X-ray Production

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.

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 (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

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.

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.

Characteristic x-ray

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:

K-Characteristic Radiation

12

1

1

K

L

M

N

K K K

Photon Energy (keV)

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)

X-ray Tube

اشعه كامل المپ با xساختماندوار آند

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