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X-ray Production M A Oghabian Medical Physics (PhD) Tehran University of medical Sciences www.Oghabian.net Crookes’ Tube Roentgen’s discovery based on Crookes’ tube r PHOTOGRAPHIC PLATE PLATES FOR VERTICAL DEFLECTION PLATES FOR HORIZONTAL DEFLECTION ELECTRON BEAM FLUORESCENT SCREEN ELECTRON GUN 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. EM Radiation 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=ln n=c/l Ek = h n0 n=c/l Ek = hc / l h = 4.15 * 10-15 ev-Sec چقدراستKeV 70 فركانس يك فتون پرتو ايكس با انرژي: مثال ؟ E 70 103 ev 19 1 E = hn , n = , n= = 1.69 10 Sec h 4.15 10 15 evSec n = 1.69 1019 Hz Electromagnetic Radiation Spectrum 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 Bremsstrahlung (brake radiation, general radiation) when a charged particle undergoes acceleration or deceleration, it emits photons. Bremmstrahlung Most electrons that strike the target give up their energy by interactions with a number of 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. Bremmstrahlung For monoenergetic electrons passing through a thick target, the energy flounce spectrum is: •kinetic energy = Ek= h n0 n=c/l lmin=12.4/kVp A0 •X-ray tube voltage is expressed as the peak )kilovoltage (kVp). (eg; 100 kVp حداك ــثر انرژي فتون xزماني اتفاق مي افتــد كه الكترون توسط اولين اتم متوقف شود درنتيجـه فوتون ــي با حداقل طول مــوج ايجاد مي شود و بستگ ــي به اختالف پتانسيل ) (High Voltageيعني KVpدارد. بعنوان مثال طول موج X-rayبا KVp=100برابراست با : 1.24 1.24 o = = = 0.0124 nm = 0.124 A kvp 100 l min 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. Characteristic x-ray 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. K-Characteristic Radiation K a2 b1 a1 g1 L M Photon Energy (keV) N Ka Kb Kg ُ فاكتورهائي كه درطيف اشعه ايكس موثرهستند عبارتند از : -1 جنس مادى هدف -2 ولتاژاعمال شده ) ( KVp -3 جريان المپ اشعة ايكس -4 تأثيرفيلتر( صافي ) اضافي -5 اثرلبة K -6 تأثيرشكل موجي ولتاژ X- rays Tube Bombardment must take place in a vacuum to prevent •Ionization of air. Tungsten Target Electrons Anode Evacuated Envelope Cathode (heated tungsten filament) X-ray Tube ساختمان المپ كامل اشعه xبا آند دوار 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. What Is Happening In Cathode (--) electrons cathode Thermo ionic emission: when a metal is heated its atoms absorb thermal energy and some electrons escape the metal surface. (1) Focusing cup. For large focal spot For small focal spot focusing cup (2) Tungsten filament : Filament is made of tungsten wire with high melting point (33700 C), low vaporization, and lasting strength. (3) Electric circuit to 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 per second from the filament to anode. What Is Happening In Anode High-speed electrons collide with the tungsten atoms. Tungsten Target Electrons (+) (--) cathode X-Rays Angulation of anode Tungsten Target Electrons (+) Cu (--) cathode Titling angle q Sin20° = 0.342, Sin16.5 =0.284 X-Rays Apparent focal spot size allows to deposit electron energy over larger surface of anode while maintaining smaller apparent focal spot size. (Typical anode angle 6- to 20-degree). 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). 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. 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 Full-wave and half-wave rectification input wave form half-wave rectification full-wave rectification X-ray Tube Chart Heat Unit (HU) = kV.mA.S For example a 70 kV @100 mA Single phase Constant potential Heat Units 700 980 A X-ray tube chart is used to determine the safe limit to operate the x-ray tube Xray Tube Rating Chart For example, a 35-kW X-ray tube can only accept 500 mA for 0.1 sec exposure at 70 kVp. Useful Formulas keV, kVP , S (sec), and mA are the units particularly suited to x-ray physics. E(keV) = 12.4/l (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