\)

Extensive work has been carried out to improve the betatron electromagnet. Electromagnets of a symmetrical multilayer design have been developed; in comparison with U-shaped magnets, they have improved magnetic, mechanical, and weight characteristics. Increasing the mechanical rigidity of the electromagnet improved the stability of the magnetic characteristics and reduced the acoustic-noise level of the electromagnet.

In designing betatron electromagnets, one must strive to ensure that, with high operating reliability in the specified mode, the mass of the electromagnet and its cost are minimal. To perform calculations of betatrons with optimal parameters, a calculation method was developed at NII EI and a program was compiled for calculations on the M-20 electronic digital computer. The calculation reduces to determining the dependences of the main parameters of betatrons on the final energy of the accelerated electrons at a specified dose rate of bremsstrahlung radiation.

Figures 1 and 2 show graphs illustrating the dependence of the weight of the betatron electromagnet \(G_{\mathrm{em}}\) and the weight of the capacitor bank \(G_b\) on the maximum energy and the dose rate of \(\gamma\)-radiation, for specified constant values of the injection voltage and induction in the central part of the electromagnet. A characteristic feature of the graphs is the presence of minima on more or less smooth curves, which makes it possible to choose such parameters for which the betatron will be optimal with respect to one or

…another characteristic (the weight of the installation as a whole or of its individual units, the reactive power of the capacitor bank, the dose power per unit weight, electric-power consumption, etc.) or group of characteristics.

On the basis of the developed method, the B-18 betatron, distinguished by high specific parameters, was calculated and manufactured.

The technical characteristics of four models of betatrons being developed and manufactured at NII EI at the present time are given in Table 1.

Table 1

The B-25/10, B-30, and B-35/8 betatrons have been introduced into production; in particular, one of the B-25/10 betatrons has been successfully operated for a number of years at the Barnaul Boiler Plant. At present NII EI is manufacturing a second B-25/10 betatron for BKZ. The B-30 betatrons have been introduced at a number of enterprises; the B-35/8 betatron has been introduced at the Izhora Plant named after Zhdanov, and two more such betatrons have been manufactured for Uralkhimmash (Sverdlovsk) and the Ordzhonikidze Plant (Podolsk). The B-30 betatron has a Ш-shaped electromagnet design. A general view of the betatron is shown in Fig. 2a. The B-30 betatrons are used in laboratory research, in flaw detection, and in medicine. A betatron moving mechanism has been developed for moving the betatron within a laboratory or shop. At present

Fig. 2a. General view of the B-30 betatron

the B-30 betatron can be equipped with a sealed-off vacuum chamber. On the basis of the B-30 betatron, a variant of a two-beam betatron has been developed that produces, in one vacuum chamber, two parallel or converging beams of γ-radiation. This makes it possible to radiograph simultaneously two sections of the object being inspected or to obtain stereo images of defective regions.

Fig. 3. General view of the B-25/10 betatron

The development of the B-25/10 betatron was undertaken with the aim of creating an installation meeting the requirements imposed on modern betatrons for flaw detection and medicine. On this betatron, a number of fundamentally new technical solutions were tested, which found further development in later betatron designs B-35/8 and B-18.

Fig. 3 shows a general view of the B-25/10 betatron. The betatron has a symmetrical ten-pole magnetic circuit. The advantages of such an electromagnet design are the reduction in overall dimensions and weight compared with a Ш-shaped design, a more uniform static and phase azimuthal structure of the magnetic field, and the possibility of using the electromagnet posts as shielding elements against scattered radiation. The mechanical rigidity of the design is increased by assembling the halves of the magnetic circuit from solid-stamped plates, which are assembled into packets, tightened with studs, and fastened to silumin plates. Fig. 4 shows a general view of an assembled half of the magnetic circuit. Special press molds have been developed for assembling the posts and the electromagnet. Thanks to the use of press molds, the dimensions and

Fig. 4. Magnetic circuit of the B-25/10 betatron in the process of assembly

the steel filling factor of the individual electromagnet stacks have a minimal scatter. Good reproducibility of the magnetic characteristics of the individual stacks and high accuracy in assembling the magnetic circuit have been achieved. As a result, a satisfactory natural phase structure of the magnetic field has been obtained, requiring almost no correction. Figure 5 shows the azimuthal phase structure of the magnetic field of the B-25/10 betatron.

Fig. 5. Azimuthal phase structure of the B-25/10 betatron

The structure has a clearly expressed 10th harmonic; the contribution of the most dangerous lower harmonics is insignificant. The absolute magnitude of the phase nonuniformity is small.

Cooling of the electromagnet is carried out by an SVM-4M axial fan mounted on the upper plate. The air-duct system on the plates provides blowing of the magnetizing coils and the vacuum chamber.

The betatron is equipped with a sealed-off vacuum chamber of the BUK-25 type, developed by the Leningrad Special Design Bureau “Svetlana.” The use of a sealed-off vacuum chamber considerably simplifies operation of the betatron and increases its reliability.

The B-35/8 betatron, with a maximum accelerated-electron energy of 35 MeV and a gamma-radiation dose rate of 250 rad/min at 1 m, is intended mainly for industrial flaw detection. With this betatron it is possible to inspect the quality of steel products up to 600 mm thick, and products made of low-density materials (1.8–2.5 g/cm³) up to 2000 mm thick. The weight and overall dimensions of the installation have been reduced in comparison with the preceding model—the B-30 betatron—by better use of active materials (steel and copper of the electromagnet) and a number of design improvements. Figure 6 shows the general appearance of the B-35/8 betatron in its casing.

Fig. 6. General view of the B-35/8 betatron.

The compactness of the installation has been achieved through the symmetrical design of the magnetic circuit and the rational placement of the pulse circuits and cooling system. A sealed-off BUK-35 vacuum chamber has been developed for the betatron (Fig. 7). The service life of the injector cathode in tested chamber samples is about 5000 hours. The operating injection voltage is 70 kV. The recommended range of thicknesses for radiography is up to 450 mm for steel or 2.5 t/m² for concrete and reinforced concrete.

The B-18.5 betatron is intended for flaw detection of industrial products, welded joints, building components, and structures by the radiographic inspection method in laboratories, shop-floor conditions, or at a construction site. To satisfy these conditions, the installation must be transportable. The use, in the calculation and design of the betatron, of the above-described method of calculation for optimal characteristics made it possible to obtain sufficiently high

Fig. 7. Sealed-off vacuum chamber BUK-35

specific performance parameters of the betatron, with the weight of individual units not exceeding 0.5 t. The total weight of the installation does not exceed 1.2 t. The betatron is designed with allowance for the possibility of its transportation. The weight and dimensions of the installation make it possible to place it in the body of a truck or in a towed motor van. A design is being developed for a mobile flaw-detection laboratory, which includes a B-18 betatron. In the betatron, either a sealed-off vacuum chamber or a porcelain vacuum chamber with a titanium pump may be used; the betatron electromagnet is housed in a cast silumin casing, and the injection pulse transformer and the fans of the cooling system are housed in the same casing. The recommended range of thicknesses for radiography is up to 300 mm for steel, or 2.5 t/m² for concrete.

At present, work is continuing at NII EI on improving the existing betatrons and on developing new designs.