X-ray production and properties of x-ray

Dr . Sameeha Khan

X-rays were first discovered in 1895 by the German physicist William Roentgen, when using a Crookes tube

He called them ‘x’ rays, ‘x’ for ‘unknown’.

Wilhelm Conrad Roentgen (1845-1923)

The first x-ray photograph: Roentgen’s wife Bertha’s hand

Electromagnetic spectrum

Unit of measurement in x-ray region is Å and nm. 1 Å = 10-10 m, 1 nm = 10 Å = 10-9 m X-ray wavelengths are in the range 0.5 – 2.5 Å. Wavelength of visible light ~ 6000 Å.

X-rays are electromagnetic radiation of exactly the same nature as light but of very much shorter wavelength

Properties of electromagnetic radiation

• Electromagnetic radiation → wavelike fluctuation of electric and magnetic fields set up in space by oscillating electrons

Electromagnetic radiation

According to the classical theory Electromagnetic

radiation can be considered as wave motion .

According to the quantum theory electromagnetic radiation can also be

considered as a particles called photons

Wave concept of electromagnetic radiation

•All EM radiations travel with the speed of light 186000miles/sec, 3×10ˆ8 m/sec but they differ in wavelength •Wavelength (λ) – distance between 2 successive crests / trough •Frequency (ν) – number of crests /cycle per second (Hz) • (λ) wavelength ↓ (ν) frequency ↑•EM travel with the speed of light c , c=λν•Wave concept of EMR explains why radiation may be reflected , refracted, diffracted and polarized .

If each wave has length λ and ν waves pass a given point in unit time

velocity of wave is v = λ× ν

Particle concept of electromagnetic radiation

•Short EM waves like XRAYS react with matter as if they are particles rather than waves.•These particles are discrete bundles of energy and each bundle is called quantum /photon.•Photon travel at the speed of light.•Amount of energy carried by each photon depends on frequency of radiation. •If frequency doubled energy doubled .•Particle concept can explain the interaction with matter like photoelectric and Compton effect .

Energy calculated E=hν h= Planck's constant (4.13×10 ˆ-18 Kev sec )

Relationship between wavelength and energy

Relationship between wavelength and frequencyν= c/λ

c – velocity of light (~3×108 m/s) also E= hνInstead of ν

E =hc/λ ( h×c = 12.4)

• Energy of photon =ev

•X-ray measured in kilo ev , 1Kev = 1000 ev

E= 12.4/λ

Source of x-rays in radiology

History:Gas x-ray tubes Cold cathode tubes

X-ray tube

Basic elements of an X Ray source assembly

Glass enclosure

•Vacuum: to control the number and speed of the accelerated electrons independently.•Pyrex glass : connecting wires same coefficient of linear expansion as glass.

Cathode --------

•Negative terminal of the x-ray tube is called cathode or filament. •Along with filament 2 other elements : connecting wires and focusing cup

Filament made of tungsten wire 0.2 mm diameter coiled to form a vertical spiral 0.2 cm diameter and 1 cm length

When Current flows – wire heated

Absorbs thermal energy – electrons move a small distance from the surface of metal

This escape is referred to as thermionic emission

Thermionic emission

Thermionic emission

Emission of electrons resulting from the absorption of thermal energy – thermionic emission Electron cloud surrounding the filament produced by thermionic emission is termed “Edison effect”

Space charge

•Collection of negatively charged electrons in the vicinity of filament when no voltage applied btw cathode and anode – space charge •Number of electrons in space charge remain constant •Tendency of space charge to limit the emission of more electrons from the filament is called space charge effect

Filament current →filament temperature →rate of thermionic emission

Space charge cloud

Space charge cloud shield the electric field for tube voltages of 40kvp and less ( space charge limited ) , above 40kvp space charge cloud is

overcome by voltage applied

Temperature limited

Tungsten

Z # 74

MELTING POINT- 3,370 DEG. CELSIUS

1. Thin wire 2. Strong 3. High melting point 4. Less tendency to vaporize 5. Long life expectancy

Filament and focusing cup ( Nickel )

• Modern tubes have two filaments

1. Long One : higher current/lower resolution, larger exposure

2. Short One : lower current/higher resolution.

At one point only one is used

Focusing cup maintained at same negative potential as the filament .

