30 MeV Two-Beam Betatron

Yu. M. Akimov, G. D. Braslavskii, V. I. Gorbunov

At present, the method of nondestructive testing of materials and various products using X-rays and gamma rays is widely used in industry. The sources of these radiations are X-ray installations, radioactive isotopes, and accelerators, of which the betatron is the most widely used.

The use of a betatron as a source of hard gamma radiation with high penetrating power and high intensity makes it possible to inspect materials of great thickness with less time expenditure than with X-ray installations and radioactive isotopes.

Reliability, increased efficiency and economy, determination of the nature, dimensions, and depth of occurrence of defects, and volumetric introscopy—these are the basic requirements that industry imposes on betatron installations.

In seeking to satisfy these requirements, a two-beam 30 MeV betatron was designed, manufactured, and put into operation for radiation at the Tomsk Polytechnic Institute.

A brief description of the betatron installation is presented below.

In the betatron B-30/2 described here, unlike previous domestic betatrons, two acceleration cycles take place during one period of the electromagnet supply current: the first in the positive half-period, the second in the negative one. The use of the first and third quarters of the period makes it possible to obtain two beams of gamma radiation from the betatron. Depending on the arrangement of the targets and the deflection variants, the radiation beams of the two-beam betatron may travel parallel in one direction, parallel in opposite directions, or intersect.

Implementation of such a “two-beam” mode of operation of the installation leads to certain advantages in comparison with a single-beam mode and makes it possible:

to irradiate two objects simultaneously (the hardness of each beam can be regulated);

to increase the irradiation field twofold;

to reduce the irradiation exposure by a factor of two;

to obtain stereo images of an object in order to determine the nature, dimensions, and depth of occurrence of a defect;

to provide volumetric viewing of a defect (stereoscopy).

The main units of the B-30/2 betatron installation are:

1. The betatron electromagnet.

1. Capacitor bank.
2. Translation mechanism.
3. Control console.
4. Vacuum system, injector.

The electromagnet of the betatron installation consists of a magnetic core and magnetizing coils. The magnetic core is made of two yokes of П-shaped form with rectangular cross section, two poles, and central inserts located between the poles. All parts of the magnetic core are made of electrical steel E-330 with a sheet thickness of 0.35 mm.

The inter-pole space is characterized by the following parameters:

1. Magnetic-field falloff index with radius — 0.75.
2. Radius of the equilibrium orbit — 24.5 cm.
3. Gap at the radius of the equilibrium orbit — 8.5 cm.
4. Radius of the central pole plateau — 17 cm.
5. Gap between the poles in the central part — 6.5 cm.
6. Size of the working zone in radius — 15 cm.
7. Induction at the equilibrium orbit — 4300 G.
8. Induction in the central part — 13500 G.

Cooling of the betatron electromagnet is carried out by means of a fan attached to the lower yoke of the electromagnet.

The capacitor bank is intended for compensation of the reactive power of the electromagnet. The total power of the capacitor bank is 1600–2000 kvar.

The betatron is mounted on a translation mechanism that provides translation of the betatron in the vertical direction over a distance of 1 m;

rotation about a horizontal axis through an angle of ±30°;

translation of the betatron along a rail track in the direction of the gamma-radiation beams.

The control console of the betatron installation is intended for placement of the measuring and control apparatus and the controls for operation of the betatron; it is made in the form of a two-pedestal table with a vertical panel, and is connected to the betatron and to the translation mechanism by shielded cables.

The vacuum system consists of a vacuum chamber and a pumping device. The vacuum chamber, in which the electrons are accelerated, has the form of a toroid with an outer diameter of 670 mm and an inner diameter of 350 mm. The chamber is made of porcelain sectors glued together with epoxy resin. The wall thickness of the chamber is 6–6.5 mm. The vacuum chamber has 6 branch pipes for connection to the pumping system, installation of injectors, targets, probes, and other devices.

A semiconducting layer of tin oxide is applied to the inner surface of the chamber. The pumping system consists of an N-5 type diffusion pump, an RVN-20 type fore-vacuum pump, and a titanium pump. The working vacuum in the chamber is \(2 \div 5 \times 10^{-6}\) mm Hg.

