Question 13: Why was a 2-hour half-life chosen as a time
of reference for noble gases or short-lived radionuclides,
as used in 10 CFR Part 20, Appendix B and its footnotes?
radionuclides that have half-lives of less than 2 hours
"might include a significant contribution from external
exposure." "Significant contribution from external
exposure" in this footnote means that the contribution to
the dose equivalent from external irradiation exceeds that
from inhalation. Two hours is the half-life value below
which the contribution to the dose equivalent from external
exposure exceeds that from inhalation for virtually all
radionuclides.
radionuclide, the ratio of the dose from external
irradiation to that from internal irradiation (from
inhalation) depends on the half-life of the radionuclide,
the characteristics of the radiations emitted in the decay
of the radionuclide, the physical and chemical properties
of the radioactive material containing the radionuclides,
and the physiological response of the body to intakes of
this material. However, considering the effect of
half-life alone, and in general, the value of this ratio
increases as the half-life decreases. The Statement from
the 1983 Meeting of the ICRP includes the following
paragraph:
exposure to short-lived nuclides (other than isotopes of
noble gases) are based on the dose equivalent to organs and
tissues as the result of inhalation. The Commission wishes
to draw attention to the fact that there is an additional
contribution to these dose equivalents from external
irradiation. In situations where short-lived materials are
widely distributed in the workplace, this additional
contribution may be greater than that due to inhalation by
a factor that increases from about 1 to 100 as the
half-life of the radionuclide decreases from 1 day to 10
min. Such contributions should be assessed as part of the
external irradiation."
hours, the values of this factor fall within the range of
about 1 to 10. Thus, for virtually all radionuclides with
half-lives less than 2 hours, the value of this factor is
greater than one. Values of this factor greater than one
were selected as values indicating "a significant
contribution from external exposure." (References: Part 20
Appendix B Footnote 2)