The x-ray tube
By
Dr. Mohsen Dashti
357 Radiologic Imaging & Processing
Lecture notes # 3
Lecture outline
• The cathode assembly.
• The anode assembly.
• The glass envelope.
• Protective housing.
The cathode assembly
• It is a fact that in order to produce x-rays three major
elements MUST take place:
1. A source of electrons.
2. An appropriate target material.
3. A high voltage and vacuum presence.
-
The x-ray tube is the environment that permits these
conditions to exist and with it allow x-ray photons to
come into existence.
The cathode assembly
• The x-ray tube consists of cathode and an anode
enclosed within glass envelope and then encased in a
protective housing making up the necessary elements
to produce x-rays.
• The cathode:
- The cathode is the negative side of the x-ray tube that
consists of filament(s), focusing cup and loops of wires.
The cathode assembly
• The cathode has three major functions:
1. Produce thermionic cloud.
2. Conduct the high voltage to the gap between cathode
and anode.
3. Focus the electron stream as it heads for the anode.
- The filament within the focusing cup of the cathode
assembly is a small coil of thin tungsten wire. Why
tungsten?
-- Because it has high melting point (3,370oC) and very
difficult to vaporize or turn into gas.
The cathode assembly
- The high melting point available in the tungsten wire
permits the filament to operate at the high temperatures
required of an x-ray tube.
- Tungsten wires make the filament which provides
sufficient resistance to the flow of electrons so that heat
produced will cause thermionic emission to occur.
- And since a tungsten filament do not exhibit significant
thermionic emission below 2,200oC then this will cause
electrons to leave the surface of the filament wire and
form the necessary thermionic cloud that will be driven
towards the anode target where x-ray photons will be
produced.
The cathode assembly
• Continuous production of thermionic clouds by the
filament will eventually cause vaporization of the
tungsten wires that contribute to the reduction of
vacuum capability of an x-ray tube.
• Vaporization of tungsten wires is also gradually deposited
on the inner surface of the glass envelope, causing old
tubes to have mirrored appearance and high-voltage
arcing that eventually will destroy the x-ray tube.
• Vaporization will cause the filament to break over time
similar to a light bulb with continuous usage.
The cathode assembly
• The filament is housed in a shallow depression in the
cathode assembly known as focusing cup.
• Focusing cup is made of nickel and its main purpose is to
narrow the thermionic cloud as it is driven toward the
anode.
• Focusing cup helps the negative electrons to travel into
straight line toward the anode instead of diverging into
different lines because of their similar charges.
The anode assembly
• The anode is the positive side of the x-ray tube and has
three major functions:
1. Serves as a target surface for the high-voltage electrons
from the filament, thereby becoming the source of the
x-ray photons.
2. Conducts the high-voltage from the cathode back into
the x-ray generator circuitry.
3. Serves as the primary thermal conductor.
The anode assembly
• The anode assembly consists of the anode, stator and
rotor.
1. The anode:
- Anodes are divided into stationary and rotating. The
rotating anodes turn during the exposure, thus
presenting a much larger target area. The faster the
anode rotates, the better the heat dissipation that help
in preserving a longer tube life. How?...
- Stationary anodes have become of less use and limited
to low-power functions, such as dental units.
The anode assembly
• Rotating anode disks range from 5-12 cm in diameter and
are composed of molybdenum.
• Molybdenum is a group of 6 chemical element that does
not occur as free metal in nature and has the ability to
stand high-voltage.
• For the reason it is used in the make up of anode disks to
help dissipate heat in heavy load situations.
• Specialized x-ray tubes for mammography utilize
molybdenum as the primary material due to its ability to
emit a more uniform range of lower energy photons.
The anode assembly
• One major problem in the anode assembly is the use of
line-focus principle, which in fact permits the best
resolution of detail while permitting as large an actual
area as possible to increase thermal conductivity. So
where is the problem?
- Anode Heel Effect.
- Because of the geometry of an angled anode target, the
radiation intensity is greater on the cathode side.
- This allows the cathode end to have 20% more photons
and 25% less photons at the anode end, a total of 45%
variation exists parallel to the anode-cathode axis.
The anode assembly
• Anode Heel Effect:
- The 45% variation is significant enough to cause a visible
difference in film density during radiographic
examinations when large film sizes are used at short
distances.
- Because the cathode end of the x-ray tube has a more
intense beam, it should be positioned toward the denser
(thicker) part of the body.
The anode assembly
2. The stator:
- The stator is the induction-motor electromagnet that
turns the anode and is the only part of the cathode or
anode assemblies that is located outside the vacuum of
the glass envelope.
- If the stator fails, the rotor will cease to turn the anode,
resulting in immediate melting of a spot on the target
that will damage the anode.
3. The Rotor:
- The rotor is located inside the stator and inside the
glass envelope. It is composed of a hallow copper
cylinder attached to the anode disk and affected by the
electromagnetic field of the stator, causing the anode to
turn.
The anode assembly
• The rotor contains silver plated steel ball bearings that
help the anode to turn. When these bearings go bad
caused by long use at high temperature, they become
imperfectly round. This leads to grinding noise and
wobbling of the rotor that eventually throws the focal
track of the anode off center and tube efficiency drops
dramatically.
• Another effect of stator or rotor failure is that the
electron stream overheats the target area of the anode
focal track.
The glass envelope
• The entire cathode and anode assemblies except the
stator are enclosed within a glass envelope commonly
called the tube.
The glass envelope
• A common glass envelope is made by sculpting several
different types heat-resistant Pyrex glass.
• The primary function of the glass envelope is to maintain
the vacuum between the cathode and the anode.
• After construction, the air is removed from the tube by
the use of special vacuum and then permanently sealed.
• The removal of air permits electrons to flow from
cathode to anode without encountering the gas atoms of
air and greatly increases the efficiency of x-ray tube
operation.
Protective housing
• Modern x-ray tubes must be mounted inside a protective
housing that controls leakage and scatter radiation as
well as isolates the high voltages and provides a means
to cool the x-ray tube.
A: anode
B: Expansion bellows
(provide space for oil to
expand)
C: cathode (and
heating-coil)
E: tube envelope
(evacuated)
H: tube housing
O: cooling dielectric oil
R: rotor
S: induction stator
T: anode target
W: tube window
Time to relax