x-ray tube · pdf file21.12.2016 · 12/20/16 1 produced by accelerating electrons...
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12/20/16 1
X-Ray TubeProduced by accelerating electrons with high voltage and allowing them to collide with metal target (anode), e.g, Tungsten.• Three Events (Two types of x-ray)a) Heat
b)bremsstrahlung(brakingradiation)c)characteristicradiationemission,
Photoabsorption orphotoelectricabsorptionisthedominantinteractionmechanisminthesoftX-rayregimeandforthelowerhardX-rayenergies.Athigherenergies,Comptonscatteringdominates.Which of the materials should be used to produce soft X-ray and hard x rays?
Ba (Z = 56)W (Z = 74) preferred targetMo (Z = 42)Pb (Z = 82)Pd (Z = 64)
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What is the minimum energy of the e -beam in the X-ray tube
IfanelectronisejectedfromtheK – shell.ThevacancyisfilledwithasecondelectronfromtheM – shell.Theproducedphotonisreferredtoas:
Thedirectinteractionofanincidentelectronwiththenucleusofanatomresultsin
X-rayproductiong -rayproduction
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The Interaction of X-Rays with Matter
AttenuationA process by which radiation loses power (reduce intensity) as it travels through matter due to the interaction with it.half-value-layer (HVL) is the thickness at which the intensity/energy is reduced by halfDeferential AbsorptionThe deference of absorption of radiation (x-ray) by deferent body tissues.e.g., image of the bone is produced because more x-rays are absorbed by bone than by the surrounding soft tissue. Resulted from - Transmitted x-ray through tissue.- Compton scattering by tissue.- Photoelectric effect.Depends on1) Incident radiation (x-rays) energy.2) Tissue atomic number.3) Tissue mass density
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The Interaction of X-Rays (high energy photon) with Matter
Attenuation1) Asx-rayenergyincreases
- LessComptonscattering- Slightincreaseinphotoelectricinteraction- Moretransmissionthroughtissue
Absorption edge: when the photon energy reaches the binding energy level for, e.g. EK, interaction with the K-shell electron becomes possible and the probability of photoelectric absorption jumps to a higher value;
The K-absorption edge occurs at different photon energies with different materials.
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The Interaction of X-Rays with Matter
Attenuation2)Astissueatomicnumberincreases- VeryslighteffectonCompton
interaction- Morephotoelectriceffect- Lessx-raytransmission
Theydependonnumberofelectrons,ratherthan,
themassnumberofthematter,thevalency ofatargetmatter,thefreebindingenergy–EK (keV),andtheMolecularconfiguration.
3)Astissuemassdensityincreases- IncreaseinComptoninteraction- Increaseinphotoelectric
interaction- Reductionintransmissionthrough
tissue
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Exponentialdecay
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Ionizing and Non-Ionizing Radiation photons
Non-ionizingRadiationHasenoughenergytomoveatomsinthemolecularrangeorcausevibrationalorrotationalexcitations,butnotenoughtoremoveelectrons.
Uses- Microwaveradiationfor
telecommunicationsandheatingfood- Infraredtoprovideheat- Radiowavesforbroadcasting
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Secondary Electrons and Ionization
Photons with enough energy can ionize atoms through the photoelectricand Compton effectsSecondary electron: the recoiled electrons and photoelectrons set moving in the material;- Secondary electrons will have enough energy to ionize more atoms. - As they travel through the material they interact with the outer shells of the atoms and excite (ionize) them;
Effects of Ionization- Air and gases become electrically conducting (the measurement of X-and gamma-rays);- Cause biological damage;- The excitation of atoms makes them emit light (luminescence);- Photographic effect (silver, bromide);- Heating of the material
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• Non-ionizingradiationmaycauseelectricalshocksandburns• Ionizingradiationproducegeneralhealtheffects,withvariationwithdifferent
tissuetypes.- damagetheprocessofnormalcellleadingtocancer
• Naturalsourcesofradiationintheenvironment
- Backgroundradiation:naturallyoccursinourenvironmentfromearth,sun,ourgalaxy,othergalaxies,anduniversebackground.Itisverylowintensity.OneCTscancanbeequaltoseveralyearsofbackgroundradiationexposure
-Radon:colorless,odorlessgasthatcomesfromdecayingrocksandsoil;Accountsforhalfofannualexposuretoradiationofaverageindividual.Secondleadingcauseoflungcanceraftersmoking.
- Cosmicandterrestrialradiation:8%ofouraverageyearlyexposuretoradation,e.g.,UVrays.UVrayscandamageDNAintheskinandcausecancer.