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Chapter 12

SURVIVOR DOSIMETRY

Part A. Fluence-to-Kerma Conversion Coefficients

George D. Kerr, Joseph V. Pace III, Stephen D. Egbert

Introduction

An important step in the dosimetry evaluation is to relate the radiation passing through a unit volume of a material of interest (fluence) to the energy release (kerma) in the material, which determines the absorbed dose. The fluence-to-kerma conversion coefficients or "kerma coefficients" used in the Dosimetry System 1986 (DS86) are taken from Kerr (1982). These kerma coefficients are based on body tissue compositions for Reference Man from the International Commission on Radiological Protection (1975) and Kerr (1982), the mass energyabsorption coefficients for photons from Hubbell (1982), and the elemental kerma coefficients for neutrons from Caswell et al. (1980). Hence, the kerma coefficients used in DS86 are approximately 20 years old. In order to provide an updated set of kerma coefficients for use in the Dosimetry System 2002 (DS02), a new evaluation has been completed. This new evaluation considered recently suggested changes in the composition of soft tissues of the body in ICRU Report 44 (International Commission on Radiation Units and Measurements 1989), the mass energyabsorption coefficients for photons by Hubbell and Seltzer (1996), and the elemental kerma coefficients for neutrons in ICRU Report 63 (International Commission on Radiation Units and Measurements 2000). The new DS02 kerma coefficients for soft tissue are presented as both point-wise data for use in Monte Carlo radiation transport calculations and multigroup data for use in discrete ordinates radiation transport calculations.

Elemental Composition of Soft Tissue

Various approximations to the elemental composition of the tissues and organs of the human body have been used in both kerma and dose calculations for photons and neutrons. Some

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calculations have considered as few as the four major elements of the body (Auxier et al. 1968, Snyder 1972) and others as many as 15 elements (Singh 1982) or more (White and Fitzgerald 1977). The DS86 kerma and dose calculations for both neutrons and photons are based on a twelve-element approximation for the organs and tissues of ICRP-1975 Reference Man. It includes the eleven most abundant elements of the total body (i.e., the skeleton and total soft tissues) and iron, which is the one of the most abundant trace elements in organs and tissues of special interest such as the lungs and bone marrow. A summary of the twelve-element approximation used for total soft tissues of the body in the DS86 kerma and dose calculations is provided in Table 1 (Kerr 1982)

White et al. (1987) and the International Commission on Radiation Units and Measurements (ICRU 1989, 1992) recently recommended some changes to the elemental composition of the organs and tissues of ICRP-1975 Reference Man (Table 1). These changes involve only data for elements that contribute more than 0.1% by mass to the composition of any organ or tissue of Reference Man. Thus, the data are limited to the nine most abundant elements in the total soft tissues of the human body, and there appears to be no compelling reason for adopting these suggested revisions to ICRP-1975 Reference Man over those already in use in DS86 (Kerr 1982). For example, the kerma or dose from neutrons depends primarily on the amount of hydrogen in the tissue or organ of interest, and essentially no differences are found among the various hydrogen values of Table 1. The slight differences in hydrogen and other elemental abundances of Table 1 also have a very small impact on the overall mass energy-absorption coefficients for photons in the soft tissues of the body.

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Kerma Coefficients for Soft Tissue

Kerma is the sum of the initial energies of all charged particles liberated by indirectly ionizing radiations such as neutrons and protons in a small volume element of a specified material divided by the mass of material in that volume element (Roesch and Attix 1968). It is a useful quantity in radiation dosimetry when charged particle equilibrium exists at the position and in the material of interest, and bremsstrahlung losses of the charged particles are negligible. In this case, kerma and absorbed dose can be equated. Absorbed dose is the energy imparted by charged particles in the small volume element of the specified material divided by the mass of the material within that volume element. Units of absorbed dose and kerma can be either rad or gray. One rad is equal to 100 erg per gram of the specified material, and 1 gray (Gy) is equal to one joule per kilogram of the specified material (or 100 rad). The kerma coefficients given here are the kerma in soft tissue of the body per unit particle fluence of either neutrons or photons at a specified energy. In many practical applications, the tissue volume of interest may be located in the body or in another medium. For example, the intensity of a radiation field of neutrons and photons incident on the body may be specified in terms of the kerma in air. The tissue kerma in air or so-called free-in-air (FIA) tissue kerma from photons and neutrons is closely related to the maximum absorbed dose in the body (i.e., the maximum absorbed dose to the skin of the body). If the particle fluence involves a broad energy spectrum of neutrons or photons, then an appropriately weighted mean value must be calculated. The mean value would be weighted by the energy spectrum of the particles in air if the quantity of interest is the FIA tissue kerma and by the energy spectrum of the particles in the body if the quantity of interest is the organ dose (i.e., the absorbed dose in the organ). The energy spectrum of the particles within the organ of interest in the body can be calculated using Monte Carlo radiation transport codes. The absorbed dose to critical organs and tissues of the body is the quantity of interest in the radiation dosimetry for the atomic-bomb survivors. Because bremsstrahlung losses by charged particles are negligible in the soft tissues of the body and charged particle equilibrium exists at the interfaces of the soft tissues, absorbed dose and kerma can be equated in most soft tissues of the body once the self shielding by overlying body tissues is taken into account. This approach is used in calculating absorbed doses (or organ doses) for the bladder, brain, breasts, eyes, intestinal tract, kidneys, liver, lungs, ovaries, pancreas, stomach, testes, and thyroid. In the skeleton, the critical tissues are considered to be the red marrow and the osteogenic cells, especially those on the endosteal surfaces of bone. However, absorbed dose and kerma cannot be equated in these tissues because charged particle equilibrium may not exist near a soft tissue-bone interface. The calculation of the absorbed doses to the soft tissue of bone is a more difficult problem which requires that the charged particles produced by the photons and neutrons be tracked in detail as they deposit their energies in the complex intermixture of bone and soft tissues of the skeleton (Kerr and Eckerman 1987).

