| Filament
Assembly
The filament is the source of electrons to be accelerated towards
the anode when heated to around 2000°C by passing a current through
the filament. The temperature of the filament determines the number
of electrons produced. The filament assembly is constructed as an
electromagnetic lens so that it focuses the accelerated electrons to
a small area of the anode, the focal spot.
There are usually two filaments, a small one with low output for
better geometric resolution filaments and a larger filament for
higher output capacity, with wire diameters of 0.22 mm and 0.3 mm
diameter respectively. The filament is constructed as a spiral with
dimensions calculated to maximise the even density of the electrons
produced. An alternative electron source is the flat emitter instead
of a helix. Used for some modern mammography tubes and allows a
better x-ray intensity distribution than with the spiral therefore,
the image quality increases.
The filament is generally made of tungsten as it is dense and hard,
relatively easy to work and it has a low vapor pressure, is a good
thermionic emitter and high melting temperature 3410 °C. Tungsten’s
low coefficient of linear thermal expansion ensures the dimensions
change little when it is heated, and the low vapour pressure ensures
little tungsten is vaporised, which when deposited on the inside of
the glass tube reduces output and increases the possibility of
arcing. The addition of between one and to percentage of thorium to
the tungsten improves thermionic emission.
The filaments are mounted in a focussing cup, the purpose of which
is by electrostatic means to focus the electrons produced by the
filament on the focal spot of the anode.
The arrangement of
the two helixes depends on the tube type. Different manufacturers’
tubes vary as well in the form of their focusing cup. There are
general differences which are sometimes expressed in the tube names.
Siemens for example have Pantix™ tubes where both filaments are
mounted parallel, or just one single filament and Biangulix™ where
both filaments are in line in-line and finally a specialist tube
Megalix™ with two parallel helixes or with three, two in-line and
the third parallel. In tubes with two parallel filaments both
electron beams are focussed on the same area of the anode, where the
filaments are in line they are focussed on different areas of the
anode.
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