42 Chapter 1. Properties and Sources of Radiation
B.1 SourcesofElectrons
Production of electron beams is a relatively simple process and a number of devices
have been developed for the purpose. We will discuss here some of the commonly
used sources of electrons.
Electron Gun
These are used to produce intense beams of high energy electrons. Two types of
electron guns are in common use: thermionic electron gun and field emission electron
gun. A third type, photo emission electron gun, is now gaining popularity specially
in high energy physics research.
The basic principle of an electron gun is the process in which an electron is
provided enough kinetic energy by some external agent to break away from the
overall electric field of the material. The three types mentioned above differ in the
manner in which a material is stimulated for this ejection. The process is easiest in
metals in which almost free electrons are available in abundance. These electrons
are so loosely bound that a simple heating of the metal can break them loose. Each
metal has a different threshold energy needed to overcome the internal attractive
force of the nuclei. This energy is calledwork functionand is generally denoted by
W.
In a thermionic electron gun, the electrons in a metal are provided energy in
the form of heat. Generally tungsten is used in the form of a thin wire as the
source due to its low work function (4.5eV). An electrical current through this
thin wire (calledfilament) produces heat and consequently loosely bound electrons
leave the metal and accumulate nearby forming the so calledelectron cloud.To
make a beam of electrons out of this electron cloud a high electrical potential is
applied between two electrodes. The negative electrode (cathode) is placed near the
electron cloud while the positive electrode (anode) is placed away from it. Cathode
is in the form of a grid so that it could allow the electrons from the filament to pass
through it when they experience attractive pull of the anode. The intensity of the
electron beam is proportional to the number of electrons emitted by the filament,
which in turn depends on the temperature of the filament. Since this temperature
is proportional to the current passing through the filament, the current can be used
to control the intensity of the electron beam. In practice, as the filament current is
raised the electron beam intensity rises until a saturation state is reached in which
the intensity remains constant even at higher currents (see Fig.1.6.8). The intensity
of the electron beam is generally quoted in Ampere, which is a unit of electrical
current and represents the charge passing through a certain point per second.
In a field emission electron gun, the electrons are extracted from a metal using a
very high electric field of the order of 10^9 V/m. This does not require the source of
electrons to be heated but safe application of such a high electrical potential requires
a very good vacuum.
It is also possible to liberate electrons from the surface of a metal by illuminating
it with photons, a process called photoelectric emission. In order to obtain intense
electron beams, this process requires utilization of intense light sources. Currently
lasers are being used for this purpose. These photo emission electron tubes are ca-
pable of producing very high intensity electron beam pulses.