Biomedical Engineering Reference
In-Depth Information
12
Radiation Dosimetry
12.1
Introduction
Radiation dosimetry is the branch of science that attempts to quantitatively relate
specific measurements made in a radiation field to physical, chemical, and/or bio-
logical changes that the radiation would produce in a target. Dosimetry is essential
for quantifying the incidence of various biological changes as a function of the
amount of radiation received (dose-effect relationships), for comparing different
experiments, for monitoring the radiation exposure of individuals, and for surveil-
lance of the environment. In this chapter we describe the principal concepts upon
which radiation dosimetry is based and present methods for their practical utiliza-
tion.
When radiation interacts with a target it produces excited and ionized atoms and
molecules as well as large numbers of secondary electrons. The secondary electrons
can produce additional ionizations and excitations until, finally, the energies of all
electrons fall below the threshold necessary for exciting the medium. As we shall
see in detail in the next chapter, the initial electronic transitions, which produce
chemically active species, are completed in very short times (
10
-15
s) in local re-
gions within the path traversed by a charged particle. These changes, which require
the direct absorption of energy from the incident radiation by the target, represent
the initial physical perturbations fromwhich subsequent radiation effects evolve. It
is natural therefore to consider measurements of ionization and energy absorption
as the basis for radiation dosimetry.
As experience and knowledge have been gained through the years, basic ideas,
philosophy, and concepts behind radiation protection and dosimetry have continu-
ally evolved. This process continues today. On a world scale, the recommendations
of the International Commission on Radiological Protection (ICRP) have played a
major role in establishing protection criteria at many facilities that deal with radia-
tion. In the United States, recommendations of the National Council on Radiation
protection and Measurements (NCRP) have provided similar guidance. There is
close cooperation among these two bodies and also the International Commission
on Radiation Units and Measurements (ICRU). As a practical matter, there is a cer-
tain time delay between the publication of the recommendations and the official
