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EQUIPMENT FOR X- & GAMMA RAY RADIOGRAPHY

EM SPECTRUM

Generation of X-Rays

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eElectron streamScattered electronTarget atomX-rays

Three basic requirementsA source of electronsa heated filament

A means of directing and accelerating the electronsa high voltage supply

A target for the electrons to bombarda heavy metalHot Body

++++++++CathodeAnodeelectronsHigh positive charge

Mechanism of X-ray ProductionWhen the target is bombarded with high-speed electrons, two kinds of interactions occur, thus producing two type of X-rays.Characteristic X-raysContinuous or Bremsstrahlung X-rays

Generation of characteristic X-ray

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X-rayHigh-speed electronLower energy ringHigher energy ring

Generation of continuous X-ray

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X-rayHigh-speed electronLower energy ringHigher energy ring

X-ray Spectrum

X-ray EquipmentX-ray equipment comprises three main elements:An X-ray tube:Emit radiation;Transformers:Convert the mains voltage and current to the type required by the X-ray tube;Regulators:Control the voltage and current.

X-ray tube

Regulators

Transformers

Control unit

X-Ray Tube

Evacuated glass envelopeFocusing cupTungsten filamentX-raysTungsten targetCopper anodeElectron stream

An X-ray tube consists of a glass outer casing which is sealed in a vacuum, inside which there is:A positive electrode, the anode, andA negative electrode, the cathode.Tube envelope is constructed of glass with a high melting point to withstand the intense heat generated at the anode.A high vacuum environment for the tube elements is necessary to prevent oxidation of the electrode materials, to permit ready passage of the electron beam without ionization of gas within the tube and to provide electrical insulation between the electrodes.

Cathode incorporates a focusing cup and filament.The focusing cup functions as a lens to direct the electrons in a beam towards the anode.The filament is usually a coil of tungsten wire. Any change in the voltage applied to the filament varies the filament current and the number of electrons emitted.Anode is a metallic electrode of high electrical and thermal conductivity. It consists a button of tungsten, gold or platinum as a target imbedded in a mass of copper to conduct much of the heat generated.

X-Ray Tube CoolingMost of the energy of the high speed electrons is converted to heat at the target.Several methods of cooling the X-ray tube have been developed.Cooling by radiating heatCooling by liquidCooling by air or gas

Tube HeadTo prevent X-rays from becoming a hazard and create scattered radiation problem, the X-ray tube is shielded with lead (the window remains unshielded).The tube head usually contains the X-ray tube, high voltage and filament transformers and insulating oil or gas.In addition, metal casing is earthed to reduce possible electrical hazards due to the high voltage used.

Focal SpotThe focal spot in an X-ray tube is the projected beam from the focal area of the target which, when bombarded by electrons, produces the X-rays.The small focal spot requirement increases the quality of radiograph by improving the definition.

Cross section of electron beamSurface of optical focusFocal spot anticathode

Tube WindowThe X-ray beam emerges from the tube through a window.The window is usually made of a less radiation absorbent material such as a light metal of low atomic number.e.g. Beryllium

Duty CycleThe duty cycle is generally expressed as a percentage of exposure time versus total time.100% means continuous service.50% means that it is necessary to provide equal rest time after each exposure and so on.By following the duty cycle as specified for a particular tube, the life of the X-ray tube can be lengthened.Tube life depends on the effectiveness of its cooling.Tube life can be increased by Preheating (Warm up).

Causes of X-ray Tube FailuresMechanical fracture, Mechanical shockPuncturing of glass-electric spark, Faulty mounting, bubbles in glassFilament burn-out, Excessive current on the filament, Excessive voltage on the filamentInverse filament burn-out, Backfiring of tubes using alternating voltageMelted focal spot, Excessive loading and time control failureMelted anode, Heat storage capacity exceededGassy tube, Tube overloaded or poor out-gassed

TransformersA transformer is a device for changing the voltage of alternating current to a higher or lower voltage.Generally, two transformers are required for X-ray equipment.The auto transformerThe step-up transformer (high voltage transformer)The step-down transformer (filament transformer)

The auto transformer (compensator) is used to change or compensate for incorrect voltages supplied by commercial sources.The step-up transformer, also called the high voltage transformer, will get its power from the auto transformer and then step it up to the voltage required on the anode of the X-ray tube.E.g. step-up transformer will furnish 100 kV from 110 V The step-down transformer also called the filament transformer can get its power from the auto transformer and step it down to the voltage required on the X-ray tube filament.E.g.step-down transformer will furnish the 110 V to 10 V for heating of wire filament.

Transformers for an X-ray tube

Auto transformerVoltmeterMilliammeter Choke coilAC MainFilament transformerX-ray

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X-ray Control PanelThe X-ray control panel houses the current and voltage controls and meters, indicating lights, and a timing device.

