X Ray Tube production

1. X RAY TUBE AND PRODUCTION
OF
X RAYS
.
DR RAKESH NALLAM

2. • X-rays were discovered by accident by Wilhelm
Conrad Roentgen (1895) while he was
experimenting with the passage of electricity
through a gas at a very low pressure.
• He named those rays as x rays --x as a symbol of
the unknown quantity
• X rays are produced by energy conversion when a
fast moving stream of electrons is suddenly
decelerated in the target anode of an x ray tube.

3. X-ray tube is made
of Pyrex glass that
encloses a vacuum
containing two
electrodes - anode
and cathode.

4. Glass enclosure
• It is necessary to seal the two electrodes of the x
ray tube in vacuum because if gas were present
inside the tube the electrons that were being
accelerated toward the anode target would
collide with the gas molecules, lose energy and
cause secondary electrons to be ejected from the
gas molecules.
• So the presence of secondary electrons would
result in variation in the number and in the
reduced speed of electrons impinging on the
target.

5. • So the purpose of vacuum in the modern x ray
tube is to allow the number and speed of
accelerated electrons to be controlled
independently.

7. CATHODE
:• negative electrode
• 3 parts-
1.Filament
2.Connecting wire- supplies voltage (~10 V) & amperage
(~3-5 A)
3.Metallic focussing cup

8. FILAMENT
• Made of Tungsten wire.
• Diameter is about 0.2 mm
• Coiled to form vertical spiral of 0.2
cm in diameter and 1 cm or less in
length.
• Filament is source of electrons.

9. ADVANTAGES OF TUNGSTEN
• High melting point (3370o C).
• Little tendency to vaporize.
• Ductility & Stability
• Thin and strong
• Long life expectancy.
• Good absorption and rapid dissipation
of heat

10. FOCUSING CUP
• The forces of mutual repulsion and the large
number of electrons results in bombardment
of large area on anode of the x ray tube.
• This is prevented by a structure called
focussing cup. It is made of nickel.
• Electrical forces of focussing cup cause the
electron beam to converge on to target anode
in required size and shape.

11. • When a metal is heated its atoms absorb
thermal energy and some of the electrons in
the metal acquire enough energy to allow
them to escape from the surface of the metal
by a process of THERMIONIC EMISSION.
• So it is defined as the emission of electrons
resulting from the absorbtion of thermal
energy.

12. • The electron cloud surrounding the filament,
produced by thermionic emission has been
termed the EDISON EFFECT.
• Electrons emitted from the tungsten filament
form a small cloud in the immediate vicinty of
the filament. This collection of negatively
charged electrons forms the SPACE CHARGE.

13. • This cloud of negative charges tends to
prevent other electrons from being emitted
from the filament until they have acquired
sufficient thermal energy to overcome the
force caused by the space charge.
• The tendency of the space charge to limit the
emission of more electrons from the filament
is called as SPACE CHARGE EFFECT.

14. Heating of the
filament
High speed electron comes from the filament by the
process of thermionic emission
electron cloud formation(space charge)
leads to space charge effect
there is development of static equilibrium
There is flow of electrons is current is in one direction
only.

15. Electrons are accelerated towards
the anode by applying potential
across the tube
Sudden deceleration of the electrons take place by
the target
Production of x rays occur

16. LINE FOCUS
PRINCIPLE
• It is the principle which is used to decrease
the effective area of focal spot.
• The focal spot is the area of the tungsten
target (anode) that is bombarded by electrons
from the cathode.
• Most of the energy of the electrons is
converted into heat, with less than 1 % being
converted into X rays.

17. • Because the heat is uniformly distributed over
the focal spot, a large focal spot allows the
accumulation of larger amounts of heat before
damage to tungsten target occurs.
• The problems posed by the need for a large
focal spot to allow greater heat loading and
the conflicting need for a small focal area to
produce good radiographic detail is resolved
by the LINE FOCUS PRINCIPLE.

18. • THE ELECTRON beam bombards the target,
the surface of which is inclined so that it forms
an angle with the plane perpendicular to the
incident beam. This angle is the anode angle.

21. • The anode angle differs according to the
individual tube design and may vary from 6 to
20 degree.
• Because of this angulation when the slanted
surface of the focal spot is viewed from the
direction in which x rays emerge from the x
ray tube , the surface is foreshortened and
appears small.

