Experimental Evaluation of the Physical Characteristics of a 45-MEV Medical Linear Electron Accelerator

Abstract

T He Specific components designed for the therapeutic application of the 45-MEV linear electron accelerator installed at Michael Reese Hospital and the general principles of operation of this apparatus have previously been reported (1). An electron beam was first released from the accelerator tube on March 6,1954, but measurements of the physical characteristics of the entire system could be carried out only after the magnet system had been designed, manufactured, and installed. Theoretically, a linear electron accelerator could be designed in such a manner as to produce a practically monoenergetic electron beam of definite size with energy controlled by the amount of input power or frequency. In the present stage of development, however, an external magnet system is required for the control of the energy and other physical properties of the output electron beam. The performance of the entire system depends as much on the magnets as on the accelerator. In this paper, some basic characteristics of the accelerator itself will be discussed; the principles of design and the measured performance of the magnet system will be explained; finally, experimental results obtained with the composite system will be presented. Only the physical characteristics pertaining to therapeutic applications will be emphasized. Operational Characteristics of the Accelerator Briefly, a traveling-wave linear electron accelerator consists of a disk-loaded copper tube in which an electromagnetic wave with a strong axial electric field and a velocity close to that of light is excited. Mounted at the input end of the tube is an electron gun which injects a beam of electrons into the tube along the axis. These electrons are accelerated by the electric field and pick-up energy until they eventually travel in phase with the wave at a velocity approaching that of light; when they reach the output end of the tube, they have acquired their maximum energy, which is approximately proportional to the square root of the input microwave power used to excite the traveling electromagnetic fields. The injected electrons are uniformly spaced during the injection pulses, but the time phase of the electromagnetic wave varies continuously. The ultimate fate of each electron will therefore depend upon the initial relative phase angle. It can be shown that, for axial electrons with fixed velocity and electromagnetic fields of constant strength; the greatest number of accelerated electrons will assume a certain asymptotic phase angle which determines the peak energy of a long accelerator; other accelerated electrons will assume an asymptotic phase angle between that value and —180o and therefore assume different energies (2). The energy spectra of our accelerator were determined, and the half-intensity width was found to be broader for lower energies. The spectra were obtained by varying the magnetic field of the analyzing magnet for different energies and registering the output current with an ion chamber while the energy-defining slit was adjusted for 1 per cent energy spread.