X-ray Production

kVp KINETIC ENERGY OF ELECTRONS

CATHODE --------

MADE OF TUNGSTEN + 1%-3% THORIUM

TUNGSTEN

Z # 74

MELTING POINT- 3,410 DEG. CELSIUS

THORIUM

Z # 90

THERMIONIC EMISSION

CATHODE HEATED UP TO AT LEAST 2,200 DEG. CELSIUS

ANODE +++++

TUNGSTENTARGET

TUNGTEN AS TARGET

HIGH Z# - 74------EFFICIENCY OF X-RAY PRODUCTION

HIGH MELTING POINT –3,410 ° C– TARGET HEATED TO 2,000 ° C

X-RAY PRODUCTION

• BREMSSTRAHLUNG RADIATION

• CHARACTERISTIC RADIATION

BREMSSTRAHLUNG RADIATION

If an incoming free electron gets close to the nucleus of a target atom, the strong electric field of the nucleus will attract the electron, thus changing direction and speed of the electron. The Electron looses energy which will be emitted as an X-ray photon. The energy of this photon will depend on the degree of interaction between nucleus and electron, i.e. the passing distance. Several subsequent interactions between one and the same electron and different nuclei are possible. X-rays originating from this process are called bremsstrahlung. Bemsstrahlung is a German word directly describing the process: "Strahlung" means "radiation", and "Bremse" means "brake

Bremsstrahlung Radiation

BREMS

DIFFERENT DEGREES OF DECCELERATION

X-RAYS

HEAT

BREMS RADIATION IS:

POLYENERGETIC !

90% OF X-RAYS ARE PRODUCED THROUGH BREMS INTERACTIONS WHEN 80-100 KVP APPLIED

BREMS EMISSION-CONTINUOUS

The high energy electron can also cause an electron close to the nucleus in a metal atom to be knocked out from its place. This vacancy is filled by an electron further out from the nucleus. The well defined difference in binding energy, characteristic of the material, is emitted as a monoenergetic photon. When detected this X-ray photon gives rise to a characteristic X-ray line in the energy spectrum.

Characteristic X-rays

Characteristic Radiation

KE OF PROJECTILE ELECTRON > BINDING ENERGYORBITAL ELECTRON

CHARACTERISTIC CASCADE

TUNGSTEN-74

K-70 KEVL-12 KEV

M-2.8 KEV

BINDING ENERGIESOF DIFFERENT SHELL ELECTRONS

CHARACTERISTIC X-RAYS

L K 70-12 = 58 keV

M K 70-3 = 67 keV

ML 12-3 = 9 keV

TRANSFER OF ELECTRONS BETWEEN OUTER SHELLS

RESULTS IN:

HEAT PRODUCTION !

EACH CHARACTERISTIC RADIATION

( ex. K TO L TRANSFER) IS:

MONOENERGETIC !

THERE ARE MANY CHARACTERISTIC RADIATION

PRODUCED IN ONE ATOMTHEREFORE CHARACTERISTIC

RADIATION IS ALSO POLYENERGETIC !

CHARACTERISTIC EMISSION-LESS POLYENERGETIC!

X-RAYS PRODUCTION AN INEFFICIENT PROCESS

80 KVP

99.4% HEAT 0.6% X-RAYS

% EFFICIENCY OF X-RAY PRODUCTION

% EFFICIENCY= K * Z * kVp

K = 1 * 10-4

X-RAY EMISSION

• ISOTROPICAL- IN EVERY DIRECTION

USEFUL RADIATION – PROJECTED TOWARD THE

PATIENT

LEAKAGE RADIATION

HOUSING

OFF-FOCUS RADIATION

USEFUL OFF-FOCUS