x-ray interaction with matter. collimate or not to collimate?! scatter
TRANSCRIPT
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X-RAY INTERACTION WITH MATTER
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COLLIMATE OR NOT TO COLLIMATE?!
SCATTER
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X-RAYS WITH LOW ENERGIES –APPROX. 10 KEV INTERACT WITH MATTER
THROUGH COHERENT SCATTERING
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Thomson scattering (R), also known as Rayleigh, coherent, or classical scattering, occurs when the x-ray photon interacts with
the whole atom so that the photon is scattered with no change in internal energy
to the scattering atom, nor to the x-ray photon. Thomson scattering is never more than a minor contributor to the absorption coefficient. The scattering occurs without the loss of energy. Scattering is mainly in
the forward direction.
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Coherent Scattering
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Λ1= Λ2
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μ1= μ2
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APPROX. 5% OF X-RAYS UNDERGO COHERENT
SCATTERING AT 70 KVP
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SCATTER
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COMPTON EFFECT-INCOHERENT SCATTERING
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Compton Scattering (C), also known a incoherent scattering, occurs when the incident x-ray photon ejects a electron from an atom and an x-
ray photon of lower energy is scattered from the atom. Relativistic energy and momentum are
conserved in this process and the scattered x-ray photon has less energy and therefore greater
wavelength than the incident photon. Compton Scattering is important for low atomic number
specimens. At energies of 100 keV -- 10 MeV the absorption of radiation is mainly due to the
Compton effect.
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MOST LIKELY TO OCCUR WITH
OUTER SHELL ELECTRONS
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ANGLES OF DEFLECTION OF PHOTONS
0-180 DEGREES
0 DEGREE DEFLECTION0 DEGREE DEFLECTION0 ENERGY TRANSFER0 ENERGY TRANSFER
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BACKSCATTER
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COMPTON
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PROBABILITY=
1E
Energy Probability
Atomic number No effect on Probability!!
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ENERGY TRANSFER=ENERGY TRANSFER=
THE ENERGY OF SCATTERED X-RAY +
THE ELECTRON BINDING ENERGY +
THE KINETIC ENERGY OF RECOIL ELECTRON
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Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is absorbed resulting in the
ejection of electrons from the inner shell of the atom, resulting in the ionization of the atom. Subsequently, the ionized atom returns to the
neutral state with the emission of an x-ray characteristic of the atom. This subsequent emission of lower energy photons is generally absorbed and does not contribute to (or hinder) the image making process. Photoelectron absorption is the dominant
process for x-ray absorption up to energies of about 500 KeV. Photoelectron absorption is also
dominant for atoms of high atomic numbers.
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PROBABILITY=
Z3
E3
Atomic number Probability
Energy Probability
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ENERGY TRANSFER
ENERGY OF INCIDENT PHOTON=ENERGY OF INCIDENT PHOTON=
THE ELECTRON BINDING ENERGY+
THE KINETIC ENERGY OF THE ELECTRON
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THE PHOTOELECTRIC EFFECT IS AN X-RAY
ABSORPTION INTERACTION
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PROBABILITY OF COMPTON INTERACTION AND
PHOTOELECTRIC INTERACTION IS THE SAME:
• IN SOFT TISSUE AT 20 keV
• IN BONE AT 40 keV
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Compton vs Photoelectric
Compton
Photoelectric
Probability of
interaction
X-ray energy
bone
s. tissue
40 keV
20 keV
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Pair Production (PP) can occur when the x-ray photon energy is greater than 1.02 MeV, when an electron and positron are created with the annihilation of the x-ray photon. Positrons are very short lived and disappear (positron annihilation) with the formation of two photons of 0.51 MeV energy. Pair production is of particular
importance when high-energy photons pass through materials of a high atomic number.
Energy: > 1.02 MeV
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ANNIHILATION
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PROBABILITY=
E
Energy Probability
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Photodisintegration (PD) is the process by which the x-ray photon is captured by the nucleus of the atom
with the ejection of a particle from the nucleus when all the energy of the x-ray is given to the nucleus. Because of the
enormously high energies involved, this process may be neglected for the
energies of x-rays used in radiography
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ENERGY > 10 MeV
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