As a new imaging method, single neutron imaging based on neutron capture events can significantly improve spatial resolution. However, current imaging methods do not fully utilize their spatial resolution performance. This work studies the key processes of particle transport and fluorescence transport in 6LiF-ZnS scintillation screens, proving that spatial resolution still has the potential to be improved. On particle transport, Monte Carlo simulation is used to quantitatively calculate the transport behavior of secondary particles produced by nuclear reactions in scintillation screen. Essential information related to fluorescence, such as the range, magnitude, and shape of the energy distribution, is provided. On fluorescence transport, a theoretical modeling of the transmission of fluorescence photons inside the scintillator was conducted. A new method for calculating point spread functions based on secondary particle transport is proposed. By combining particle transport simulation and fluorescence transport calculation, a more accurate point diffusion function is obtained, which can be used to replace the traditional point light source point diffusion function. This work proves that single neutron imaging based on 6LiF-ZnS scintillation screen for neutron event location is a feasible technical route. The physical factors limiting the imaging method and the credible value of the optimal resolution are given. It is further shown that single neutron imaging based on 6LiF-ZnS scintillation screen still has the potential to improve the resolution, and has the possibility of better than 9.506μm.
The spontaneous imbibition of water in variable saturated porous media is a common phenomenon in various geotechnical applications. The relationship between the traveled distance of wetting front and imbibition time can be characterized by the sorptivity however, few researches have been carried out to investigate the sorptivity model in the spontaneous imbibition process. In this paper, based on the fractal dimension of pore mass and the formula of total flow rate of cross section of medium without considering gravity, a model was derived to calculate the sorptivity of sandstone. To validate the model, quantitative data on the imbibition of water in the matrix of unsaturated sandstone were obtained by neutron radiography a powerful imaging tool to research the dynamic water imbibition behaviour in rocks. The consequences display that there is the obvious linear relation between the traveled distance of wetting front and the square root of imbibition time in four monitoring lines and the linear coefficient is regarded as the experimental value of the sorptivity. Compared the mean of the experimental value, the theoretical value computed by the sorptivity model can reflect the experimental value to a large extent and, thus the sorptivity model can be applied to estimate the sorptivity of sandstone in the process of spontaneous imbibition.
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