Pulsed neutron experiments in graphite

Engineering, Pulsed neutron, graphite

McFarlane, Harold Finley ; Corngold, Noel Robert

University:California Institute of Technology

Department:Engineering and Applied Science

关键词: Engineering, Pulsed neutron, graphite;   

Others  :  https://thesis.library.caltech.edu/8536/1/McFarlane-hf-1971.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Detailed pulsed neutron measurements have been performedin graphite assemblies ranging in size from 30.48 cm x 38.10 cm x 38.10 cm to 91.44 cm x 66.67 cm x 66.67 cm. Results of the measurement have been compared to a modeled theoretical computation.

In the first set of experiments, we measured the effective decay constant of the neutron population in ten graphite stacks as a function of time after the source burst. We found the decay to be non-exponential in the six smallest assemblies, while in three larger assemblies the decay was exponential over a significant portion of the total measuring interval. The decay in the largeststack was exponential over the entire ten millisecond measuring interval. The non-exponential decay mode occurred when the effective decay constant exceeded 1600 sec^( -1).

In a second set of experiments, we measured the spatialdependence of the neutron population in four graphite stacks as a function of time after the source pulse. By doing an harmonic analysis of the spatial shape of the neutron distribution, we were able to compute the effective decay constants of the first two spatial modes. In addition, we were able to compute the time dependenteffective wave number of neutron distribution in the stacks.

Finally, we used a Laplace transform technique and asimple modeled scattering kernel to solve a diffusion equation for the time and energy dependence of the neutron distribution in the graphite stacks. Comparison of these theoretical results with the results of the first set of experiments indicated that more exact theoretical analysis would be required to adequately describe the experiments.

The implications of our experimental results for the theory of pulsed neutron experiments in polycrystalline media are discussedin the last chapter.