Download Lecture 2: x ray Production (PPT

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