1) The document discusses the components and functioning of an x-ray tube, including the cathode, thermionic emission, space charge effect, focussing cup, anode, and grid control. 2) It describes how increasing the voltage across the x-ray tube increases the kinetic energy of electrons, producing x-rays via bremsstrahlung and characteristic radiation processes. 3) Rotating and stationary anodes are discussed as ways to dissipate heat generated during x-ray production and allow higher power outputs from the tube.

1. X RAY TUBES &
PRODUCTION OF X-RAYS
MODERATOR-DR
PRESENTER-DR.JAYA ADITYA.P

3. Glass Enclosure
• It is necessary to seal the two electrodes of the x-ray tube in vacuum.
• If gas were present inside the tube the electrons accelerated toward
the anode target would collide with the gas molecules, lose energy
and cause secondary electrons to be ejected from the gas molecules.
• This results in variation in the number and in the reduced speed of
the electrons impinging on the target. This would cause a wide
variation in the energy of the x rays produced.

4. CATHODE
• The negative terminal of the x-ray tube.
• In addition to the filament, the cathode has two other elements.
• The connecting wires, which supply both the voltage and the
amperage and focussing cup.

5. THERMIONIC EMISSION
• Defined as the emission of electrons resulting from the absorption of
thermal energy.
• 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.

6. EDISON EFFECT SPACE CHARGE
• The electron cloud surrounding
the filament,produced by
thermionic emission.
• 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.

7. SPACE CHARGE EFFECT
• The 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.

8. FOCUSSING CUP
• Surrounds the filament.
• Because of the forces of mutual repulsion, this electron stream would
tend to spread itself out and result in bombardment of an
unacceptably large area on the anode.
• This is prevented by focussing cup which is maintained at negative
potential.

9. 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.
• This heat is uniformly distributed over the focal spot, a large focal
spot allows the accumulation of larger amounts of heat.

11. • 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.
• Anode angle- The electron stream bombards the target, the surface
of which is inclined so that it forms an angle with the plane
perpendicular to the incident beam.
• The anode angle differs according to individual tube design and may
vary from 6 to 20°.

12. • Size of the projected focal spot is directly related to the sine of the
angle of the anode.
• Sine 20° = 0.342 and sine l6.SO = 0.284, an anode angle of 16.5° will
produce a smaller focal spot.

13. ANODE
• Positive electrode
• Tungsten is chosen as the target material because high atomic
number, which makes it more efficient for the production of x rays.
• And high melting point (good material for the absorption of heat and
for the rapid dissipation of the heat).
• Types of anode- Stationary and Rotating.

14. Stationary Anode
• It consists of a small plate of tungsten, 2-3 mm thick that is
embedded in a large mass of copper which aids 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.
• The actual size of the tungsten target is considerably larger than that
the area bombarded by the electron stream (because of the relatively
low melting point of copper).

16. ROTATING ANODE

17. • The rotating anode principle is used to produce X ray tubes capable of
withstanding the heat generated by large exposures.
• Rotating anode tube consists of a large disc of tungsten, or an alloy of
tungsten.
• The tungsten disc has a beveled edge. The angle of the bevel may
vary from 6 to 20 degree.
• The purpose of the rotating anode is to spread the heat produced
during an exposure over a large area of the anode.

18. • If the filament and focusing cup of the x ray tube produce an electron
beam that covers an area of anode 7 mm high and 2 mm wide, the
area of the anode bombarded by electrons is 14 sq mm rectangle.
• If we assume the average radius of the bombarded area of the
tungsten disc to be 40 mm, the circumference of the disc at a radius
equal to 40 mm will be----251 mm (2 π r).
And Area = average circumference of disc X height of e- stream
• = 251 x 7
• = 1757 sq mm

20. • Now at a speed of 3600 rpm (60 rps), any given area on the tungsten
disc is found opposite the e- stream only once every 1/60 sec and the
remainder of the time- generated heat can be dissipated.
• During 1/60 sec exposure , the entire circumference of the tungsten
disc will be exposed to the electron beam.
• 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.

