CAPINTEC, INC CRC
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In nuclear medicine, we are interested in photon energies of 20keV 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 4
th
power of the
atomic number and is inversely proportional to the atomic weight of the medium
(Z
4
/A).
Compton Effect
The Compton Effect is a collision between a photon and an electron that can be
considered unbound. An electron can be considered to be unbound (or “free”) if the
energy of the incident photon is much greater than the binding energy of the electron.
The kinetic energy of the scattered electron is not constant, but is a function of the
angle through which it is scattered. The scattered photon must interact again in order
to impart all of its energy to the medium.
The Compton Effect is the dominant process for photon energies from 100keV to
about 10MeV in the region of the atomic numbers for detector materials. At 100keV,
the maximum kinetic energy of the scattered electron is about 30 percent of that of
the incident photon; at 1MeV, it is about 80 percent; and at 10MeV, it is about 98%.
The number of Compton interactions per unit mass varies directly as the atomic
number and inversely as the atomic weight of the medium (Z/A).
Pair Production
The process of pair production is difficult to comprehend because it is strictly a
relativistic quantum mechanical effect. What is observed to take place is that in the
presence of the electric field of a nucleus, the incident photon disappears and an
electron and a positron appear. (A positron is a particle with the same properties as
an electron, except that it has a positive charge.)
In order to produce an electron-positron pair, the incident photon must have an
energy of at least twice the mass of an electron, i.e., 1.022MeV. This process
dominates for very high energies, that is, above about 10MeV. The number of pair
production interactions per unit mass is proportional to the square of the atomic
number and inversely proportional to the atomic weight of the medium (Z
2
/A).
IONIZATION CHAMBER MEASURING PROCESS
An ionization chamber consists of two or more electrodes. The electrodes confine a volume
of gas and collect the charge (ions) produced by radiation within the volume. Thus, ionization
chambers can be used to measure radiation fields if the relationship between the radiation
field and the charge produced is known.
The radiation enters the chamber through the chamber wall and interacts with the gas in the
chamber or with the chamber wall. It must be pointed out that photons cannot produce
ionization directly, but must first interact with the chamber material (gas and wall) producing
June 09 APPENDIX I A1 - 5
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