0202X-ray TubeX-ray Tube

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Rotating anode tube is Rotating anode tube is similar in many respects to similar in many respects to the stationary anode X-ray the stationary anode X-ray tube, plus a number of tube, plus a number of additional features (Figure).additional features (Figure).

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Figure: Rotating anode tube and casing

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Rotating anode X-ray tube Rotating anode X-ray tube produce higher intensities of X-produce higher intensities of X-ray beams than the stationary ray beams than the stationary anode tube. This is due to 2 anode tube. This is due to 2 factors:factors:The heat deposited on the The heat deposited on the

anode is spread over a much anode is spread over a much larger area.larger area.

The cooling characteristics of The cooling characteristics of the rotating anode are superior. the rotating anode are superior.

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Figure: Rotating anode tube

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Construction of the rotating Construction of the rotating anode X-ray tube. Cathode has anode X-ray tube. Cathode has the same basic construction the same basic construction except that it is off-set from the except that it is off-set from the central axis of the tube in order central axis of the tube in order that electrons emitted from it that electrons emitted from it strike the strike the bevelledbevelledsurface of surface of the anode. the anode.

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Anode AssemblyAnode AssemblyAnode is made of a tungsten Anode is made of a tungsten

or molybdenum disk with an or molybdenum disk with an accurately bevelled edge.accurately bevelled edge.

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The anode has a central hole The anode has a central hole through which it is connected through which it is connected to a beryllium anode stem and to a beryllium anode stem and hence to the hence to the rotor of the rotor of the induction induction motor motor (Figure).(Figure).

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The diagrammatic representation The diagrammatic representation of the anode and rotor assembly of the anode and rotor assembly in figure below illustrates the in figure below illustrates the arrangement.arrangement.

Figure: X-ray tube insertand stator

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Insert tiub sinar-x dan stator

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The stator is composed of 2 The stator is composed of 2 windings set into a circular windings set into a circular iron core (figures below).iron core (figures below).

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Figure: Stator core and windings

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Figure: Photograph of stator core

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Rotor, stem and anode disk are Rotor, stem and anode disk are accurately balanced so that no accurately balanced so that no wobbling occurs when the whole wobbling occurs when the whole assembly rotates.assembly rotates.

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In the more modern rotating In the more modern rotating anode tubes, the anode is anode tubes, the anode is made of a solid block of made of a solid block of molybdenum molybdenum with a thin with a thin coating of coating of an alloy of an alloy of tungsten andtungsten andrhenium onrhenium onthe surface. the surface.

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Molybdenum has double the Molybdenum has double the specific heat capacity of specific heat capacity of tungsten and so produces an tungsten and so produces an anode of much anode of much higher heat higher heat storage capacitystorage capacityin terms of Heatin terms of Heatunits.units.

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Rhenium has an atomic Rhenium has an atomic number of 75 (tungsten, 74) number of 75 (tungsten, 74) and so has a good conversion and so has a good conversion of electron energy to X-rays. It of electron energy to X-rays. It also has the advantage of also has the advantage of slowing down the ageing slowing down the ageing process on the anode surface process on the anode surface caused by the inevitable pitting caused by the inevitable pitting of the surface during of the surface during exposures. exposures.

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Induction MotorInduction MotorInduction motor isInduction motor is used to drive used to drive

the rotorthe rotor. The motor work on the . The motor work on the principles of electromagnetic principles of electromagnetic induction, particularly Lenz’s induction, particularly Lenz’s law. The principle of this type of law. The principle of this type of motor is depicted in Figure motor is depicted in Figure below.below.

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Figure below shows more Figure below shows more efficient system, where more efficient system, where more magnets are used and a higher magnets are used and a higher flux linkage is obtained. flux linkage is obtained.

The copper drumThe copper drum(the rotor) follows(the rotor) followsthe direction of the direction of rotation of the rotation of the magnets.magnets.

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The magnetic field from the The magnetic field from the solenoids penetrates the glass solenoids penetrates the glass envelope of the X-ray tube and envelope of the X-ray tube and interacts with the copper rotor on interacts with the copper rotor on the anode assembly (Figures the anode assembly (Figures below).below).

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Electron FocusingElectron FocusingDuring conduction, the anode of During conduction, the anode of

the X-ray tube is positively the X-ray tube is positively charged and the cathode is charged and the cathode is negatively charged (Figure). negatively charged (Figure).

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The electron space charge The electron space charge emitted by the heated filament emitted by the heated filament is thus repelled from the is thus repelled from the cathode and attracted towards cathode and attracted towards the anode, since electrons are the anode, since electrons are themselves negatively themselves negatively charged.charged.

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In the case of a rotating In the case of a rotating anode tube, the heat anode tube, the heat caused by conversion of caused by conversion of their energy is spread over their energy is spread over a much larger area, the a much larger area, the disc’s focal track. For a disc’s focal track. For a stationary anode, electrons stationary anode, electrons bombard a small, bombard a small, rectangular area.rectangular area.

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Figure: A conventional rotating anode disc.(a) Face view; (b) In profile.

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If both the anode and cathode were If both the anode and cathode were flat plates, the electric field consists flat plates, the electric field consists of parallel lines starting from the of parallel lines starting from the anode and finishing on the cathode anode and finishing on the cathode (Figure). (Figure).

Figure: Focusing of theFigure: Focusing of theelectrons in an X-ray tubes. electrons in an X-ray tubes. The concave focusing cupThe concave focusing cupdirects the electrons from directs the electrons from thermionic emitter F toward the centralthermionic emitter F toward the centralaxis, so that they strike the anode over axis, so that they strike the anode over a small area.a small area.

