Lesson 19 - Human as targetIn this lesson, the response function for use in human beings is discussed. For the most part, shielding analysts have response functions specified for them, either due to regulatory requirements, instrument manufacturer specifications, or from experimental or calculated data for particular situations.
Nevertheless, it is a valuable exercise to look at how the various different response functions are determined in order to better judge which situations call for which response functions. The distinguishing concepts that we will discuss are:
Free-field flux versus local flux response functionsUp to now, we have been calculating the response functions based on the interaction coefficients (cross sections) of the actual materials making up the "detector". In practice, however, this is not done in everyday shielding work -- the determination of response functions is a specialty and the specification of which response functions to use in a given situation is the subject of debate and regulations.
In shielding practice, then, one does not determine effective dose rates directly, but instead determines the neutral particle flux or fluence rates (by particle type, energy, and position) in the absense of the detector. These fluxes are called free field fluxes. The energy-dependent response functions are specially formulated to deliver the desired response when used with free-field fluxes.
This makes the shielding analyst's job much easier, since the free-field
flux can be determined, for example, for all points in a vault in a single
calculation and then the dose field can be infered at all points in the
field. This is much easier, of course, than performing repeated calculations
with an explicitly-modeled detector in various places in the vault.
Simple phantoms versus anthropomorphic phantomsA phantom is a model of a human being. We will deal with two types: simple phantom and anthropomorphic phantoms.
A simple phantom is a homogeneous tissue-like substance in a spherical (typically 30 cm diameter) or cylindrical geometry. This is the simplest possible model of a human -- it takes no account of the location of organs, so only is used for maximum dose determination or for dose versus depth in the body. Simple phantoms are conservative and are used for determining operational dose quantities well below exposure limits.
An anthropomorphic phantom (as illustrated in Figure 5.5 in the
text) is a more complicated model of a human that does model the
location of organs, although in an average sense. Still, the extra
anatomical detail allows for more detailed and accurate organ-specific
doses. Response functions based on anthropomorphic phantoms are weighted
averages of these organ doses.
Differences in response functions for simple phantomsFor simple phantoms, response functions are variously determined based on two characteristics -- depth in the phantom and irradiation geometry (i.e., direction that the flux comes from).
The depth variations that you will need to know are:
The irradiation geometries that you should be able to identify are PAR, OPP, ROT, and ISO. These correspond to:Par
The PAR geometry results in the maximum doses, so is the conservative choice.
Differences in response functions for anthropomorphic phantomsFor the anthropomorphic phantoms the distinguishing characteristics of the response functions are: (1) whether the function is based on the effective dose equivalent or effective dose methodologies and (2) (again) the irradiation geometry.
The first of these concerns with whether a given response function is based on the newer "effective dose" model (recommended by the ICRP in 1991) or the pre-1991 "effective dose equivalent" model. The two differences that I want you to remember are:
Because of the layout of our internal organs, the AP irradiation results in the highest doses, so is the conservative choice.
Return to Course Outline © 1999 by Ronald E. Pevey. All rights reserved.