Tube VoltageUsually expressed in kVIt determines the penetrating power or energy of the X-rayAn increase in the tube voltage increases the speed of the electrons striking the targetThen, produce short wavelength (high energy) X-raysTube voltage can control by adjusting kV knob on the control panel

Tube CurrentUsually expressed in mAIt determines the current flowing from the cathode to the anodeIt is indicated by milli-ammeter on the control panelTube current controls the intensity of the X-ray beam

Exposure TimeThe length of time during which radiation is emitted is called exposure time.It is recorded and controlled by an interlocked timer on the control panel

Quantity and Quality of X-raysMuch of the energy of the impinging electrons is dissipated as heat.0.1% of the energy of the electron beam is converted into X-rays in very low voltage tubes.The quantity or the intensity of the X-rays produced depends on the tube current.The quality or the energy of X-rays produced depends upon the energy of electrons striking the target.

Effect of mA and kV on X-ray output

WAVELENGTHWAVELENGTHINTENSITYINTENSITY4 mA2 mA100 kV50 kV20 kV10 kV

Modern X-ray EquipmentDirectional X-ray UnitsFully portableKilo voltage 100 kV to 400 kVkV can be varied continuously or in stepsTube current - 5 mA to 20 mASize of the focal spot - 0.50.5 mm to 44 mm

Modern X-ray EquipmentPanoramic X-ray UnitsNormally available up to 300 kVEmergent beam angle - 3600300Focal spot is elliptical in shapeEffective focal spot sizes - 41, 4.81, 51.5 mm, etc.Tube current up to 15 mA

Modern X-ray EquipmentRadiographic Linear AcceleratorsFor the radiography of thick samplesX-ray energy in the MeV rangeFocal spot size 2.52.5 mm10 cm steel can be radiographed in one minuteBeam angles 150, 300, 450, and 3600Electron beam current 0.01 mA to 0.25 mAMax. focal spot diameter is 2 mm

Modern X-ray EquipmentMicrofocus X-ray SystemDo not permit large SFDVery small focal spot (0.5 to 0.05 mm)Tube current - maximum 1 mA for 100 kV

Modern X-ray EquipmentFlash X-ray TubesBetatronMicrotron

Gamma Ray EquipmentSource AssemblyThe source of radiation used in radiography is extremely small and enclosed used in a sealed protective metal covering Type of Radioisotopesradioisotopes most commonly used for radiography purposes are Ir-192, Co 60 and Cs-137.

Selection Criteria for a Gamma ray SourceActivityThe activity determines the exposure timeEnergyEnergy of Gamma rays from the source must be sufficient to penetrate the thickness of the specimenHalf lifeThe source should have half-life long enough to complete the job.AvailabilityThe source should be easily available and preferably at low cost.

Gamma Ray ProjectorsRemovable Plug Type UnitUnits of this type are available with capacities up to 2 Ci of Co-60 (or 100 Ci of Ir-192).A conical step plug can be removed from the main body when a beam of radiation is required.The steps are designed to avoid streaming

D type UnitSuitable for pipe radiography (DWSI)Available with capacities up to 7.5 Ci of Ir-192 or 1 Ci of Cs-137The unit can be clamped onto a pipe with the help of a chain and can also be rotated to any desired position when required.

Remote control UnitThese units can be operated from a remote distance which makes them suitable when high activity sources are to be used.The source can be pushed out of the shielding container to a desired position via guide tubes and brought back when the exposure is completed.

Clutch Wire Cable TypeThis consist of a shielding container with all the safety locks and devices, etc.A remote control system that controls the movement of the source with the help of a clutch wire cablePneumatic TypeThis consists of a shielding container and a pneumatic remote control system.The source is pushed out of the container to the exposure site by a compression stroke and sucked back into the container by a suction stroke

Common Sources Used For Industrial RadiographyIsotopeCobaltCo-60RadiumRa-226CesiumCs-137IridiumIr-192ThuliumTm-170YtterbiumYb-169Half-life5.3 y1600 y30 y74 d130 y30 dEnergy(MeV)1.251.220.660.3550.0720.053X-ray equivalent(MeV)2 - 31 - 20.6 - 1.50.3 - 0.80.1 - 0.30.1 - 0.4Radiation LevelRHM/Curie1.330.830.370.50.00250.125Optimum thickness range (mm) of Steel50-20050-15020-10010-702.5-122.5-12

Merits and Demerits of X- and Gamma RaysTechnical AspectX-RaysGamma RaysPower supplyElectrical power requiredNo power supply neededSafety supervisionRequired during operationAt all timeWeight and dimensionsLarge, bulky and heavyRelatively light (except Co-60)Manipulation for source positioningSetting up time consuming and difficultBased on manipulationRadiation hazardsOnly during operationContinuous hazardsRadiation penetrationCan be adjusted (mat. & t)No adjustment for given sourceRadiation intensityHigh Low Focal spotVery fine focal spot possibleDepends upon the specific activityShape of beamUnidirectional and PanoramicUnidirectional and PanoramicExposure timeShort Generally longRadiographic contrastHigh LowCostInitialLow High Repair & MaintenanceLowHigh

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