The injectors are three-electrode systems (cathode, focusing cylinder, and anode—Kerst injector).

The two-beam operating mode of the betatron installation is provided by two identical, electrically unconnected channels. Each channel includes:

1. A zero-field sensor (picker).
2. A synchronization circuit with stabilization of radiation intensity and energy.
3. An injection circuit and injector.

1. Electron displacement circuit.
2. Device for indicating the gamma-radiation beam.

Such a distribution of channels facilitates operation of the installation in the two-beam mode, makes it possible to operate in a single-beam mode, and, in the event of a fault in any channel, easily to switch to operation of the second channel. In this sense one may speak of an increase in the reliability of the betatron installation as a whole.

The principal difficulties in starting up the betatron in the two-beam mode arise during the injection of electrons and their displacement from the equilibrium orbit onto the targets. The process of electron acceleration itself, when the betatron operates in the two-beam mode, does not differ substantially from electron acceleration in the single-beam mode.

In order to ensure electron acceleration during both the negative and positive half-periods of the supply voltage, it is necessary to inject the electrons in opposite directions: in one half-period clockwise, and in the other counterclockwise.

Such injection is carried out by using two independent injectors. The use of a single complex injector consisting of an anode with two slits, two focusing cylinders, and two cathodes complicates operation of the installation and, in order to obtain the rated intensity, requires additional control of the injector optics.

The radial position of the injectors is selected on the basis of starting the accelerator in the single-beam mode and is refined experimentally during operation in the two-beam mode (the injector installation radius is 290 mm).

The azimuthal position of the injectors is determined by the specified convergence of the beams. To obtain radiation beams converging at a distance of 1 m from the center of the electromagnet, the angle between the injectors must be 148°; at a distance of 3 m, 169°.

Such injection by means of two independent injectors leads to a certain deterioration in the conditions for trapping electrons into acceleration and to an approximately 10% reduction in intensity in each beam in comparison with the single-beam mode; this is compensated for by the convenience of adjustment for radiation and of accelerator operation.

Displacement of electrons from the equilibrium orbit in the single-beam mode (with one injector in the chamber) is readily accomplished by passing a current pulse through the central displacement winding located on the insert block.

As a result of the redistribution of the magnetic field, the electrons, spiraling, inevitably strike the injector target. When the accelerator operates in the two-beam mode (with two injectors in the chamber), the use of central displacement windings results in electrons displaced from the equilibrium orbit striking the targets of both injectors. By selecting the magnitude of the current pulses through the winding, it is possible to direct most of the electrons of each beam to its corresponding target, but it is not possible to obtain a clean separation of the beams.

The use of sector displacement windings leads to defocusing of the electron beam, redistribution of the radiation in space, and a significant decrease in the intensity along the axis of the accelerator beam.

The combined use of central and sector displacement windings makes it possible to separate the electron beams by targets. The windings are connected in series; the ratio of the numbers of turns of the windings is selected experimentally. The central windings, with sectors extending over 120°, are placed above and below the accelerating chamber, with the centers of the sector windings lying approximately at the azimuths of the injectors.

At present, the B-30/2 two-beam betatron is operated with the following principal technical characteristics:

1. The betatron produces two independent gamma-radiation beams.

2. The maximum bremsstrahlung energy is 30 MeV. Provision is made for smoothly varying the radiation energy from 5 to 30 MeV with an accuracy of ±10%.

3. The dose rate of the bremsstrahlung radiation at a distance of 1 m from the targets is not less than 150 R/min for each beam.

4. Radiation-beam geometry: the distance between the targets is 580 mm; the angle between the beam axes is 11°; the axes of the radiation beams intersect at a distance of 3000 mm from the center of the electromagnet.

5. The stability of the dose rate and radiation energy is no worse than 5% when the supply voltage varies by ±10% from nominal.

6. Operating duration: in continuous operation, at an ambient-air temperature not exceeding 30° and a relative humidity of 70%, not less than 10 hours.

7. Power consumption: 40 kW.