PRODUCTION OF XRAYS

Sneha Susanna George

HISTORICAL BACKGROUND

• German physicist• Discovered Xrays (Nov 8,1895)• 1ST Nobel Prize in Physics in

1901• IUPAC named Element 111”

roentgenium” in honour of his accomplishments

Wilhelm Conrad Roentgen (1845-1923)

HOW XRAYS WERE PRODUCED????• He was working on Hittorf- Crookes tube(black

cardboard covering it) through which an electric current from a battery was flowing

• This tube was kept in a darkened room with many fluorescent plates

• He noticed something from the tube causing the fluorescent plates to glow

• He first experimented by placing his wife’s hands on the photographic fluorescent plate for 15 minutes

• When he developed the plate, the outline of the bones of her hand could be seen

• He named these unknown rays as ‘X’rays

Dr Charles.L.Leonard

• Demonstrated the power of Roentgen rays to alter the characters of malignant cells, prevent spread and development

THE XRAY TUBE(Coolidge tube)

• Glass tube – evacuated to high vaccuum

• Cathode(-) – contains filament and focusing cup

• Anode(+) – contains x-ray target

• Why is it evacuated???- Prevents the collision of the moving electrons

with the air molecules- Prevent the oxidation and burnout of the

filaments

THE CATHODE

– Tungsten filament – Focusing cup– Connecting wires

TUNGSTEN(Z=74)

• High melting point – 3370 degree celsius

• Thin wire• Strong• Less tendency to

vaporise• Long life expectancy

THERMIONIC EMISSION

• Emission of electrons resulting from the absorption of thermal energy

• “EDISON EFFECT” – Electron cloud surrounding the filament produced by thermionic emission

SPACE CHARGE • Space Charge Collection of negatively charged electrons in

the vicinity of the filament when no voltage is applied between the 2 electrodes

• No of electrons is constant• Space Charge Effect Tendency of the space charge to limit the

emission of more electrons from the filament

FILAMENTS

• 2 Filaments are used- Long one – higher current/ lower resolution ,

larger exposure- Short one – lower current/higher resolution• At one point , only one is used• Seperately for small and large focal spots

FOCUSING CUP

• Prevents the spread out of the electron stream

• Forces electrons to converge on the anode in the required shape and size

THE ANODE• Tungsten target with a high melting point to

withstand the intense heat produced by the electronic bombardment

• Copper anode for removing the heat from the target where it is cooled by oil, water or air

• Copper hood for preventing the electrons from striking the walls

90% Tungsten and 10% Rhenium ?????- Increased resistance to surface roughening - Increased thermal capacityModifications of tube- Short stem length- Bearings as far as possible- Decreased weight (Alloy used)

Focal spot• True focal spot – Area of the tungsten target ( anode)

that is bombarded by electrons from the cathode

• Size and shape determined by the electron stream

• Heat uniformly distributed on the focal spot

• Apparent focal spots– 0.1 – 2.0 mm in diagnostic tubes, – 5 – 7 mm in therapy tubes

• The anode angle(6 – 30) is defined as the angle of the target surface to the central axis of the X-ray tube.

• The anode angle q determines the effective focal spot size

• The angle q also determines the X-ray field size coverage. For small angles the X-ray field extension is limited due to absorption and attenuation effects of X-ray photons parallel to the anode surface.

LINE FOCUS PRINCIPLE

• Effective focal spot size is the length and width of the focal spot projected down the central ray in the Xray field

• Helps in adjusting the size of the focal spot• Effective focal length = focal length • sinq• Large focal spot =greater heat loading• Small focal spot = good radiographic detail

HEEL EFFECT

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

• Intensity of exposure on the anode side < cathode side

• Less noticeable with large focus film distance• Less with smaller films

BASIC XRAY CIRCUIT

High voltage circuit Low voltage circuit

(Accelerating power) (Heating current)

• Filament temperature controls the current in the circuit due to flow of electrons across the tube and hence the Xray intensity

HIGH VOLTAGE CIRCUIT• X-ray tube with the cathode and anode• Step-up transformer to supply high voltage and low

current to the Xray tube• Autotransformer for stepwise adjustment in voltage• Voltage selector switch for selecting the turn of a

coil in autotransformer• Rheostat to introduce desired resistance in the

circuit and vary the voltage in a continous manner• Voltmeter for recording voltage to the Xray tube• Milliammeter for recording the tube current

LOW VOLTAGE CIRCUIT

• Xray tube with cathode and anode• Step down transformer to supply low voltage

and high current to the filament for electron emission

• Choke coil filament control to control the current from the main power line to the filament

Self rectified unit

The tube current and the Xrays are generated only during the half-cycle when the anode is positive

VOLTAGE RECTIFICATION• To solve the problem during inverse voltage cycle:- When anode is negative relative to cathode, no xrays are

produced- When target gets hot and emits electrons which will flow

from the anode to the cathode, bombard cathode filament and destroy it

• Can be done in 2 ways- Half wave rectification- Full wave rectification

Half wave rectification

• Rectifiers placed in series in the high voltage part of the circuit prevents conduction during the inverse voltage cycle

• 2 types of high voltage rectifiers

- Valve state type- Solid state type

Full wave rectification

• Four rectifiers are arranged in the high voltage part of the circuit so that cathode is negative and anode is positive in both cycles

• Electrons flow from the filament to the target in both cycles

• In megavoltage x-ray tubes the electrons bombard the target from one side and the x-ray beam is obtained on the other side

• In low voltage x-ray tubes it is advantageous to obtain the x-ray beam on the same side of the target

Physics of X-ray production

• Produced by two different mechanisms

BREMSSTRAHLUNG CHARACTERISTIC RADIATION RADIATIONIncident electron interacts Incident electron interactswith the nucleus of with an orbital electron of the target atom the target atom

Bremsstrahlung radiation• “Braking radiation”• Electron comes within the proximity of the

nucleus- Coloumbic forces attract and decelerate the

electron- Loss of kinetic energy and change in the

trajectory• An x-ray photon with energy equal to the

kinetic energy lost by the electron is produced

• Distance between the bombarding electron and the nucleus determines the energy lost

- Closer it gets to the nucleus, greater is the energy loss

• Part or all of the energy of the electron is dissipated from it and propogates in space as electromagnetic radiation

Characteristic X-rays

• Incident electron with energy > binding energy ejects the electron from the inner orbits

• Electron from the outer shell loses energy and replaces the inner shell electron

• The energy lost by this electron is emitted as x-ray photon

• Energy of the x-ray photon depends on the element and shell and not on the energy of the incident electron

Characteristic radiation• The wavelength of the x-rays produced are characteristic of

the atom that has been ionised and is not changed by the kVp used( The applied kilovoltage must be high enough to excite the characteristic radiation)

• The quantity of x-rays generated is proportional to the atomic nummber of the target material(Z), the square of the kilovoltage[(kVp)2] and the milliamperes of x-ray tube current(mA)

• The quality(energy) of thee x-rays generated depends almost entirely on the x-ray tube potential(kVp)