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In contrast with the original Maxwellian distribution of thermionically emitted electrons, there appear in electron beams so-called anomalous shirtings and broadenings as well as symmetrizations of the energy distribution (BOERSCH t954). These anomalies are ascribed to a phenomenon of relaxation with the energy distribution changing to a new equilibrium. A relation is given to connect beginning relaxation with cathode temperature, accelerating voltage and beam current density ~'s" The relative energy broadening is found to be proportional to js ~. This is verified experimentally. It is shown that strong relaxation is already acting in the accelerating region. The influence of source geometry, especially source aperture, is discussed. The mechanism of relaxation is derived from general principles. Therefore it could turn out to be of importance for other corpuscular beams also.

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