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X-ray Production

X-ray Production. M A Oghabian Medical Physics (PhD) Tehran University of medical Sciences www.Oghabian.net. PHOTOGRAPHIC PLATE. PLATES FOR VERTICAL DEFLECTION. PLATES FOR HORIZONTAL DEFLECTION. ELECTRON BEAM. FLUORESCENT SCREEN. ELECTRON GUN. Crookes’ Tube.

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X-ray Production

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  1. X-ray Production M A Oghabian Medical Physics (PhD) Tehran University of medical Sciences www.Oghabian.net

  2. PHOTOGRAPHIC PLATE PLATES FOR VERTICAL DEFLECTION PLATES FOR HORIZONTAL DEFLECTION ELECTRON BEAM FLUORESCENT SCREEN ELECTRON GUN Crookes’ Tube Roentgen’s discovery based on Crookes’ tube r

  3. Electromagnetic Radiation Radiation may be defined as energy in transit from one location to another. • The Nature of X-Rays: • X-rays, light, radio waves, ultraviolet rays, are electromagnetic radiation; • electromagnetic Radiation can be produced by accelerating an electric charge; • . It has an associated electric and magnetic field.

  4. EM Radiation

  5. Particle-wave duality of x-ray • X-rays, as well as all other electromagnetic radiation, have the wave-particle duality; • This wave can be described by its frequency n (or wavelength l) and traveling velocity c. • c=3 x 108 m/sec (velocity in a vacuum) l c = l n n = c / l

  6. مثال : فركانس يك فتون پرتو ايكس با انرژي KeV 70 چقدر است ؟ Ek = h n0 n = c / l Ek= hc /l h = 4.15 * 10-15 ev-Sec

  7. Electromagnetic Radiation Spectrum

  8. X rays generation X rays are generated by interactions between the energetic electrons and atoms of the target. The interactions result x rays in two ways: (1)Bremsstrahlung (brake radiation, general radiation); (2) Characteristic radiation

  9. Bremsstrahlung (brake radiation, general radiation) when a charged particle undergoes acceleration or deceleration, it emits photons.

  10. Bremmstrahlung Most electrons that strike the target give up their energy by interactions witha numberof atoms; Conclusion:The bremsstrahlung produces inherently continuous spectrum of x rays. 99%of the radiation are absorbed by target and the walls of the x-ray tube to produce only heat.

  11. Bremmstrahlung For monoenergeticelectrons passing through athick target, the energy flounce spectrum is: • kinetic energy = Ek= h n0 • = c / l lmin=12.4/kVp A0

  12. X-ray tube voltage is expressed as the peak • kilovoltage (kVp). (eg; 100 kVp) • حداكـــثر انرژي فتون x زماني اتفاق مي افتــد كه الكترون توسط اولين اتم متوقف شود در نتيجـه فوتونـــي با حداقل طول مــوج ايجاد مي شود • و بستگـــي به اختلاف پتانسيل (High Voltage) يعني KVp دارد. • بعنوان مثال طول موج X-ray با KVp=100 برابر است با :

  13. Characteristic radiation • The minimum energy required to “knocking out” an electron in a specific orbit depends on binding energy. • Removal of an electron cause the target atom to a • higher energy state with 2 productions: • 1) Positive charged ion • 2) Negative charged ion • Two ways for an ionized atom returning to its normal • (lower energy) state: • (1) Characteristic x-ray radiation; • (2) Emission of Auger electrons.

  14. Characteristic x-ray

  15. Characteristic radiation: K-shell filling is the most important for diagnostic x rays. For tungsten: This energy is ~70 keV for K shell. L-characteristic x-ray is ~11 keV. Ek-L= 59 (keV). - It is ~ 10% of total spectrum -The low-energy photons are absorbed by the target and the walls of the x-ray tube to produce heat.

  16. K-Characteristic Radiation Ka Kb Kg K b1 a2 a1 g1 L M N Photon Energy (keV)

  17. فاكتورهائي كه در طيف اشعه ايكس موُثر هستند عبارتند از : • 1- جنس مادى هدف • 2- ولتاژ اعمال شده ( KVp ) • 3- جريان لامپ اشعة ايكس • 4- تأثير فيلتر ( صافي ) اضافي • 5- اثر لبة K • 6- تأثير شكل موجي ولتاژ

  18. Tungsten Target Electrons Anode Evacuated Envelope Cathode (heated tungsten filament) X- rays Tube • Bombardment must take place in a vacuum to prevent • Ionization of air.

  19. X-ray Tube

  20. ساختمان لامپ كامل اشعه x با آند دوار

  21. Rotating Anode: 7 mm 2 mm Disk-shaped, rotating anode allows heat to be spread over a much larger surface without compromising apparent size of focal spot.

  22. (--) cathode electrons For large focal spot For small focal spot focusing cup What Is Happening In Cathode Thermo ionic emission: when a metal is heated its atoms absorb thermal energy and some electrons escape the metal surface. (1) Focusing cup.

  23. (2) Tungsten filament : Filament is made of tungsten wire with high melting point (33700 C), low vaporization, and lasting strength. (3) Electric circuitto provide the heating currents. This is filament circuit which is different from the x-ray tube current. (4) Electrons are accelerated towards the anode. The x-ray tube current, measured in mA , refers to the number of electrons flowing persecond from the filament to anode.

  24. TungstenTarget Electrons (--) (+) cathode X-Rays What Is Happening In Anode High-speed electrons collide with the tungsten atoms.

  25. Angulation of anode TungstenTarget Electrons (+) (--) Cu cathode Titling angle q Sin20° = 0.342, Sin16.5 =0.284 Apparent focal spot size X-Rays • allows to deposit electron energy over larger surface of • anode while maintaining smaller apparent focal spot • size. (Typical anode angle 6- to 20-degree).

  26. Heel effect x-ray beam produced in target is attenuated in anode body, therefore its intensity is not constant over the entire field of coverage. (X-ray intensity is diminished toward the direction of anode).

  27. X-ray Generator • Electric power is needed in a x-ray tube for three objectives: • (1) To boil off the electrons from the filament; • (2) To accelerate electrons; • (3) To control the exposure time. • The x-ray generator has a circuit for each of these objectives: • (1) Filament circuit; • (2) High-voltage circuit; • (3) Time circuit • Two compartments • Control panel : Exposure switching, Exposure timer. • Transformer assembly : Voltage transformers, Current rectifiers.

  28. X-ray generator • Voltage transformers provide low-voltage (~ 10 V ) for the filament current • and high-voltage ( ~ 150, 000 V) for the cathode-anode tube. Vp/Vs=Np/Ns ~ primary coil secondary coil V Principle of transformer: Changing magnetic field induces electric currents

  29. input wave form Full-wave and half-wave rectification half-wave rectification full-wave rectification

  30. X-ray Tube Chart Heat Unit (HU) = kV.mA.S For example a 70 kV @100 mA Heat Units Single phase 700 Constant potential 980 A X-ray tube chart is used to determine the safe limit to operate the x-ray tube

  31. Xray Tube Rating ChartFor example, a 35-kW X-ray tube can only accept 500 mA for0.1 sec exposure at 70 kVp.

  32. Useful Formulas keV, kVP , S (sec), and mA are the units particularly suited to x-ray physics. E(keV) = 12.4/(angstrom) 1 angstrom =10-10 m 1 eV = 1.6 X 10-19 joules 1 e = 1.6 X 10-19 coulombs 1 joule = 1 coulomb X 1V 1 cal = 4.184 joule 1 ampere = 1 coulomb/sec

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