1: X-ray Generation Flashcards
1901, first Nobel prize in physics
Röntgen with discovery of x-ray
x-ray generation three components
1) thermal emission
2) electron acceleration
3) x-ray generation in target
thermal emission takes place in
cathode
x-ray generation takes place in
anode
medical imaging kV range
30-140 kV
thermal emission: #electrons depend on 2 things
Fermi Energy level
Temperature
richardson dushman eqn
explains how much electron goes out of cathode
J = A0 T^2 exp( -W / k T)
current density = Richardson const * temp^2 * exp( work function / boltzman const Temp)
<acceleration in the vacuum depends on 2 things
acceleration voltage
temperature
explain three different regimes in the electron beam current plot
beginning: leakage current
linear part: increases linearly with acc voltage
saturation part: almost flat, higher temp, higher saturation current level
x-ray generation in target: 2 principles
bremsstrahlung: continuous spectrum
characteristic emission of x-ray: material specific property
bremsstrahlung
electron beam gets closer to nucleus, bends and changes the direction, emits continuous x-rays
closer to nucleus, higher the energy of x-ray
max energy of emitted x-ray in bremsstrahlung is equal to
kinetic energy of the incoming electron
how to estimate the min wavelength, max energy pf the emitted brensstrahlubg
Ephoton=Ekinetic,electron
hc = eU
lambda,min = hc/eU
h:planck const
c: light velocity
e: elementary charge
U: acceleration voltage
bremsstrahlung spectrum shape
energy vs relative output
filtered: like a hill, low energy part absorbed by the window material AND also self absorption in metal target
unfiltered: linear decreasing line
bremsstrahlung spectrum shape for different acc voltage
photons increases as kV increases
peak shifts to right, peak #electrons emitted at higher energy level
difference between x-ray and gamma ray
1) produced ,n the electronic processes
2) produced in the nuclei
who proved x-rays are lights and got nobel prize in 1914
max von laue
characteristic x-ray emission physical process
incoming electron collides and releases an inner shell electron
another electron from higher shell transitions to lower one and releases energy by geenrating a characteristic x-ray photon
characteristic x-ray law
moseley law
moseley law: wavelength depends on
rydberg const
atomic number in periodic table
shielding constant
main quantum numbers of shells
why tungsten target material
high melting point, gives lots of electron before it melts
other target materials
iron —– x-ray diffraction
cupper —– x-ray diffraction
molybdenum —– mammography
rhodium —– mammography
tungsten —– radiography / CT
why rotating anodes?
for more power
distribution of heat is more even,
equipment provides more precise focal spot for high resolution
microfocus transmission tubes
