6.6 Accurate Calculation of Effective Dose for Environmental Gamma Rays Using Human Phantoms

Fig. 6-15 Typical source distributions in the environment investigated in simulation calculation
1. Source distribution
(a) uniformly distributed semi-infinite sources in the air
(b) uniformly distributed plane source in the ground
(c) uniformly distributed natural sources in the ground
2. Human models and posture
Four models of different body size (baby, infant, adults: male and female)
Standing position and prone position

Fig. 6-16 Comparison of the angular distribution of absorbed doses in air in typical environmental source distribution
In source (a), the dominant gamma rays come almost isotropically from the upper 2 pi directions, while a small amount of scattered gamma rays come from the lower 2 pi directions. In source (b), a large proportion of the gamma rays comes from the horizontal directions. Source (c) shows the inverse tendency of case (a) where the angular distribution is nearly uniform in the lower 2 pi directions with small components of scattered gamma rays stemming from the upper 2 pi directions.

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Fig. 6-17 Variation of the effective dose due to age for a plane source in the ground
The ratios of effective dose and absorbed dose in air at 1 m above the ground in case (b) are shown. The difference of effective dose between a baby (8 weeks) and adults (mean of male and female) is at most a factor of 2.

The effective dose E is a very important, convenient quantity for expressing quantitatively the stochastic effect of radiation to man, and is widely used in dose evaluation for nuclear work as well as for the public in the environment. But in principle effective dose is a quantity which is impossible to measure directly because it is the sum of organ and tissue equivalent doses multiplied by appropriate weighting factors. Therefore, the effective dose is usually evaluated indirectly from the measurements of the absorbed dose in air, by the use of conversion factors obtained based on theoretical calculation. Therefore, the conversion factors and the characteristics should be precisely investigated by a theoretical simulation calculation using human phantoms under practical irradiation conditions.
In JAERI the investigation of various conversion factors has been carried out by modeling typical distribution conditions of natural sources of the 238U series, 232Th series, and 40K in the environment, as well as of artificial sources released from nuclear facilities to the environment (Fig. 6-15). By simulation calculation, the behavior of radiation scattering and absorption in the environment was reproduced precisely and the energy and directional distributions were calculated. In addition, organ and tissue doses in a human body exposed under various irradiation conditions were calculated by using mathematical human phantoms, and the effective doses were evaluated.
Through these investigations the characteristics of effective dose were elucidated in detail e.g. dependences on differences of sources and their distribution, the size and posture of the human body, etc. and the accurate conversion factors for various irradiation conditions were prepared. Figures 6-16 and 6-17 are representative examples of the results obtained for directional dose distributions (under typical source conditions) and for the conversion factors to obtain the effective dose from the air absorbed dose. From this study it has become possible to accurately evaluate effective doses in the environment.

Reference
K. Saito et al., Calculation of the Effective Dose and Its Variation from Environmental Gamma Ray Sources, Health Phys., 74 (6), 698 (1998).

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