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The plume of electrons from the The plume of electrons from the thermionic emitter F, being thermionic emitter F, being charged negatively, travel charged negatively, travel against the direction of the against the direction of the electric field, striking electric field, striking the anode over a the anode over a width, W (W>F). The width, W (W>F). The situation situation produces producesan unacceptably large an unacceptably large focal area on the anode. focal area on the anode.

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In the case of focussing-cup In the case of focussing-cup cathode, the thermionic electrons cathode, the thermionic electrons from F now experience a force from F now experience a force which is always towards the which is always towards the central axis (as well as towards central axis (as well as towards the anode) owing to the shape of the anode) owing to the shape of the electric lines of the electric lines of force produced by force produced by the focusing cup the focusing cup (Figure). (Figure).

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The plume of electrons is The plume of electrons is spread over a much smaller spread over a much smaller width, w, on the anode (w<F) width, w, on the anode (w<F) and the electron beam is said to and the electron beam is said to have been focused. A line-focus have been focused. A line-focus is produced. is produced.

Figure: The focusing cup directsFigure: The focusing cup directsthe electrons from thermionic the electrons from thermionic emitter F toward the centralemitter F toward the centralaxis, so that they strike theaxis, so that they strike theanode over a small area.anode over a small area.

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Anode Heel EffectAnode Heel EffectFigure below shows crazing or Figure below shows crazing or

roughening of the target surface. roughening of the target surface. Such crazing results in a loss of Such crazing results in a loss of radiation output from the tube. radiation output from the tube. Photons of radiation being Photons of radiation being absorbed to a greater extent as absorbed to a greater extent as they must now penetrate a they must now penetrate a greater thickness of metal. greater thickness of metal.

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Roughening of the target Roughening of the target surface, due to cumulative surface, due to cumulative effects of heat stress, effects of heat stress, creates pitting, the formation creates pitting, the formation of small crevices into which of small crevices into which electrons from the filament electrons from the filament may enter. may enter.

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X-ray photons so produced X-ray photons so produced have a greater thickness of have a greater thickness of anode substance to anode substance to penetrate before they penetrate before they emerge from its surface. emerge from its surface. This reduces the beam This reduces the beam intensity, compared with the intensity, compared with the output from a new, smooth output from a new, smooth target surface (Figure).target surface (Figure).

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Figure: Effects of target surface pittingFigure: Effects of target surface pitting

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FigureFigure(a) Effect of (a) Effect of

anode anode crazing.crazing.(b) Photographs (b) Photographs of crazed of crazed anodes.anodes.

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Care of the X-ray tubeCare of the X-ray tubeAn X-ray tube is an An X-ray tube is an

expensive and vulnerable expensive and vulnerable piece of equipment. If piece of equipment. If damaged, it is unlikely to be damaged, it is unlikely to be repairable. repairable.

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Tube damage may be either Tube damage may be either physical or thermal. Physical physical or thermal. Physical damage is avoided simply by damage is avoided simply by ensuring that the tube is never ensuring that the tube is never allowed to collide with any other allowed to collide with any other object, and that all movement is object, and that all movement is reasonably smooth and steady. reasonably smooth and steady. Thermal damage is minimized Thermal damage is minimized by:by:

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Keeping the rate of heat Keeping the rate of heat production within the safe production within the safe limits, determined by the tube limits, determined by the tube manufacturer.manufacturer.

Ensuring, before making each Ensuring, before making each exposure, that the anode can exposure, that the anode can safely accept the extra heat safely accept the extra heat which the exposure will bring.which the exposure will bring.

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X-ray Tube RatingX-ray Tube RatingDefinitionDefinition

The rating of an X-ray unit is The rating of an X-ray unit is that combination of exposure that combination of exposure settings which the unit can settings which the unit can just withstand without just withstand without incurring unacceptable incurring unacceptable damage.damage.

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Any exposure gives rise to some Any exposure gives rise to some ‘damage’ to the X-ray tube, since ‘damage’ to the X-ray tube, since the anode becomes slightly more the anode becomes slightly more pitted and the filament becomes pitted and the filament becomes slightly thinner. However, in this slightly thinner. However, in this context unacceptable damage context unacceptable damage means that amount of damage means that amount of damage which will seriously impair the which will seriously impair the performance of the unit for further performance of the unit for further exposures or even to make the unit exposures or even to make the unit completely inoperative.completely inoperative.

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Single Exposure FactorsSingle Exposure FactorsThe exposure factors which The exposure factors which are under the control of the are under the control of the operator are:operator are:Selectable FactorsSelectable FactorsNon-selectable factorsNon-selectable factors

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Selectable FactorsSelectable FactorskVkVpp, mA, exposure , mA, exposure

time/mAs, focal spot time/mAs, focal spot size, single / multiple size, single / multiple exposures, radiographyexposures, radiography/ screening./ screening.

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Non-selectable factorsNon-selectable factorsStationary Anode TubeStationary Anode Tube

Rectification, thermal Rectification, thermal capacity of anode/shield, capacity of anode/shield, efficiency of heat loss from efficiency of heat loss from anode/shield, anode angle, anode/shield, anode angle, filtration, rating of high-filtration, rating of high-tension cables and tension cables and transformers.transformers.

Rotating Anode TubeRotating Anode TubeAs above, plus: anode As above, plus: anode diameter, anode rotation diameter, anode rotation speed.speed.

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Rating Chart Rating Chart Stationary Anode TubesStationary Anode TubesRating chart shows the Rating chart shows the

effect of varying the effect of varying the quantities in the selectable quantities in the selectable factors list, i.e. those which factors list, i.e. those which may be altered.may be altered.

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For any particular X-ray For any particular X-ray unit, the quantities in the unit, the quantities in the non-selectable factors list non-selectable factors list is unalterable, and a rating is unalterable, and a rating chart is used which is chart is used which is applicable only to that unit.applicable only to that unit.