22. • It is evident therefore that the side of the
effective or apparent focal spot is considerably
smaller than that of actual focal spot.
• f= F sin θ
• where f is apparent focal spot
• F is actual focal spot
• Θ is the anode angle.
• Thus as the angle of the anode is made
smaller the apparent focal spot also becomes
smaller.

24. ANODE
:• positive electrode
• Made up of Tungsten
• Alloy of Tungsten preferred
 90% Tungsten + 10% Rhenium
 More resistant to surface roughening and pitting
 Higher thermal capacity
• 2 types-
1. Stationary
2. Rotating

26. STATIONARY ANODE
• It consists of a small plate of tungsten, 2-3 mm
thick that is embedded in a large mass of copper.
• The anode angle is usually 15 – 20 degrees.
• The small tungsten is embedded /bonded to the
much larger copper portion of the anode to
facilitate heat dissipation.
• Copper is a better conductor of heat than
tungsten, so the massive copper anode acts to
increase the total thermal capacity of the anode
and so speeds its rate of cooling.

27. • The actual size of the tungsten target is
considerably larger than that the area
bombarded by the electron stream.
• This is necessary because of the relatively low
melting point of copper (1070 deg c).
• A single x ray exposure may raise the
temperature of the bombarded area of the
tungsten target by 1000 deg c or more.
• So if the tungsten target were not sufficiently
large to allow for some cooling around the
edges of the focal spot , the heat produced
would melt the copper in the vicinity of the
target.

28. Rotating anode x- ray tube
• Large disc of tungsten/ alloy of tungsten
• Beveled edge
• Carbon coating
• Stem
• Rotor
• Stator coils
• Lubricants

30. • The ability of the X ray tube to achieve high X
ray output is limited by the heat generated at
the anode.
• The rotating anode principle is used to
produce X ray tubes capable of withstanding
the heat generated by large exposures.
• The anode of the rotating anode tube consists
of a large disc of tungsten which rotates at
about 3000 rpms.

31. • The tungsten disc has a beveled edge. The
angle of the bevel may vary from 6 to 20
degree (same principle as line focus principle).

32. • Purpose Of Rotating Anode is to spread
the heat produced during an exposure
over a large area of anode
• If the target is made to rotate at a
speed of 3600 rpm, however , the
electrons will bombard a constantly
changing area of the target.

34. • The power to effect rotation is provided by a
magnetic field produced by stator coils that
surround the neck of the x ray tube outside
the envelope.
• The magnetic field produced by the stator
coils induces a current in the copper rotor of
the induction motor and this induced current
provides the power to rotate the anode
assembly.

35. • Heat generated in a solid tungsten disc is
dissipated by radiating through the vacuum to
the wall of the tube and then into the
surrounding oil and tube housing.
• ( in stationary anode heat is dissipated by
absorption and conductivity is provided by the
massive copper anode.)
• But in the rotating anode tube, absorption of
heat by the anode assembly is undesirable
because heat absorbed by the bearings of the
anode assembly would cause them to expand
and bind.

36. • Because of this problem the stem which
connects the tungsten target to the remainder
of the anode assembly is made of
molybdenum.
• Mo has high melting point (2600deg C) and is
a poor conductor of heat.
• Thus the Mo stem provides a partial heat
barrier between the tungsten disc and the
bearings of the anode assembly.

37. • If the speed of the rotation is increased,the ability
of the anode to withstand heat will become
greater.
• Three modifications to increase velocity:
1. Length of the anode stem to be made short
2. Bearings to be placed as far apart as possible
3. Inertia of the anode is reduced by decreasing the
weight of the anode.

38. GRID CONTROLLED XRAY TUBES
• A grid controlled xray tube contains its own
switch,which allows the xray tube to be turned on
and off rapidly,as is required with cinefluorography.
• A third electrode is used in the grid controlled tube
to control the flow of electrons from the filament to
the target. This third electrode is the focussing cup
that surrounds the filament.

39. SATURATION VOLTAGE
• If the potential applied across the tube is insufficient
to cause almost all electrons to be pulled away from
the filament the instant they are emitted,a residual
space charge will exist about the filament.

