The betatron and its medical application.

It is well known, that high-energy electrons can be used for tumor therapy. The so-called conventionel therapy with 100 through 250keV x· rays causes a great part of the x.rays to be scattered and absorbed in the sane tissue. In spite of the medicamental radiation prophylaxis additional radiation diseaes result by those compton scattered rays. By application of fast electrons and hard x.rays (so called gamma. rays) one tries to diminish those undesired side-effects and at the same time to increase the therapeutical effect of the ray treatment. As radiation source for fast electrons and hard gamma.rays one uses the Betatron, which was developed by NBRST in 1941 after preliminary operation of SLEPIAN, WALTON, WIDEROE and STEENDECK. The following statements are based on the references (1) through (6). ∗PMID: 4247895 [PubMed indexed for MEDLINE] Copyright c ©OKAYAMA UNIVERSITY MEDICAL SCHOOL Acta Merl. Okayama 24, 237-247 (1970) THE BETATRON AND ITS MEDICAL APPLICATION E. SZIRMAI and P. ROYL Department of Nuclear Hematology and Radiation Biology, Institute of Nuclear Energy, University of Stuttgart, Stuttgart, Germany Received for publication, October 31, 1969 It is well known, that high-energy electrons can be used for tumor therapy. The so-called conventionel therapy with 100 through 250keV x· rays causes a great part of the x.rays to be scattered and absorbed in the sane tissue. In spite of the medicamental radiation prophylaxis additional radiation diseaes result by those compton scattered rays. By application of fast electrons and hard x.rays (so-called gamma. rays) one tries to diminish those undesired side-effects and at the same time to increase the therapeutical effect of the ray treatment. As radiation source for fast electrons and hard gamma.rays one uses the Betatron, which was developed by N BRST in 1941 after preliminary operation of SLEPIAN, WALTON, WIDEROE and STEENDECK. The following statements are based on the references (1) through (6). The Principle of the Betatron. The Betatron is comparable to an ordinary transformer wherein the high voltage or secondary winding consists of an evacuated doughnutshaped tube (K) in Fig. 1 in which electrons moving at high velocity form the secondary circuit. This ring tube is placed between a C.shaped focusing magnet (St). An electron gun G (heated cathode) proiects free electrons into the evacuated tube. These are accelerated by the action of the changing magnetic field and at the same time they are caused to move in a circular orbit along the axis of the tube by virtue of the existence of a magnetic field. An alternating current is sent through the primary coils Psp of the Betatron. With increasing current the magnetic flux ep(R)= ~~Bz(r) 2rr+dx = BZ(R)R2. rr ( 1 ) increases. In equation (1) R is the radius of the circular orbit, ¢(R) the total magnetic flux of the circular disk with radius R, and Bz(R) is the mean ·value of the magnetic induction between r = 0 and R. According to Farraday's law of electromagnetic induction an electromotive force is induced at R, whose momentary value is proportional to the time rate of 237 1 Szirmai and Royl: The betatron and its medical application Produced by The Berkeley Electronic Press, 1970 238 E. SZIRMAI and P. ROYL --_ .. _--... \ I ~: I I I I r I I I Ifp: I Figure 1 Schematic View of the Betatron. ( 2 ) change of the magnetic flux in the circular disk of radius R. U d(R)= _ dep(R) = _R2j[dBz tn dt dt The electromotive force builds up an electric field whose value is related to the electromotive force Uind(R) by equation (3) ( 3 ) An electron with charge eU, which moves at the velocity v in a circular orbit of radius R is accelerated tangentially by the electric force K =eo• I~(R) I in direction of the electric field. The energy dE picked up by the electron having moved a distance ds in direction of the field during an increase dBz of the magnetic induction is given by equation (4). R ds RdE = kds = eo I€ (R) Ids = eO "7[ dt dBz(R) = eO"7[vdBz(R) ( 4 ) If one makes the electron stay on the circular orbit of R, while the induction increases from 0 toBz • it ga!ns the energy E=eo~ ~~zv dBz(R) To keep the electron on the circular orbit the centrifu~al force must 2 Acta Medica Okayama, Vol. 24 [1970], Iss. 2, Art. 9 http://escholarship.lib.okayama-u.ac.jp/amo/vol24/iss2/9 The Eetatron and its Medical Application Figure 2 The Magnetic Field and the Interacting Forces t------s~ ttl -----.l 239