Photon Kerma Coefficients for Soft Tissue

Photon kerma coefficients for soft tissue are obtained by summing the products of the mass faction on an element in soft tissue, the photon energy, and the mass energy-absorption coefficient of the element for photons of that energy. The sums are calculated for discrete photon

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energies, and the kerma coefficients are referred to as "point-wise data." If the unit of photon energy is MeV, and the units of the mass energy-absorption coefficients are cm2 per g, then the sums can be multiplied by 1.602 × 10-10 to obtain kerma coefficients with units of Gy per photon per cm2. The mass factions for soft tissue were taken from the 1982 report by Kerr (see column 2 of Table 1) and the mass energy-absorption coefficients were taken from the report by Hubbell and Seltzer (1996). The newer kerma coefficients (DS02) and the older kerma coefficients (DS86) for photons in soft tissue of ICRP-1975 Reference Man are compared in Table 2. The only noticeable departure between the two sets of kerma coefficients occurs at energies above 10 MeV, with the older DS86 kerma coefficients being larger than the newer DS02 kerma coefficients. The maximum difference between the two sets of kerma coefficients occurs at 20 MeV and is approximately 7%.

Neutron Kerma Coefficients for Soft Tissue

The DS86 studies used kerma coefficients for neutrons in 19 different isotopes and elements, including the 12 elements used in the soft tissue-approximation for ICRP-Reference Man, that were tabulated by Caswell et al. (1980). Their tabulations gave the kerma coefficients for a monoenergetic "thermal neutron" energy of 0.0235 eV and for 119 contiguous energy "groups" or "bins" extending from 0.026 eV to 30 MeV. Each bin was characterized by a central or mean energy and an energy interval of a given width (Caswell et al. 1980). The kerma coefficients were calculated from cross sections averaged over the full energy width of each bin. Averaging over binned energy widths eliminated the somewhat irregular behavior of the kerma coefficients due to resonance absorption of neutrons by elements other than hydrogen. Only the tabulated data for bins with neutron energies below 20 MeV were used in the DS86 studies.

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The neutron kerma coefficients of Caswell et al. (1980) have been revised for 12 different isotopes and published in Report 63 of the International Commission on Radiation Units and Measurements (2000). Because these revisions are more limited than before (Caswell et al. 1980), the newer DS02 neutron kerma coefficients for soft tissue are based only on the four major elements of the body (hydrogen, carbon, nitrogen, and oxygen) and the mass fractions for the other eight elements (Table 1) were assigned to oxygen. This works well with the neutron energies of interest here (Figure 1). At thermal neutron energies, most of the soft-tissue kerma (and organ dose) is from protons produced by neutron capture in nitrogen, and at fast neutron energies between about 1 keV and 1 MeV, the proton recoils from elastic scattering by hydrogen contribute 90% or more to the soft tissue kerma (or organ dose). The recoil-proton contribution drops to only 80% at about 10 MeV and 70% at about 20 MeV, the highest neutron energy of interest here. The neutron kerma coefficients in ICRU Report 63 extend up to energies of 150 MeV (International Commission on Radiation Measurements and Units 2000). Both the newer (DS02) and older (DS86) neutron kerma coefficients for soft tissue are compared in Table 3. The neutron kerma coefficients only go down to neutron energies of 0.0235 eV (see Table 3) but nitrogen is a 1/v absorber at thermal neutron energies (Figure 1). Thus, it is possible to extrapolate the kerma coefficients to lower thermal neutron energies of interest in the DS02 studies using the relationship K 2 = [E 2/E 1] 1/2 K 1, where K 1 and K 2 are the kerma coefficients at thermal neutron energies E1 and E2, respectively (Figure 2). At energies between 0.0253 eV and 10 MeV, the two sets of neutron kerma coefficients are in close agreement (Table 3). However, there is a noticeable departure between the two sets at higher energies, with the older DS86 kerma coefficients being larger than the new DS02 kerma coefficients by approximately 10% at 20 MeV. These differences at higher neutron energies are mainly due to recent changes in the kerma coefficients for carbon and oxygen (International Commission on Radiation Units and Measurements 2000) rather than the use of only four elements to approximate the soft tissue of the human body in the development of the newer DS02 kerma coefficients for neutrons.

