Technical Requirements for Betatrons Used in Flaw Detection of Steel Products

V. I. Gorbunov

(Presented by the scientific seminar of the Faculty of Physics and Technology)

Numerous studies, as well as practical experience accumulated over several years, have shown that the use of a betatron in flaw detection is not only possible, but in a number of cases necessary.

The purpose of the present article is to formulate and substantiate the basic technical requirements that a modern induction electron accelerator intended for flaw-detection purposes in thick-walled steel products under factory operating conditions must meet.

Required Range of Radiation Energy, Dose Rate, Dimensions, and Weight

On the basis of the theory of the interaction of radiation with matter, taking into account the technical conditions that must be observed when radiographing steel products, it is possible to determine the required maximum radiation energy of a betatron. A number of works [1, 2] are devoted to this question. Without dwelling on it, we shall point out that, for steel, the minimum absorption of X-rays is observed at a radiation energy of about 8 MeV. But since a betatron gives a continuous spectrum, it is obvious that, in order to increase the number of quanta with an energy of 8 MeV, it is necessary to increase the maximum energy in the spectrum. Such a maximum energy in the spectrum is an energy of 25–30 MeV.

Indeed, as experience has shown [7], such an energy range provides the best results. True, this statement can be made only with one reservation: the point is that the sensitivity of the method—in other words, the detectability of a defect—is proportional to the linear attenuation coefficient of the radiation; the larger the coefficient, the smaller the defect that can be detected [2]. Consequently, at an energy on the order of 15 MeV the sensitivity will be greater than at an energy of 25–30 MeV. Therefore, the design of betatrons intended for flaw-detection purposes must provide for the possibility of changing the maximum radiation energy in the event that especially high requirements are imposed on the part being radiographed. However, in deciding the question of the choice of

of the energy it is necessary to take into account such important factors as the power of the physical radiation dose, as well as economic considerations.

It is known that the higher the energy of the X-rays, the greater, other conditions being equal, the radiation intensity; moreover, the intensity increases almost in proportion to the fourth power of the energy [1]. But since the exposure in radiography is ultimately determined by the radiation intensity, we must strive to obtain as high an intensity as possible. Consideration of this question [2] has shown that increasing the hardness of the radiation above 25–30 MeV, first, worsens the radiographic characteristics; second, only insignificantly reduces the exposure in radiography, since the coefficient of linear attenuation increases; and third, the dimensions, weight, and consequently the cost of the installation increase sharply. It follows from this that, at energies of 25–30 MeV, the radiation intensity should be increased not by a further increase in energy, but by improving the capture mechanism.

At present, betatrons with energies of 25–30 MeV provide an intensity of 50–150 r/min at a distance of 1 m from the target [3, 7]. Such an intensity makes it possible to use exposures that are quite acceptable in industrial practice. Thus, for example, for iron 100 mm thick the exposure is less than 1 minute; for thicknesses in the range from 150 to 350 mm it is on the order of 10 minutes; and, finally, for 450–500 mm of steel the exposure is 20 minutes [3, 7, 4].

A major shortcoming of modern betatrons is their bulkiness, which creates considerable inconvenience in operation and also increases the cost of betatrons. The problem of reducing the weight and dimensions of a betatron electromagnet can be solved in two directions. First, by using high-quality steel with a high saturation-induction value. This makes it possible to reduce the weight of the electromagnet considerably. Thus, for example, the electromagnet of a 15 MeV betatron made of ordinary transformer steel weighs 1.5 tons, whereas an electromagnet for the same energy but made of KhT steel weighs only 400 kg [8]. Second—which is more advanced—by developing a multistand or shielded-type electromagnet design. Calculations have shown that, by using such an electromagnet design, it is possible to reduce the weight of the electromagnet by a factor of 6–10 and, correspondingly, the cost of the installation by a factor of 2–4.

The necessity of vertical, horizontal, and circular motion of the betatron

The testing of various articles by means of betatron radiation can be carried out for steel articles with thicknesses from several centimeters up to 0.5 m at practically permissible exposures. This is a very valuable supplement to radiographic techniques using conventional X-ray installations with energies up to 300 keV, as well as radioactive isotopes, especially Co-60.

When various radiation sources (X-ray apparatus, Co-60, betatron) are used in combination for flaw-detection purposes, the latter is used for radiographing parts 100 mm thick and above; that is, one is dealing with parts whose weight ranges from several hundred kilograms to several tons. Naturally, the question arises of the possibility of radiographing the tested object from any direction.

