CAPINTEC, INC CRC
®
-25R
PHOTONS
Photon is the general term for a quantum of radiation. Photons are classified according to
their method of production.
γ-Rays
Photons resulting from nuclear transitions, nuclear reaction or annihilation of particles (e.g.,
electron-positron annihilation) are called Gamma-rays (γ-rays). Radioisotope sources
(radionuclides) are the most common means of γ-ray production. Radioisotope γ-sources
emit photons of one or more discrete energies.
X-Rays
X-rays are associated with the deceleration of electrons or with orbital electron transitions in
atoms.
The radiation from a γ-source is often accompanied by characteristic x-rays from transitions
of the orbital electrons in the daughter atom.
Bremsstrahlung
When very fast electrons are brought to rest in a medium (or pass through media) a
continuous low energy photon spectrum occurs. This is called Bremsstrahlung (“stopping or
braking radiation”).
The intensity and the energy spectrum of Bremsstrahlung are highly dependent upon the
source configuration and media surrounding the sample.
In this manual, the term photon will be used when the method of production of the radiation
has no bearing on the discussion.
Interactions of Photons with Matter
There are three mechanisms by which photons can interact with matter and, thus, deposit
their energy. These mechanisms are: Photoelectric effect, Compton Effect, and, pair
production. The energy of the photon determines which process (or processes) is possible.
Photoelectric Effect
The photoelectric effect is an interaction between a photon and an electron that is
bound to an atom. In the photoelectric process, the photon is absorbed by the atom
and a bound electron is ejected. The kinetic energy of the ejected electron is equal to
the photon energy minus the binding energy of the electron. The binding energy of an
electron is the energy that must be supplied in order to remove the electron from the
atom.
A1 - 4 APPENDIX I June 09
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