| Summary: | 博士 === 國立清華大學 === 工程與系統科學系 === 90 === This study investigated the characteristics of the cosmic-ray neutron field near air/ground and air/water interfaces with emphases on the energy and angular distributions. The cosmic-ray neutron fluence rate at sea level has been reported at the order of 10-3 cm-2s-1. In order to properly measure the low intensity cosmic-ray neutrons, two sets of high-efficiency neutron detecting systems were developed. The first one, called the Bonner Cylinders, was used for measurements of the energy information. The other one, referred to as the 8CND (eight-channel neutron detector), was used to characterize the angular information of the neutron field. The measured results were used to normalize and confirm one-dimensional transport calculations for cosmic-ray neutrons near interfaces. According to these measurements and calculations, both energy spectra and angular distributions of cosmic-ray neutrons near interfaces were resolved. The neutron flux change near interface is owing to the discontinuity in both the neutron interaction and production cross sections. Besides, the instruments and methods developed here were also used to study the neutron field outside the shielding wall of high-energy electron accelerator.
In the present work, annual sea level neutron effective doses in Taiwan were determined to be 26 (air/ground) and 22 Sv (air/water), corresponding to the neutron fluence rates of 5.610-3 (air/ground) and 3.710-3 n cm-2 s-1 (air/water). The dependence of the cosmic-ray neutrons on the atmospheric depth can be represented well by the exponential function of exp(-d/L) with the effective attenuation length L = 175.4 g cm-2 in Taiwan. Since Taiwan is located near the geomagnetic equator, the vertical cutoff of geomagnetic rigidity is almost the highest in the world. The latitude effect, as compared with that reported elsewhere, results in appreciable differences both in absolute values and the altitude dependence. In addition, the energy spectra of cosmic-ray neutrons near air/ground and air/water interfaces were also unfolded. The results show that the neutron fluence rate near the air/ground interface is greater than that near the air/water interface, while the neutron spectrum near the air/water interface is somewhat harder than that near the air/ground interface. Although there is a clear distinction between energy distributions of upward neutrons and downward neutrons near interfaces, the angular distribution of the total neutrons near the air/ground interface is almost isotropic while near the air/water interface more neutrons come downward from the air than upward from the water. In comparison with the in-flight spectrum, the interface has a significant effect on the low-energy part of the spectrum, especially in the thermal region. However, these low-energy neutrons do not make significant contribution to the effective dose rate. On the contrary, high-energy neutrons dominate the effective dose rate from cosmic-ray neutrons.
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