In a cathode-ray tube, Fig. 1, electrons Figure 1 . The cathode-ray tube. Electrons from the cathode C pass through the slits at A and B and strike a phosphorescent screen. The electron is one of the elementary particles, called leptons, which include muons and neutrinos among others. The energy of the electron volt is given in units of electron volts with the symbol eV. One electron-volt is a unit of energy equal to the work done on an electron moving it through a potential difference of one volt.
Electrons occupy well-defined energy levels around the positively charged nucleus of an atom.
The positive charge is determined by the number of protons and gives the atomic number, Z, of the atom. Vacancies in the inner electron levels of the atom can be produced by irradiation with X-rays or energetic electrons or protons. The excited atoms can release their energy by emission of photons such as X-rays or by emission of electrons. This last process, discovered by Pierre Auger in 1927, is known as Auger emission and is the dominant de-excitation process for low atomic number elements such as boron or carbon. Figure 2 illustrates the Auger process.