40. 40 kvp defines the
location of the
saturation point
of this xray tube.
below 40kvp,the
current flowing in
the tube is limited
by the space
charge effect

41. HEEL EFFECT
• The intensity of the X ray beam that leaves the
X ray tube is not uniform through out all the
portions of the beam.
• Rather it depends on the angle at which the X
rays are emitted from the focal spot. This
variation is termed as HEEL EFFECT. ( variation
in the intensity of the X ray beam after it
leaves the x ray tube).

43. • The above fig shows that the intensity of the
beam towards the anode side of the tube is
less than that which angles toward the
cathode.
• The decreased intensity of the X ray beam that
is emitted more nearly parallel to the surface
of the angled target is caused by absorption of
some of the X ray photons by the target itself.

44. • This is used to advantage when imaging
anatomical parts that are unequal in thickness
and densities throughout their respective
lengths. Eg thoracic vertebrae , humerus,
femur, tibia, fibula etc.
• The thicker portion of the anatomical part is
placed beneath the cathode end of the x ray
tube.

45. • Three clinically important aspects of the heel
effect are----
• 1) The intensity of film exposure on the anode
side of the X ray tube is significantly less than
that on the cathode side of the tube.
• 2) the heel effect is less noticeable when
larger focus film distances are used.
• 3) For equal target film distances , the heel
effect will be less for smaller films.
• This is because the intensity of X ray beam
nearest the central ray is more uniform than
that toward the periphery of the beam.

46. TUBE HOUSING
• X ray tube is always mounted inside a lead
lined protective housing that is designed to
-Prevent excessive radiation exposure
-Prevent electric shock to the patient and
operator.
• Contains oil in which the tube is bathed.

47. PROCESS OF XRAY GENERATION
Xrays are generated by two different processes
When the high speed electrons lose energy in the
target of the xray tube.

48. GENERAL/BREMSSTRAHLUNG
RADIATIONS
• These radiations are produced when the electrons
react with the nucleus of the tungsten
atoms,producing xrays termed as general
/bremsstrahlung radiations.

49. • The electrons when coming towards the
anode nucleus are deflected from their path
due to positive charge on the nucleus. The
electrons may lose energy and be slowed
down when their direction changes.The kinetic
energy lost by the electron is directly emitted
in the form of photon of radiation.

50. • The highest energy xray photon leaving the xray
tube depends on kvp used.
• The lowest energy xray photon leaving the xray
tube does not depend on kvp,instead is
determined by the filter used.
• The wavelength of xrays in the continuous
spectrum varies.The variation is produced by the
different energies with which the electrons reach
the target,and by the fact that most electrons
give up their energies in stages.

51. CHARACTERISTIC
RADIATIONS
• These radiations result when the electrons
bombarding the target eject electrons from
the inner orbits of the target atoms.
• Xrays produced are called characteristic
because these are characteristic of the atom
that has been ionised.

53. • Characteristic photon energy is equal to the
difference between binding energy of the
electron shells involved.(Ex. If l shell electron
fills k shell vacancy then k-l =characteristic
photon energy)
• When outer shell electrons fill inner shell
vacancies ,a characteristic cascade occurs.This
produces several xray photons of different
energies.

54. • Any outer shell electron can fill an inner shell
vacancy,the most likely it is the adjacent shell.
• K shell emissions are highest in energy and are the
only emissions useful to us.
• Toget k characteristics we must use at least 70kv(k
shell binding energy of tungsten is 69.5kev).

55. INTENSITY OF THE XRAY
BEAMS
• The intensity of the xray beam is defined as
the number of photons in the beam multiplied
by the energy of each photon.
• The higher the atomic number of the target
atoms,the greater will be the efficiency of the
production of xrays.

56. EFFECT OF kVp ONXRAY
BEAM
THE kVp DETERMINES THE
MAXIMUM ENERGY OF
THE XRAYS PRODUCED.
INTENSITY IS
PROPORTIONAL TO (kVp)2

57. EFFECT OF TUBE CURRENT
ONXRAY BEAM
TH NUMBER OF
ELECTRONS DEPEND
DIRECTLY ON THE
TUBE CURRENT (mA)
USED.THE GREATER
THE CURRENT ,THE
MORE ELECTRONS
WILL BE PRODUCED
AND CONSEQUENTLY
MORE X RAYS WILL BE
PRODUCED.

58. •THANK YOU