GENERATION OF A BEAM OF ULTRAFAST ELECTRONS FOR MEDICAL PURPOSES

A. A. VOROB'EV, B. A. KONONOV, V. A. MOSKALEV, L. S. SOKOLOV

Ultra-hard X-ray radiation, arising when electrons accelerated in a betatron are braked, is being successfully used in modern medicine for the treatment of deeply located tumors. The use for this purpose of the direct beam of accelerated electrons makes it possible to reduce the irradiation time many times over, with minimal injury to adjacent healthy tissues. In our work we consider the extraction of electrons accelerated to an energy of 15 MeV from the betatron chamber.

In a betatron, the accelerated electrons move along an equilibrium orbit, the radius of which is determined by the condition \(A = 300 B \cdot r\), where \(A\) is the electron energy in eV, \(B\) is the magnetic induction on the orbit in gauss, and \(r\) is the radius of the orbit in cm. By changing the value of the magnetic induction \(B\) for a given value of \(A\), we correspondingly change the radius of the electron orbit. In this way one can increase the radius of the electron orbit up to the inner surface of the side wall of the chamber and extract the electrons outward through a window. In our betatron \(r = 13\) cm and \(B = 4000\) gauss at an energy of 15 MeV.

At the Tomsk Polytechnic Institute, extraction of accelerated electrons from the chamber of a 15 MeV induction accelerator was carried out in three variants.

1. By the method of increasing the radius of the equilibrium orbit of the electrons with the aid of a special “extra-orbital” displacing coil.

2. By the method of removing electrons beyond the action of the betatron magnetic field with the aid of a nonmagnetic channel.

3. By the method of deflecting accelerated electrons in the electric field of a capacitor.

A displacing device was constructed with the aid of which electrons accelerated in the betatron to 10 MeV were deflected onto the wall of the chamber.

The electron beam, striking glass, caused it to luminesce. In these experiments the dimensions of the coil were \(20 \div 70^\circ\) in azimuth, and the radial dimension was 35 mm. The width of the current pulse in the coil was 30 μsec. The coil was located at the azimuth of the injector; in this case the luminous spot on the chamber wall appeared at an azimuth of 135°.

The second method of extracting accelerated electrons from the betatron chamber consists of the following. A nonmagnetic channel is created inside a magnetic shunt made of sheet transformer steel E4AA. The shunt is placed at a radius of 165 mm in the direction tangent to the instantaneous orbit at this azimuth. At the end of electron acceleration, the current pulse of the displacing winding disturbs the relation between the magnetic fluxes at the center and on the orbit. As a result, the electrons begin to move along an unwinding spiral and enter the nonmagnetic channel of the shunt. The magnetic field inside the channel is very weak (tens of oersteds), and the electrons,

moving in it practically along a rectilinear trajectory, pass beyond the limits of the accelerator’s magnetic field. The electrons are released into the atmosphere through an aluminum window \(0.2\) mm thick, with dimensions \(10 \times 40\) mm. The shunt brings out electrons of any energy equally well within the range from 5 to 15 MeV.

Adjustment of the position of the magnetic shunt makes it possible, within known limits, to change the direction of the electron beam in space.

The extracted electron current in the beam, measured at a distance of 5 cm from the window with the aid of a Faraday cylinder and a dc amplifier, is \(10^{-11}\) A.

In the third variant, the plates of the deflecting capacitor are located at a radius of 163 mm. The deflecting capacitor consists of 2 plates 47 mm long, curved in accordance with the radius of curvature of the electron orbit. The inner (cutoff) plate is made of tantalum sheet, the thickness of which is chosen from the conditions of avoiding electron interception of its front surface and of mechanical and electrical strength. In our case the thickness of the cutoff plate is 50 microns. The outer (deflecting) plate is made of stainless steel 1 mm thick. The distance between the plates is 2 mm. The accelerated electrons are led away from the orbit by means of an ordinary expansion winding fed by a pulsed current. The electrons move along an unwinding spiral, the pitch of which, at the radius where the deflecting capacitor is located, reaches a considerable value; the electrons thus become able to enter the capacitor. By the moment the electrons are displaced, a positive voltage pulse of up to 25 kV is applied to the deflecting plate of the capacitor, providing a field strength between the plates of about 80 kV/cm. Under the action of this electric field, the electrons acquire an additional acceleration in the radial direction and leave the accelerating chamber. The electrons leave the betatron chamber through aluminum foil 30 microns thick. The extracted electron beam with electron energy of 5 MeV is well focused and has a cross-sectional size of \(5 \times 8\) mm at a distance of 50 mm from the exit window. The dimensions of the electron beam at a distance of 50 cm from the window increase to 12 cm in width and 6 cm in height.

For extracting electrons accelerated to an energy of 10 MeV, the length of the plates of the deflecting capacitor was somewhat increased.

The electron current in the beam extracted by this method was measured with the aid of a Faraday cylinder and a galvanometer and amounts to not less than \(4 \cdot 10^{-9}\) A; the radiation intensity in roentgens is then 3300 r/min in the immediate vicinity of the exit window.

It is interesting to note that, for a certain position of the deflecting capacitor, a stable extraction of the electron beam with electron energies of both 5 MeV and 10 MeV is observed even in the absence of voltage on the deflecting plate, owing to the interaction of the electron beam with the capacitor plate. The cross-sectional dimensions of the electron beam then remain practically unchanged, with sufficiently good focusing. However, the electron current in this beam is lower, by approximately a factor of 20, than in the case of extracting electrons by the electric field of the capacitor.

In both variants of extracting the electron beam from the accelerator chamber, the injector is located outside the plane of the equilibrium orbit.

The electron beam obtained can be successfully used in medicine for electron therapy of malignant tumors.

At present, work is under way toward improving the methods, increasing the current in the extracted electron beam, and also extracting electrons accelerated to an energy of 25 MeV.