Operating Manual

4
Radiation sources
33
4.1 X-Ray tube
The X-ray tube, see figure 1-4, consists of a glass (or ceramic) envelope containing a positive
electrode (the anode) and a negative electrode (the cathode) evacuated to an ultra high
vacuum [10
- 9
hPa (hectoPascal)].
The cathode comprises a filament that generates electrons. Under the effect of the electri-
cal tension set up between the anode and the cathode (the tube voltage) the electrons from
the cathode are attracted to the anode, which accelerates their speed.
This stream of electrons is concentrated into a beam by a “cylinder” or “focusing cup”.
When the accelerated electrons collide with a target on the anode, part of their energy is
converted to X-radiation, know as X-rays.
4.2 The anode
The target is generally made of tungsten. Not only because it has a high atomic number, but
also because of its high melting point (approx. 3400˚C). It is essential to use a material with
a high melting point because of the substantial amount of heat dissipated as the electron-
“bombardment” is concentrated (focused) on a very small surface. Only a part (approx. 0.1
% at 30 keV; 1 % at 200 keV; 40 % at 30 to 40 MeV) of the kinetic energy of the electrons
is converted into X-radiation; the remainder is transformed into heat.
Cooling the anode
The heat which accompanies the production of X-radiation is quite considerable, so that
the anode has to be cooled. This can be done in a variety of ways :
1. by natural radiation
2. by convection
3. by forced circulation of liquid or gas
4. by conduction
The focal spot
The area of the target which is struck by the electrons, see figure 2-4, is called the focal spot
or “the focus”. It is essential that this area is sufficiently large to avoid local overheating,
which might damage the anode.
From the radiographic point of view, however, the focus has to be as small as possible in
order to achieve maximum sharpness in the radiographic image. This “focal loading” is
expressed in Joule/mm
2
. A tungsten target can take a maximum loading of 200 Joule/mm
2
.
A higher loading might damage the anode.
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Fig 1-4. Glass envelope X-ray tube
cathode
focusing
cylinder
or cup
filament
electron beam
X-ray beam
anode
glass
target