Knock-on Electron

  Also called delta rays , knock-on electrons are emitted from atoms by the passage of charged particles through matter. Any charged particle traversing a medium transfers energy to that medium via the process of ionization or excitation of the constituent atoms. Due to the statistical fluctuations in energy loss, there is some probability of transmitting energy in excess of a few keV; the precise cutoff energy has to be defined as a function of the detector: they become detectable tracks. Knock-on electrons have enough energy to produce, themselves, fresh ions in traversing the medium (secondary ionization).

The probability density function for the energy transfer is approximately given by , with X the particle path length, and E the kinetic energy; for a more detailed discussion of the energy distribution, see [Barnett96].

In cosmic ray physics using photographic emulsions, -rays served to determine the charge of the observed particle. In bubble chamber physics they were used for particle identification, which is possible because the kinematics for - ray production vary drastically with the mass of the traversing particle. Also, the -ray laboratory angle of emission differs for different particles. -rays up to a few keV are emitted more or less perpendicularly to the incident track, but their mean free path is only of the order of a few microns even at atmospheric pressure.

In wire chambers, knock-on electrons can distort the signal recorded on the sense wire, and cause occasional outliers (viz. very large charges), particularly in drift chambers.