The analysis of beams of moving charged particles by a magnetic field

Abstract

The practice of analysing beams of charged particles moving with different velocities by means of the magnetic field is now well established. Among the many important physical quantities which have been determined in this way must be included the value of e/m , the velocities, intensities and charge carried by the homogeneous β-ray groups of radioactive elements, the masses of iso­topes, and, recently, it has been applied to the analysis of very slow electrons. In most of this work it has only been necessary to find the energies of the various groups of charged particles, and for this purpose no detailed consideration of the focussing action of the magnetic field was necessary. Problems relating to the relative numbers of particles in the homogeneous groups cannot, however, be solved without more accurate knowledge of the action of the magnetic field, and it was in connection with the relative intensities of the β-ray groups of radium B and radium C that the work here described was started. In carrying out these calculations, it soon became evident that they had a much wider application than simply to the problem to which they owed their origin. In addition to furnishing a method of obtaining the structure of the line produced by the focussing action of the magnetic field for any kind of source of charged particles, these calculations indicate the best design for the apparatus containing the source of charged particles. Further, when applied to particles which have traversed thin layers of stopping material, the analysis leads to a knowledge of the velocity distribution of the retarded particles, Up to the present no method has been devised of finding this velocity distribution experimentally, and although the results obtained are only approximate, yet they show definitely that the method indicated here is quite practicable.