Focusing cup

Current across tube one direction only

Mutual repulsion

↑Number of electrons

Electron stream spread out

Bombarding Large area of

anode

Prevented by focusing cup – forces the electron stream to converge on the anode in

required shape and size

Filament vaporization

•Filament vaporization – shorten the life •Not heated for too long- filament boosting circuit •Vaporized filament usually deposited on the inner surface of glass wall •Color deepens as the tube ages- bronze colored “sunburn” •Tends to increase filtration and changes the quality of beam

Anode +++++

Stationary anode Tungsten target in copper anode

Rotating anode+++

Spread the heat produced during an exposure over a large area of anode – capable of withstanding high temperature of large exposures

Anode +++ parts

1. Anode disk –tungsten •3600rpm •Beveled edge – line focus•Target area increased but effective focal size remains the same.

2. Stator3. Rotor4. Bearings - metallic

lubricants (silver )

5. Stem - molybdenum

90%tungsten W and 10 % rhenium Re- ↑resistance to surface roughening - ↑thermal capacity

Anode +++

Modification of tube to improve speed of rotation and in turn increased ability to withstand heat .

•As short as possible

•Decrease inertia

1.Stem length

•2 sets as far as possible

2.Bearings

•Decrease weight ( molybdenum + W Re alloy )

•Reduced inertia

3.weight

Focal spot

•True focal spot :Area of the tungsten target (anode) that is bombarded by electrons from the cathode.•The size and shape of focal spot is determined by the size and shape of the electron stream which hits the target.•Heat uniformly distributed on focal spot

Line focus principle

•Anode angle : defined as the angle of the target surface with respect to the central ray in the x-ray field. •Anode angle range :6°- 20° •Line focus principle - Effective focal spot size is the length and width of the focal spot projected down the central ray in the x-ray field .

Line focus principle

Foreshortening of the focal spot length

Line focus principle

effective focal length = focal length • sinq Effective focal spot<actual focal spot

Anode angle

Large focal spot = greater heat loading. Small focal spot = good radiographic detail.

Heel effect

Intensity of beam depends on the angle at which the x-rays are emitted from the focal spot

Heel effect

Intensity of exposure on anode side < cathode side

of tube

Heel effect less noticeable with large focus-film

distance

Heel effect is less with smaller films

Anode Cathode ←Intensity→

Properties of xrays

1. X-rays travel in straight lines.2. X-rays are electrically neutral 3. X-rays are Polyenergetic and heterogeneous 4. X-rays travel at the speed of light -

electromagnetic radiation 5. X-rays are highly penetrating , invisible rays.

Properties of x-rays

6. X-rays cannot be deflected by electric field or magnetic field.

7. X-rays cannot be focused by lens.8. Photographic film is blackened by X-rays.9. Fluorescent materials glow when X-rays are directed

at them.10. Produce chemical and biologic changes by ionization

and excitation. 11. Liberate minute amounts of energies while passing

through matter. 12. X-rays interact with matter produce photoelectric and

Compton effect.

Processes of x-ray generation

When high speed electrons lose energy in the target of the x-ray tube

2 processes of x-ray generation

General

Characteristic

General radiation ( Bremsstrahlung) • High speed electrons with nucleus of the tungsten atom

Characteristic radiation • High sped electrons with the electrons in the shell of tungsten atoms

Degree of deceleration

+

+

e‾

e‾

0.5%time electron comes in proximity

with nucleus

Coloumbic forces attract and decelerate the

electron

Loss of kinetic energy and change in trajectory

Bremsstrahlung ( braking radiation )

Enrgy of photon = enrgy of initial ectron – enrgy of braked electron

Energy of photon E = 12.4 /λ Energy is related to the potential difference across tube or

λmin = 12.4 / kVp

Continuous spectrum

Highest energy determined by the kVpMinimum wavelength determined by the kVp

Maximum wavelength determined by the filters used

Brems radiation- Polyenergetic

Characteristic radiation results when the Electrons bombarding the target eject electrons from the inner orbits of target atoms

Characteristic radiation

Characteristic radiation

BINDING ENERGIESOF DIFFERENT SHELL ELECTRONS

K-70 KEVL-11 KEVM-2 KEV

Characteristic radiation

L K(α)70-11= 59 keV

M K (β)70-2 = 68 keV

ML 11-2 = 9 keV

Between 80 and 150 kVp , k shell characteristic contributes to about 10 %(80kVp) to 28%(150kVp) of useful beam.

Characteristic radiation

EACH CHARACTERISTIC RADIATION ( eg. K TO L TRANSFER) IS:

Monoenergetic

THERE ARE MANY CHARACTERISTIC RADIATION

PRODUCED IN ONE ATOMTHEREFORE CHARACTERISTIC

RADIATION IS ALSO POLYENERGETIC !

Characteristic radiation

Less Polyenergetic

Typical x-ray pattern

Thank you