| Summary: | 博士 === 國立清華大學 === 工程與系統科學系 === 92 === The annual dose limit of 5 mrem for the site boundary of the radiation facilities is much lower than the annual external radiation dose of about 60 mrem of natural background. The natural background radiation dose changes to a certain extent from site to site and with the meteorological condition, the surrounding environment, and time. It results in the dose contributed by the facility itself is much difficult to be identified by the monitoring system. Therefore, the accurate and reliable analysis tools for the radiation shielding design is required to make sure the dose beyond the limit. For the skyshine problems, however, the basic technique of ray analysis and the application of buildup factors are invalid, and more specialized techniques have to be employed. Most of these specialized techniques are based on some approximations, it becomes more important to determine the reasonable assumptions when these techniques are encountered.
The main research emphasis is to perform the characteristic study of the gamma-ray skyshine. The characteristic parameters of the gamma-ray skyshine were divided into three categories: the first is the characteristic of source, including the energy distribution, the angular distribution and position; the second is the characteristic of medium, including the air properties, the effect of ground scattering, and shielding structures; the third is the characteristic of detector, including the energy distribution of the scattered photon, the angular distribution of the scattered photon, and the position of the detector. The influences of the individual and combined parameters on gamma-ray skyshine phenomenon are very complicated. Therefore, these parameters were discussed in two respects, the effect on the evaluation and monitoring of skyshine dose rates. The effects of the energy and angular distribution of the equivalent point source have to be taken into consideration on the analysis of the skyshine dose rates when the volume source is encountered. The dedicated skyshine codes SKYDOSE and McSKY were revised to include the capability of dealing with these anisotropic sources.
Furthermore, a simplified method, based on the integral of the first collision kernel, is presented for performing gamma-ray skyshine calculations for the collimated sources. The first collision kernels were calculated in air for a reference air density by use of the EGS4 Monte Carlo code. These kernels can be applied to other air densities by making density corrections. The integral first collision kernel method has been used to calculate two of the ANSI/ANS skyshine benchmark problems and the results were compared with a number of other commonly used codes. Our results were generally in good agreement with others but only spend a small fraction of the computation time required by the Monte Carlo calculations. The scheme of the integral first collision kernel method for dealing with lots of source collimation geometry is also presented in this study.
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