arunmp's version from 2017-05-01 14:18

xray tube

Properties of X-rays

Question Answer
 X-rays travel instraight lines.
 X-rays cannot be deflected byelectric field or magnetic field.
 X-rays have a --- penetrating powerhigh
 Photographic film is blackened by X-rays.
 Fluorescent materialsglow when X-rays are directed at them.
 Photoelectric emission can be produced by X-rays.
 when an X-ray beam is passed through itIonization of a gas results

Production of X-rays

Question Answer
 X-rays are produced when rapidly moving electrons that have been accelerated through a potential difference of order 1 kV to 1 MV strikes a metal target
 Electrons from a hot element are accelerated onto a targetanode.
 When the electrons are suddenly decelerated on impact some of the kinetic energy is converted into EM energy, as X-rays.
 Less than 1 % of the energy supplied is converted into X-radiation during this process. The rest isconverted into the internal energy of the target.

X-ray Equipment

Question Answer
 Following are essential features of the x-ray equipmentX-ray tube+Main voltage supply+ High tension source+Kilo-voltage control+Filament circuit and milli-amperage control+Timer

X-ray Tube: Cathode

Question Answer
◦ A filament materialA small coil of thin thoriated tungsten wire. 2 mm dia. & 1 to 2 cm long.
◦ A trace of thorium in the tungsten wire increases theefficiency of the electron emission and prolongs the life of filament
◦ Other cathode or filament materialsMolybdenum , Rhenium
Three qualities for cathode or thermionic emitterHigh melting point, High mechanical strength ,high emission Efficiency
Commonly used Thermionic EmitterTungsten:
High melting point of tungesten is(3422 0C)
◦ Advantage of tungsten as cathodeThermionic emission capabilty+ High melting point + Can be drawn into very thin wire+ Greater mechanical strength, Low tendency to vaporize , Long structural & emission life.
Disadvantage of tungstenHigh operating temperature (>2500 0C)+High work function+Low emission efficiency.
the work function is the minimum energy needed toremove an electron from a solid to a point immediately outside the solid surface

Thermionic emisson

Question Answer
◦ The emission of electron from the filament based on thermionic emission-
◦ Define thermionic emissionthe metal is heated to sufficient temperature (~ 2500oC) to enablethe free electrons to leave the metal surface
The number of electrons emitted from cathode depends upon the temperature. The higher the temperature, the greater is the emission of electrons.
Doubling the temperature of an emitter may increase electron emission by more than107 times.
◦ Small changes in the work function of the emitter can produce enormous effects on emission.

Focusing Cup

Question Answer
 The filament is embedded in a metal shroud called thefocusing cup.
Function of focusing capAll electrons accelerated from cathode to anode are electricallynegative, the beam tends to spread out due to electrostatic repulsion, and someelectrons can even miss the anode completely.
 The focusing cup is negatively charged so that itcondenses the electron beam to a small area of the anode.
 There can be two filaments with focusing cups, providing different focal spots on the anode.
Cathode in a xray tube isfilament + focusing cup

Cathode is over ,Now about currents in Xray tube

Question Answer
Two electrical currents flow in an x-ray tubeThe filament current and The tube current
The filament currentIt is the flow of electrons through the filament to raise its temperature and release electrons.
 The tube currentIt is the flow of released electrons from the filament to the anode across the x-ray tube. This current varies from a few to several hundred milliamperes (mA).
 Filament current Vs Tube current
• At low tube potential (40kVp and lower), tube current will be space charge limited.
• Space charge places upper limit on tube current (space charge compensation necessary for change in tube current)
• At higher tube potential, tube current will be emission limited.
• Emission limited tube current cannot be increased by increase in tube potential.
• Emission limited tube current can be changed only by increasing the filament heating.

Now about Anode

Question Answer
 The anode is provided with positive potential in the x-ray tube.
◦ Function of AnodeReceives electrons emitted by the cathode + and Conducts them through the tube + Serves as a target surface or provides mechanical support for the target+ Also serves as the primary thermal conductor.
◦ most common anode material with tungsten as a targetCopper
◦ AnodeCopper
◦ Cathode from where electron is producedTungesten +Molybdenum
◦ Target Tungsten
 Factors considered for the design of anodeThermal conductivity+Small focal spot size for sharp images
  =We completed about cathodefocuss captube currentAnode Now about Target
Question Answer
 The target is the area of the anode struck by the electrons from the cathode.
 Tungsten is the material of choice for the target for three main reasons High melting point (~34220 C ) +High Z element (more Bremsstrahlung yield)+Substantial heat deposition capacity (resist cracking and pitting)+It does not vaporize easily+It can be machined and made smooth

Types of Anode

Question Answer
 There are two types of anodesStationary and ROTATING ANODE
Stationary consists of a tungsten insert embedded in a copper block
The copper serves a dual roleIt supports the tungsten target + It removes heat efficiently from the tungsten target
. Heat must be conducted away from target ,why? Otherwise it can melt the target
. most common anode material Copper
Fixed Anode is used where high tube current and power are not required
PrACTICAL APPLICATION OF fIXED ANODE IN OUR HOSPITALDental x-ray and portable x-ray machines where use fixed anode IS USED

Component Description


Question Answer
CathodeNegative electrode used to generate and repel electron beam towards anode target
FilamentMaterial within cathode that can release electrons upon addition of heat (thermonic emission)
Focusing cupOn cathode, shaped to focus electron beam created from fi lament onto the target
AnodePositive electrode to attract electron beam and dissipate heat generated in x-ray process
TargetRequired to be a high Z material with high melting point to facilitate x-ray production
Glass windowTo allow x-ray radiation to leave the tube but also must maintain the vacuum of the systemVacuum (cathode–anode gap)
A vacuumis required between the cathode (focusing cup) and the anode (target) in order togenerate a high-speed electron beam to facilitate electron-atomic interactions
Glass housingTo create vacuum within the x-ray tube