The proper solution of this problem must be considered to be the creation of a movable betatron design that can provide displacement of the beam relative to the horizontal axis by 180°, relative to the vertical axis ...

through 360°, and also, with the aid of an ordinary overhead traveling crane, could move in the horizontal direction. When a betatron is used for flaw-detection purposes under production conditions, precisely such a movable design is needed; with push-button control it will make it possible simply and quickly to radiograph a part of any weight in the direction we require [7].

Further improvement of the betatron for flaw-detection purposes

A betatron with a maximum energy of 25–30 MeV has a substantial shortcoming in comparison with an ordinary 200 kV X-ray installation: for one and the same distance, the irradiation field of the betatron is smaller than that of the X-ray installation. This shortcoming can be eliminated by improving the design of the betatron, namely: by using not only the first quarter of the period but also the third for accelerating electrons, it is possible, instead of one beam, to obtain two equivalent beams; in other words, it is possible simultaneously to obtain two adjacent irradiation fields. This can be achieved in two ways. In the first case, instead of one gun, two injectors are placed in the accelerating chamber, installed at 165° relative to one another. A target is placed in front of each gun, above or below the plane of the equilibrium orbit. By means of magnetic fields that vary in the radial direction, the planes of the electron orbits at the end of acceleration will be displaced so that these planes, for electrons rotating in opposite directions, will be somewhat lowered or raised [5]. This ensures that each of the two electron beams strikes the corresponding target, and both X-ray beams acquire approximately the same direction. Since the guns are arranged approximately at 165°, and the X-rays are directed along tangents to the electron orbit, two separate beams are obtained whose axes intersect at an angle of 15°.

In the second case, to obtain two beams directed to one side, a double-sided injector is used [6]. In order that both beams be directed to one side, two targets are placed in the chamber—one inside and the other outside the equilibrium orbit. Injection is carried out by means of an expansion winding, through which current pulses are passed twice during each period (at the end of the first and at the end of the third quarter-period), with the direction of the current not changing. Owing to this, these pulses alternately produce either expansion or compression of the equilibrium orbit, so that the electron beam strikes alternately the outer and the inner target.

By applying one of these two methods, it is possible to obtain radiographs of double width simultaneously. Since the exposure times for frequently encountered steel thicknesses from 100 to 150 mm are, in general, much shorter than the time required for setting up and preparing the radiographs, a considerable saving of time is obtained, which cannot be achieved by reducing the exposure time. But this is not the only advantage of a betatron with two beams. Since the targets are installed at an angle of 165° relative to one another, stereoscopic radiographs can be obtained, from which it is easy to determine the depth at which the detected defects lie, without moving either the betatron or the part.

This is especially valuable, above all, in the case of thick products, since it is necessary to know at what depth the defect lies. The method of radiography using a two-beam betatron is as follows. First, the object being tested is examined simultaneously by two beams; if, during

in this case suspicious places are detected, they are examined in greater detail by means of stereoradiography and, if required, enlarged photographs of them are obtained. It should be noted that the use of two intersecting beams significantly increases the sensitivity of the method, especially to such defects as cracks and fissures.

Thus, the creation of a betatron having two beams increases the efficiency of the installation twofold and completely solves a problem that is very important for flaw detection—the determination of the depth at which a defect lies without moving the specimen or the installation itself. If one takes into account that the sensitivity of the method increases, the importance of creating a two-beam installation for the purpose of flaw detection in thick-walled steel products becomes clear.

The creation of a betatron meeting the above requirements will undoubtedly constitute major progress in domestic betatron construction and will lead to further successes in the design and manufacture of still more advanced installations.

LITERATURE

1. Vorob'ev A. A. Accelerators of Charged Particles. Gosenergoizdatel'stvo, 1949.

2. Toruborov V. I. Modern methods of flaw detection in metallic products of great thickness. Report at the All-Union Conference on Betatrons, 1955.

3. Wilson J. I. Steel Processing, 1946, p. 780.

4. Köppers M. Bull. Schweiz Elektrotechn., 9, 152.

5. Wideröe R. Nondestructive Testing, March, 1953.

6. Wand T. Phys. Rev., 69, 42 (L), 1946.

7. Wideröe R. Braun Boveri Mitteilungen, XXVIII Jg., Sept., 301, 1951.

8. Gund K. E.T.Z., Jg. 75, H. 15, 1954.