A gamma-ray will interact with its medium in one of three different ways: photoelectric
absorption, Compton scattering, and pair production. These different interactions change
their probability of occurring depending on the energy of the gamma-ray and the atomic
number of the material.
In the photoelectric absorption, the incident photon disappears and a photoelectron is produced
from one of the electron shells of the absorber. The kinetic energy that this electron
carries off is Ee− = h − Eb, where Eb is the binding energy of the liberated electron in its
original shell. This empty spot in the electron shell is quickly filled by electron rearrange-ment. This process causes the binding energy, Eb, to be liberated as well. This energy is
liberated in the form of a characteristic X-ray or an Auger electron.
The photoelectric absorption interaction is the ideal interaction for gamma-ray spectroscopy.
The photoelectron carries away most of the gamma-ray energy and then an X-ray or Auger
electron carries away the remaining kinetic energy. Assuming an ideal detector, the sum of
these energies will equal the energy of the original gamma-ray.
This is desired for gamma-ray spectroscopy because we are interested in knowing the energies
of the various gamma-rays that are emitted by a source. In Figure 3.3, we see what the ideal
photopeak created by mono-energetic gamma-rays of a single energy looks like.
The Compton scattering interaction is the scattering of a gamma-ray off of a free or unbound
electron, thus creating a scattered gamma-ray photon and a recoil electron. The energy of
the incoming photon is divided between the scattered photon and the recoil nucleus by a
relationship that is dependent on the scattering angle.
There are two extreme cases dictated by this equation: When = 0, the scattered photon
retains all of its energy and the recoil electron gains no energy. When = , the incident
gamma-ray is backscattered and the recoil electron moves along the direction of incidence.
This case is the case with the maximum energy transfer between the incoming gamma-ray
and the electron.
In the detector, all scattering angles from 0 to will occur. Because of this, a continuum of
energies can be transferred to the electron. This
energy has a range from 0 all the way to the maximum.
Pair production is a gamma-ray that turns into an electron-positron pair. This occurs when
the gamma-ray is in the intense electric field near the nuclei of the absorbing material.
There is a minimum amount of gamma-ray energy that is required for this process to take
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