*X-rays
• Short wavelength (high energy) light.
. 1 nm (E & 1 keV)
• Applications in medical imaging (X-ray, CT)
*X-ray Production
• The photoelectric effect showed that light is absorbed
in the form of photons.
• Early evidence for emission of light in form of photons
using X-ray tubes:
• 7 Thermionic emission of e- from cathode.
• Large V (& 103 V) accelerates e- across tube to
collide with anode.
• X-rays are produced!
Heating up the metal releases
electrons
*X-ray Production
• The photoelectric effect showed that light is absorbed
in the form of photons.
• Early evidence for emission of light in form of photons
using X-ray tubes:
• First done in 1895 (Wilhelm Röntgen).
• First Nobel prize in physics.
*Bremsstrahlung
• Accelerating charges emit electromagnetic radiation:
braking radiation (bremsstrahlung).
• In X-ray tube:
when e- reach anode, they interact with the anode atoms.
(collisions, deflections).
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is (Zinke-Allmang)
• Most e- braked by a series of collisions & interactions
• Bremsstrahlung produces a spectrum of wavelengths.
*Bremsstrahlung
• Wave model predicts a spectrum that includes all .
BUT, spectrum shows a sharp cutoff. disagrees
Cutoff depends on V. hill
steepest
• Photon model explains cutoff:
e- stopped in one collision,
to
giving one photon all energy.
Energy hc
e V = hfmax =
Yi min
*Characteristic X-rays
• Bremsstrahlung spectrum doesn’t depend on anode material.
• Characteristic X-rays:
sharp peaks in spectrum that depend on anode material.
(wave model doesn’t explain these at all…). Innershell maylead to x rays
• Collision of incident e- removes inner shell e- from atom.
• Outer shell e- drops down to fill gap & photon emitted.
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L
Anode atom
mm Er
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g g emits photon
Higherthy
facehigher
Eisthephoton (Zinke-Allmang)
emitted
*Characteristic X-rays
• Bremsstrahlung spectrum doesn’t depend on anode material.
• Characteristic X-rays:
sharp peaks in spectrum that depend on anode material.
(wave model doesn’t explain these at all…).
• Collision of incident e- removes inner shell e- from atom.
• Outer shell e- drops down to fill gap & photon emitted.
• Mammography
• Use a molybdenum anode:
• characteristic X-rays provide good contrast for imaging.
• Other applications
• Filter out characteristic X-rays.
• A lot of energy deposited (dose) without imaging benefits.
*X-Ray Imaging
• X-rays in medical imaging (25 to 150 keV).
High energy: they can penetrate several cm’s of solid matter.
• Imaging: patient placed between X-ray source & detector.
• Darker areas: higher X-ray exposure.
Bones are more effective absorbers & show up lighter than soft
tissue.
r aa film
x patient
(Zinke-Allmang)
*X-Ray Imaging
• Interactions between matter & X-rays:
as X-rays pass through tissue, there is a loss of intensity.
• In medical imaging: Compton scattering & photoelectric effect
dominate.
photoelectric effect
total
Mass Absorption coefficient Compton scattering
for soft tissue
Toption
mass Rayleigh scattering
coefficient
pair production
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so soo
x ray (Zinke-Allmang)
Rayleigh Scattering
• More important at lower energies E
•
•
much larger than atom.
WIG
~ 12 % of interactions for 30 keV (mammography).
g
wigging
reemitted
1. Photon absorbed by atom.
2. all e- oscillate together.
3. photon of same energy re-emitted instantaneously
(in different direction).
• Can be understood classically (wave model).
• Not all that important for typical diagnostic X-ray energies.
*Photoelectric Effect
• X-ray photon is completely absorbed by e- causing
ionization.
• Probability increases when E close to binding energy.
• Affects inner shell e-.
• More important at lower energies.
• Does not directly produce a secondary X-ray.
Less likely to intent
as EM
Ionization
(Zinke-Allmang)
*Compton Scattering
• High energy photon interacts with a valence electron.
• e- is ionized.
• scattered photon shifted to longer (shift depends on angle).
• Only occurs* if E sig. greater than binding energy of e-.
• Experimental data can be explained by collision of photon with e-.
• conservation of both (relativistic) energy and momentum.
• Another win for the Photon model (Nobel prize again…)
(Zinke-Allmang)
*Compton Scattering
• High energy photon interacts with a valence electron.
• e- is ionized.
• scattered photon shifted to longer (shift depends on angle).
• After ionization:
I
• Ionized e- interacts with matter (further ionizations).
• Photon continues to detector (bad for image quality) or
has further interactions with matter.
of 0
because
(Zinke-Allmang)
*Pair Production
• Only possible for E > 1.02 MeV .
• Outside of energies of conventional radiography.
• We will come back to this for nuclear medicine
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