Figure 1. DS02 neutron kerma coefficients for soft tissue and contributions to the total DS02 neutron kerma coefficients by the four major elements of soft tissue.

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Figure 2. DS02 calculated and extrapolated neutron kerma coefficients for soft tissue.

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Fine-Group Kerma Coefficients for Soft Tissue

The DS02 radiation-transport calculations were performed using both discrete ordinates and Monte Carlo methods (Chapter 3). The kerma coefficients provided in Tables 2 and 3 are suitable for use with the Monte Carlo codes that use continuous energy or point-wise cross-section data. The discrete ordinates method is a numerical integration technique that yields highly differential solutions to the Boltzman transport equation. This method derives its name from the replacement of the continuous angular variable by weighted values for discrete angles such that particles are only allowed to scatter along a finite number of directions rather than in all directions. The discrete representation of the spatial and energy variables in the discrete ordinates transport equation is obtained by dividing the geometry into a fine mesh system and by using multigroup cross sections. The discrete ordinates calculations of the DS02 studies were performed using ENDF/B-VI cross-section data in the multigroup format described in the documentation for the Vitamin-B6 fine-group neutron and photon cross-section library. Hence, it was also necessary to place both the neutron and photon kerma coefficients in the same energy group structure as the Vitamin-B6 fine-group cross-section library (Chapter 3). The fine-group DS02 kerma coefficients for soft tissue are provided in Table 4 for the 199neutron energy group structure and in Table 5 for the 42-photon group structure of the VitaminB6 cross-section library (Ingersoll et al. 1995). The neutron weighting factor used to obtain these neutron multigroup data was a Maxwellian thermal neutron spectrum below 0.125 eV, a E-1 slowing down spectrum between 0.125 eV and 820.8 keV, and a fission neutron spectrum between 820.8 keV and 20 MeV. The photon weighting factor used in the discrete ordinates calculations of the DS02 studies consisted of an E-1 spectrum with a roll-off of the spectrum at lower energies to represent photoelectric absorption and a drop-off at higher energies corresponding to the Q-value for neutron capture (Chapter 3). This weighting factor is referred to in both Table 5 and Figure 3 as NJOY weighting (Oak Ridge National Laboratory 1999). Additional fine-group DS02 coefficients for photons in soft tissue have been obtained using simple E-1 weighting and flat weighting (Table 5 and Figure 3). The flat weighted kerma coefficients for photons are recommended for use in calculating absorbed dose to soft tissue of the body other than the active marrow of the skeleton. The upper energy bound of Vitamin-B6 cross-section library for photons is 30 MeV rather than the 20 MeV of our point-wise data (Tables 2 and 5). A value for the photon kerma coefficient at 30 MeV was added as follows. First, photon kerma coefficients were calculated for soft tissue at photon energies of both 20 MeV and 30 MeV using the photon data from Storm and Israel (1970). These photon kerma coefficients were normalized next to the one calculated at the upper energy bound of 20 MeV for photons in Hubbell and Seltzer (1996). Finally, the 30-MeV kerma coefficient for soft tissue, which was normalized to the 20-Mev kerma coefficient of the point-wise data, was used in developing the fine-group DS02 kerma coefficients in soft tissue up to the upper energy boundary of 30 MeV for photons in the Vitamin-B6 cross-section library (Table 5 and Figure 3).

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Broad-Group Kerma Coefficients for Soft Tissue

The DS86 and DS02 computer codes both use the 37 neutron-21 photon energy group structure of the DLC31 cross-section library (Bartine et al. 1977) and shielding calculations with other computer codes also make extensive use of the 46 neutron-23 photon energy group structure of the DABL69 cross-section library (Ingersoll et al. 1989). The energy group structure of the DLC31 and DABL69 cross-section libraries are both subsets of the energy group structure used in the Vitamin-B6 cross-section library. Thus, the fine group set of kerma coefficients from this study can be collapsed to these broad group sets using the air-transported spectra as weighting factors. This was done at ground ranges of 500 m, 1,000m, 1,500 m, 2,000 m, and 2,500 m at both cities for both the prompt and delayed components and for both the above- and below-ground spectra of neutrons and photons. The kerma factors produced from the hundreds of neutron and photon spectra used as weighting factors are typically within less than 1% for each energy group. There are two DLC31 energy groups, however, where the difference was as much as 5%. These anomalous differences occurred only in Hiroshima at ground ranges less than 1,000 m and only in the very broad neutron energy groups from 0.158 to 0.550 MeV and from 0.550 to 1.11 MeV. In the DABL69 energy group structure, the differences disappear because of the finer group structure at these neutron energies. For photons, the lowest energy kerma coefficient (0.01 to 0.02 MeV) differed

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