21. • To overcome this problem the stem which connects the tungsten
target to the remainder of the anode assembly is made of
molybdenum.
• This is because Mo has high melting point and is a poor conductor of
heat.
• (ATOMIC NUMBER-42)

22. GRID CONTROLLED XRAY TUBES
• A grid controlled x-ray tube contains its own switch, which allows the
x-ray tube to be turned on and off rapidly.
• 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.

23. SATURATION VOLTAGE
• If the potential applied across the tube is insufficient to cause all
electrons to be pulled away from the filament, a residual space
charge will exist about the filament.
• This residual space charge acts to limit the number of electrons
available.

25. HEEL EFFECT
• The intensity of the x-ray beam that leaves the x-ray tube is not
uniform throughout all portions of the beam.
• The intensity of the beam depends on the angle at which the x rays
are emitted from the focal spot-Heel Effect

27. • 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.

28. Tube Shielding
• The tube housing is lined with lead -absorbs primary and secondary x-
rays that would otherwise produce a high intensity of radiation
around the tube, resulting in needless exposure.
• It also provide shielding for the high voltages required to produce x
rays.

29. Metal/Ceramic X-Ray Tubes
• This tube has a metal casing instead of the usual glass envelope, and
has three ceramic insulators.
• Two insulators provide insulation for the two (positive and negative)
high-voltage cables, and one supports the anode stem.
• Ceramic insulators are used to insulate the high voltage parts of the x-
ray tube from the metal tube envelope.

31. PROCESS OF XRAY GENERATION
• X rays are produced by energy conversion when fast-moving electrons
from the filament of the x-ray tube interact with the tungsten anode.
• E = eV
• Increasing the voltage across the x-ray tube will increase the kinetic
energy of the electron.

32. X ray s are generated by two different
processes-
• General Radiation (Bremsstrahlung)
• Characteristic Radiation

33. General Radiation (Bremsstrahlung)
• When an electron passes near the nucleus of a tungsten atom, the
positive charge of the nucleus acts on the negative charge of the
electron.
• The electron deflected from its original direction.
• The electron may lose energy and be slowed down.
• The kinetic energy lost by the electron is emitted directly in the form
of a photon of radiation.

35. Characteristic Radiation
• Characteristic radiation results when the electrons bombarding the
target eject electrons from the inner orbits of the target atoms.
• Removal of an electron from a tungsten atom causes the atom to
have an excess positive charge.
• In the process of returning to its normal state, the ionized atom of
tungsten may get rid of excess energy in one of two ways.

36. • An additional electron may be expelled by the atom and carry off the
excess energy.
• This electron is k/a Auger electron.
• The ejection of Auger electrons does not produce x rays.

37. • An alternative way to get rid of excess energy is for the atom to emit
radiation that has wavelengths within the x-ray range.
• Binding energy-The energy that an electron in a shell must be given to
raise the energy value to zero or to a positive value.
• For instance, the binding energy of an electron in K shell is 70 KeV.
• After an impinging electron uses a 70 ke V of its energy to eject the K-
shell electron, the remaining energy is shared between the initial
electron and the ejected electron and these electrons leave the atom.

38. • The ionized tungsten atom is unstable, and the K-shell electron is
rapidly replaced, usually with an electron from the L shell.
• The energy lost by the L shell electron is radiated as a single x-ray
photon.

40. Intensity of X-Ray Beams
• The intensity of an x-ray beam is defined as the number of photons in
the beam multiplied by the energy of each photon.
• The intensity is commonly measured in roentgens per minute (R/min)
• Target Material-
• The higher the atomic number of the target atoms, the greater will be
the efficiency of the production of x rays.
• The atomic number of the target material determines the energy, or
quality, of characteristic x rays produced.

41. EFFECT OF kVp ON XRAY BEAM
• The energy of the electrons is determined by the peak kilovoltage
(kVp) used.
• Determines the maximum energy (quality) of the x rays produced.

43. X-Ray Tube Current
• The number of electrons depends directly on the tube current (mA)
used.
• The greater the mA the more electrons that are produced;
consequently, more x rays will be produced .

45. THANK YOU