CAPINTEC, INC.
CRC
®
-712MX
The sensitivity curve of the high range 1 atmosphere Ionization Chamber is under
development. As Calibration Numbers are determined, users will be updated.
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 250 keV, 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 70 keV and 250 keV 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 1.2 mm), 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 3 mm quite
independently of the volume of glass vial.
The assay of radioisotopes having a significant abundance of low-energy gamma-, x-,
and/or high-energy beta-ray radiation may be affected by changes in the sample
configuration used to assay the radio-pharmaceutical if the samples are severely
different from the standard source. In such cases, an independent check or
determination of a calibration appropriate to a user's needs is advised. Fortunately,
most radioisotopes can be accurately assayed independently of the sample size.
The radioisotopes most sensitive to source configuration and type of container are
I125 and Xe133. Other radioisotopes that fall into this category are I123, Y169, Tl201,
and other radioisotopes that decay with significant low-energy photon emission. It is
not unusual to have a required correction factor of 2 if I125 is measured in a glass
vial.
January 08
APPENDIX I
A1 - 7
