CAPINTEC, INC CRC
®
-25R
electrons. That is, through a series of interactions, the photon transfers its energy to one or
more electrons.
The electron is slowed down through collisions with the chamber gas (argon). The collisions
knock electrons off the molecules producing positive ions (this is the ionization process).
The collection voltage across the chamber sets up an electric field. The positive ions will drift
towards the negative electrode and the electron (and negative ions if they are formed) will
drift towards the positive electrode, thus producing a current. The electronic circuitry then
measures either the current or the total charge produced during the period of interest.
The number of ions produced in the chamber is directly related to the energy deposited in the
chamber by the radiation.
DETAILED DISCUSSIONS
Effects of the Integral Shield
The advantage of the shield is the reduction of radiation exposure to the personnel handling
the radioisotopes, as well as reduction of the background effects on the activity
measurements.
It is important to note, however, that if a shield is placed around or near a calibrator, the
sensitivity of the ionization chamber is enhanced due to backscattering of photons by the
shielding. Above about 250keV, the scattering of photons is mainly forward and at the low
energy region, attenuation of photons by the outer wall of the chamber becomes significant.
For a CRC
®
calibrator, the backscattering effects are more significant for photons of energies
between 70keV and 250keV than photons in other energy regions.
Effects of the Container
The radioactive standard materials in the ampoules now being provided by NIST are a good
approximation to an assay of a radiopharmaceutical in a plastic syringe or in a glass syringe
(a wall thickness of about 1.2mm), even for radioisotopes that decay with a significant
abundance of low-energy photons.
The user should select, whenever possible, a standardized procedure, volume, and container
for all radioactivity measurements. The plastic syringe is convenient since it represents the
delivery vehicle to the patient in most clinical situations.
Significant errors will occur in some instances, e.g., if the radioisotope is assayed in an
appreciably different material and/or wall thickness than that of the standards.
The ampoules of recently available standards from NIST are uniform. Plastic syringes also
have a rather uniform wall thickness and absorption is low. However, a random sampling of
5, 10, 25, 50, and 125ml size multi-injection dose vials from several sources indicated that
the wall thickness varied randomly from 1 to 3mm quite independently of the volume of glass
vial.
A1 - 6 APPENDIX I June 09
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