Coulomb disintegration of weak electron fluxes and the photocounts

Formation of electron bunches in the interelectrode space in vacuum detectors of optical radiation is discussed. Such bunches give rise to pulses of the electric current, usually interpreted as photocounts, in the external circuit. It is shown that the traditional theory of photocounts, in inconsistent and, in particular, it violates causality. Calculations based on the variational method are used to show that a distributed low-density electron cloud is unstable in the presence of the Coulomb forces and that it splits into bunches. The electron bunches moving in the interelectrode space experience peaking, which is easiest to understand on the basis of the catastrophe theory. Spatial (caustics) and temporal (overtaking) catastrophes may occur in an electron flux. Numerical simulation is used to consider spherical and linear expansion of electron bunches under the action of the Coulomb forces. It is shown that sharp electron density maxima are formed and that their properties resemble those of point-like particles capable of inducing electric current peaks (photocounts) in the external circuit of a detector when they travel across the interelectrode space. Circumstances leading to a higher probability of formation of one-electron bunches are pointed out. The analysis as a whole is intended to help the understanding of the discrete nature of photocounts when a photocathode is excited by a continuous high-energy laser radiation train.