x-ray production. crookes’ tube plates for horizontal deflection plates for vertical deflection...
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Crookes’ Tube
PLATES FORHORIZONTAL DEFLECTION
PLATES FORVERTICAL DEFLECTION
ELECTRON GUN
ELECTRON BEAM
FLUORESCENTSCREEN
PHOTOGRAPHICPLATE
Roentgen’s discovery based on Crookes’ tube r
Electromagnetic RadiationRadiation may be defined as energy in transit from onelocation 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.
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 (or wavelength ) and traveling velocity c.• c=3 x 108 m/sec (velocity in a vacuum)
c = c
مثال : فركانس يك فتون پرتو ايكس با چقدر است ؟KeV 70انرژي
Hz 19101.69
Sec 101.69 evSec 104.15
ev 1070 ,
h
E , hE 119
15
3
EEk = = h c EEk = = hc / h = 4.15 * 10-15 ev-Sec
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.
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 = EEk= = h
= c / min=12.4/kVp A0
Bremmstrahlung
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 اختالف پتانسيل (High Voltage) يعني
KVp.دارد
بعنوان مثال طول موج X-ray با KVp=100 برابر است با :
oA 0.124nm 0.0124=100
1.24
kvp
1.24
min
•X-ray tube voltage is expressed as the peak kilovoltage (kVp). (eg; 100 kVp)
•The minimum energy required to “knocking out” an electron in a specific orbit depends on binding energy.
Characteristic radiation
•Removal of an electron cause the target atom to a higher energy state with 2 productions:
1) Positive charged ion2) 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.
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.
Characteristic radiation:
فاكتورهائي كه در طيف اشعه : ايكس موCثر هستند عبارتند از
1 جنس مادى هدف -2 ولتاژ اعمال شده - (KVp) 3 جريان المپ اشعة ايكس -4 تأثير فيلتر ) صافي ( اضافي -5 اثر لبة -K 6 تأثير شكل موجي ولتاژ -
X- rays Tube Bombardment must take place in a vacuum to prevent •Ionization of air.
Tungsten Target
Evacuated Envelope
AnodeElectrons
Cathode (heated tungsten filament)
Disk-shaped, rotating anode allows heat to be spread over a much larger surface without compromising apparent size of focal spot.
Rotating Anode:
2 mm
7 mm
What Is Happening In Cathode
For large focal spot
focusing cup
For small focal spot
cathodeelectrons
(--)
(1) Focusing cup.
Thermo ionic emission: when a metal is heated its atoms absorb thermal energy and some electrons escape the metal surface.
(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.
(2) Tungsten filament :
Filament is made of tungsten wire with high melting point (3370C), low vaporization, and lasting strength.
What Is Happening In Anode
High-speed electrons collide with the tungsten atoms.
cathode
(--)
(+)
X-Rays
ElectronsTungsten Target
Angulation of anode
allows to deposit electron energy over larger surface of anode while maintaining smaller apparent focal spot size. (Typical anode angle 6- to 20-degree).
cathode
(--)(+)
X-Rays
ElectronsTungsten Target
Cu
Sin20° = 0.342, Sin16.5 =0.284Titling angle
Apparent focal spot size
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.
Principle of transformer: Changing magnetic field induces electric currentsV
primary coil
secondary coil
Vp/Vs=Np/Ns
Full-wave and half-wave rectification
half-wave rectification
full-wave rectification
input wave form
X-ray Tube Chart
Heat Unit (HU) = kV.mA.S
For example a 70 kV @100 mA
Heat UnitsSingle phase 700Constant potential 980
A X-ray tube chart is used to determine the safe limit to operate the x-ray tube
Xray Tube Rating ChartFor 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/(angstrom)1 angstrom =10-10 m1 eV = 1.6 X 10-19 joules1 e = 1.6 X 10-19 coulombs1 joule = 1 coulomb X 1V1 cal = 4.184 joule1 ampere = 1 coulomb/sec