and xray Interactions with Matter

1
?? and x-ray Interactions with Matter
There are 3 dominant interaction mechanisms of ?
and x-rays with matter. These interactions are
the means by which ? and x-rays are attenuated
through material. Photoelectric
Absorption Compton Scattering Pair Production
The predominant mode of interaction depends on
the energy of the incident photons and the atomic
number of the material.
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Photoelectric Effect
Above the energy threshold of the absorption
edge, the incident photon is completely
absorbed. A photoelectron is ejected from the
electron shell with energy Ee h? - Eb The
vacancy in the shell is filled by an electron
from an outer shell with the subsequent emission
of either - characteristic x-rays - Auger
electrons
3
Compton Scattering
A photon is scattered off of an electron. The
electron recoils. The scattered photon is reduced
in energy. The energy of the scattered photon
depends on the angle ? through which it is
scattered
?
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Pair Production
Above the threshold of 1.022 MeV (twice the
electron rest mass) pair production can take
place.
In the field of the nucleus, an electron positron
pair can be created.
e -
incident ?-ray
If the energy of the ?-ray is greater than 1.022
MeV, then the excess energy is given to the
positron/electron pair as kinetic energy. This
kinetic energy is deposited in the detector
volume.
e
511 keV
annihilation
511 keV
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Pair Production
The positron slows in the detector, until it is
nearly stationary. It forms a short-lived state,
analagous to hydrogen with an electron, called
positronium. In over 95 of decays, the electron
and the positron annihilate with the emission of
two 511 keV photons at 180 degrees to each other.
e -
incident ?-ray
e
511 keV
annihilation
511 keV
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Dependence of Interactions upon ?-ray energy
The probability of an interaction being either
photoelectric absorption, Compton scattering or
pair production depends on the energy of the
?-ray Photoelectric events predominate at low
energies Compton Scattering predominates in
medium energies Pair production predominates at
high energies.
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Absorbed Dose
Absorbed Dose energy absorbed per unit mass
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Radioactive Materials Materials that give off
radiation which results in ionization of
materials Types of Ionizing Radiation x-rays and
?-rays a particles ß particles neutrons
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US Geological Survey
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US Geological Survey
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US Geological Survey
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Relative Decay of 28Al and 36Cl
Ni(t) N0e-?t
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Half-Life

• For radioactive decay, we consider the The
  half-life, t½, is the time at which Ni(t)N0/2
• N0/2N0e-?t
• ?t ln(2)
• t½ ln(2)/?

Decay of Al-28