Lesson 19 - Human as target

In 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 functions;
• Simple phantoms versus anthropomorphic phantoms;
• For simple phantoms, differences in response functions based on depth and irradiation geometry.
• For anthropomorphic phantoms, differences in response functions based on use of effective dose equivalent versus effective dose concepts and irradiation geometry.

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Free-field flux versus local flux response functions

Up 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 phantoms

A 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 phantoms

For 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:

• Shallow dose equivalent dose index,  -- the maximum dose between 0.007 cm and 0.01 cm depth in the phantom.  This corresponds to dose to the radiosensitive layer of the skin.
• Deep dose equialent dose index, -- the maximum dose inside the 14 cm radius core sphere of the spherical phantom ( i.e., > 1 cm deep in the 30 cm diameter sphere).  This corresponds to the maximum dose in internal organs.
• Ambient dose equivalent, -- maximum dose a depth of d mm in the sphere.  Remember the examples mentioned in the text -- 0.01 mm for skin, 3 mm for eye lens, 10 mm for strongly penetrating radiation.
• The irradiation geometries that you should be able to identify are PAR, OPP, ROT, and ISO.  These correspond to:Par
• Parallel (PAR) -- Radiation field consisting of a parallel beam of particles
• Opposed (OPP) --  Radiation field consisting of two parallel beams of particles 180 degrees apart.
• Rotational (ROT) -- A rotating phantom in a PAR field.
• Isotropic (ISO) -- Isotropic directional distribution of particles impinging on all points of phantom surface.
• The PAR geometry results in the maximum doses, so is the conservative choice.

Differences in response functions for anthropomorphic phantoms

For 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:

1. Although both of the methodologies use weighted averages of individual organ doses, the twho methodologies use different weights.  The weights used for the two models are given in Tables 5.4 and 5.6 in the text.
2. The newer "effective dose" uses a simpler, non-organ-specific determination of the Q factor (which is denoted by the symbol ).  The two methodologies are given by the Equations 5.44 and 5.45.
The second distinguishing characteristic -- like it was for the simple pahntoms -- is the irradiation geometry. The irradiation geometries that you should be able to identify are AP, PA, LAT,  ROT, and ISO.  These correspond to:
• Anterior-posterior (AP)-- Radiation field consisting of a parallel beam of particles impinging on the front of the phantom
• Posterior-anterior (PA) -- Radiation field consisting of a parallel beam of particles impinging on the back of the phantom
• Lateral (LAT) -- Radiation field consisting of a parallel beam of particles impinging on the side of the phantom
• Rotational (ROT) -- A rotating phantom in a PAR field.
• Isotropic (ISO) -- Isotropic directional distribution of particles impinging on all points of phantom surface.
•  Because of the layout of our internal organs, the AP irradiation results in the highest doses, so is the conservative choice.