X-ray discover by Roentgen in 1895 while studding cathode raysX –ray tube ;-The tube consist of galas envelope that has been evacuated to high vacu -cathode ;- -ve electrode -anode ;- + ve electrode both of same in the tube .The cathode is a tungsten filament that when heated emits electrons .(thermionic emission ) . The anode consists of a thick copper rod at theWhich is placed a small piece of tungesten target .When a high voltage is applied between the anode & cathode , electronEmitted from the filament are accelerated toward the anode and achieveHigh velocities before striking the target . The x-ray are produced by theSudden deflection or acceleration of the electron caused by the attractiveForce of the tungstenThe anode;- the choice of tungsten as target due to ;-• the target must have high atomic number Z =74• high melting point 3,370
the remove of the heat from the target is an important for the anodeAnd this achieved by ;- 1) in some tubes by conduction of heat through a thick copper anodeOut side where cooled by oil , water or air 2)rotating anode have also used in the diagnosis x-ray . 3)the function of the oil bath surrounding an x-ray tube is to insulateThe tube housing from high voltage applied to the tube as well asAbsorb heat from the anode .Some stationary anode are hooded by a copper and tungsten shieldTo prevent stray electrons from striking & non target component .Stray electron are 2ry electron produced from target when it is beingBombarded by the 1ry beam .The copper of the hood absorb electron where as tungsten absorbThe x-ray produced in the copper
Focal spot ;- is area in the target with optimum size . Is the area from whichX ray are emitted . Should be as small as possible for producing sharp imagesIn the therapy tube relatively larger focal spot are acceptable .The apparent size of the focal spot = sin 0 + A .In the diagnosis radiology the target angles are quite small ( 6—17 ) to produceApparent focal spot size ( 0.1 x 0.1 m m ) to 2 x2 mm .In most therapy tube however the target angle is large ( 30 degree ) & apparenFocal spot 5 x5 ---- 7 x 7 m m .Heel effect ;- is the variation across x-ray beam , due to variation in the x-rayComing from different depth . This problem can be minimized by using acom-Pensating filter .The cathode ;- consists of a wire filament . Circuit ,& negatively charge focusingCup . The function of the cathode cup is to direct the electrons toward the anodSo as to strike in a will defined area ( focal spot ) . The material of the filamentIs the tungsten ( high matlingpoint )Basic x ray circuit ;-in simplified diagram the circuit is consist of two parts 1) high voltageCircuit to provide the accelerating potential for the electron 2 ) low voltagCircuit to supply heating current to the filament .The filament supply for electron emission usually consists of 10 v at 6 aCan be cheeve by using step down transformer .High voltage to the x ray tube is supplied by the step up transformer
Voltage rectification ;-The disadvantage of the self rectified circuit is that ;-1) No x ray are generated during the inverse voltage cycle , where ( whereThe anode is negative - ve relative to the cathode So the out put of The machine is low2)When the target gets hot and emits electrons by the process of thermionEmission . During the inverse voltage cycle , these electron will flow from thAnode to the cathode .The problem of tube coduction during inverse voltage can be solved by usVoltage rectifiers , which prevent the tube from conducting during the inversvoltage cycle is called half wave rectification placed in high voltage partof the circuit .High voltage rectifiers are either valve or sold .The valve rectifier is similar principle to the x ray , has cathode & anodeSold rectifier these rectifiers consist of conductors coated by selenium ,Silicon &germanium . They conduct electron in one direction only .
Full wave rectification ;- this is achieved by arranging 4 rectifiers in the high vpart of the circuit , so that the cathode remain -ve & anode +ve during bothHalf so x ray produce during both half .
PHYSICS OF X RAY PRODUCTION ;-There are two different mechanism by which x-ray are produced one giveRise to bremsstrahlung x ray & other characteristic x rayA) Bremsstrahlung ;-AL so called braking radiation . Is the result of radiative interaction betweenA high speed electron and nucleus . The electron while passing nearMay be deflected from it is path by the action of coulomb forces ofAttraction & loss energy as bremsstrahlung [Max well general theory ] Energ Propagated through space By electromagnetic field , which is part of it oOf it is dissociated from it & propagates as electromagnetic radiation .Since electron may have one or more bremsstrahlung interaction .The direction of emission of bremsstrahlung photon depends on the energyOf the incident electron ,Below 100kev x ray are emitted more or less in all direction equally.As the kinetic energy of the electrons increase , the direction of x-ray emissBecomes forward .Transmission – type targets are used in megavoltage x-ray tube in which thElectron bombard the target from one side & the x ray beam is obtained oThe other side .
Low voltage x ray tube , it is technically advantageous to obtain the x ray beamOn the same side of the target [at 90 degree with respect to the electron beamDirection ] .The probability of bremsstrahlung production varies with the Z or the targetmaterial. -10X ray efficiency = 9 x 10 zv .V = Tube voltage .Characteristic x ray ;-The mechanism of their production ;- an electron with kinetic energy E0 mayInteract with the atoms of the target by ejection an orbital electron, such as a K,L OR M electron , leaving the atom ionized .The original electron will recede with energy = E0 –A E , where AE is the energyGiven to the orbital electron . Apart of the AE energy is spent in overcomingThe binding energy of the electron & the rest is carried by ejected electron .When a vacancy is created in an orbit, an outer orbital electron will fall down toFill that vacancy . In so doing , the energy is radiated in the form of electromagnRadiation , which called characteristic radiation ( characteristic of the atoms of theTarget ) . It is un like bremsstrahlung , characteristic radiation or x ray are emitted aDiscrete energies