X-Ray Flashcards
What is the energy range of X-radiation?
100 eV - 1 MeV
What are the components of an X-ray tube?
How does it work?
Draw it.
Components:
- Vaccuum tube made of glass
- anode and cathode
Function:
- tungsten filament (high Z, high melting point) of the cathode is heated by anode current I and emits e- by thermionic effect (no. of e- emitted depends on I)
- high voltage is applied btw the electrodes that accelerates e- towards the anode
- e- decelerated at anode, X-rays produced (if Ekin high enough also characteristic X-rays produced)
What is Bremsstrahlung?
How does its spectrum look like?
Draw the graph.
X-radiation generated by the deceleration of high speed (large Ekin) charged particles
- has a smooth, continuous spectrum with a wavelenght minimum λmin
What does the Duane-Hunt law state?
Give the formula.
Explain it using a graph.
gives minimum wavelenght λmin of X-radiation produced by Bremsstrahlung
⇒ higher accelerating U → Bremsstrahlung’s spectrum shifts towards lower λ, thus higher Ephoton
λmin = k/U
- U = accelerating voltage
-
k = h*c/e = 1230 pm*kV
- h = Planck’s constant
- c = speed of light
- e = charge of e-
The total power of Bremsstrahlung emitted by a X-ray tube depends on… ?
Refer to a formula.
integral of X-ray spectrum dependent on:
- anode current I
- accelerating voltage btw anode and cathode U
- atomic number of the anode material Z
PX = cX U2 I Z
- cX = X-ray constant = 1.1 * 10-9 V-1
How can the integral of the X-ray spectrum be changed?
- increase I → P changes proportionally (intensity is increased because greater no. of e- hits anode)
- increase U → P changes proportionally to U2 (cf. Duane-Hunt law)
- increase Z → higher efficiency of conversion of Ee- to EX-ray
What is the precondition for characteristic X-radiation?
How does its spectrum look like?
Can it be changed and if so, how?
Ekin of the accelerated e- is high enough to eject an inner shell e- from the anode with high Z
→ empty orbital filled by an outer shell e- → excess E emitted as X-radiation
- line spectrum, position characteristic for the atom
- position of lines is independent of U, only intensity (height of lines) can be changed by increasing I
What is the most important interaction type of X-radiation in diagnostics?
Explain it on molecular level.
Explain why contrast btw different tissues can be achieved w/r/t a formula.
mainly photoeffect → τM
- photon of the X-radiation absorbed by an inner atomic e-, which is consequently ejected from the atom
- τM strongly depends on Z → even small differences btw diff. tissues types lead to large attenuation effects
τM = C λ3Z3
⇒ also explains why high λ X-radiation used for elements with low Z (e.g. mammography)
What is another fairly important way of X-ray attenuation?
When is it especially important and why?
Compton scattering
important for atoms with low Z bc attenuation directly proportional to Z, not Z3 like in case of photoeffect attenuation
What is the minimum wavelenght λmin?
corresponds to the one-step process when the total Ekin of the e- is converted to one X-ray photon with E=hfmax (fmax because λmin)
Ekin e- = eU = U * I = hfmax = h c/λmin
Bragg diffraction
NOCH AUSSTEHEND
How was the Duane-Hunt law verified?
- X-ray spectrum was measured at different U
- different λmin could be observed → changing shape
- plot λ vs. 1/v → good linear fit → Duane-Hunt constant k
- calculation of Duane-Hunt constant
λmin = k/U
How was the relation btw intensity of X-radiation and increasing current verified?
- the spectrum was integrated at different anode currents
- plot P vs. I → good linear fit → P proportional to increasing I
How was the Z-dependence of the τM verified?
NOCH AUSSTEHEND
