x-ray production. kvp kinetic energy of electrons
TRANSCRIPT
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