Capintec CRC-712MH, Руководство пользователя

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CAPINTEC, INC. CRC
®
-712MH
(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. (See Appendix of this manual for
more detailed discussion on Bremsstrahlung.)
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.
In nuclear medicine, we are interested in photon energies of 20 keV or greater. At
these energies, all the electrons in the materials used for the chambers are able to
participate in the photoelectric process. The photoelectric effect is the most important
process at low energies. However, for photon energies much greater than electron
binding energies, the processes described below become more important and the
number of photoelectric interactions occurring becomes small. At a given energy, the
number of photoelectric interactions per unit mass varies as the 4th power of the
atomic number and is inversely proportional to the atomic weight of the medium
(Z
4
/A).
A1 - 4 APPENDIX I January 08