Effects of Spatial Harmonics on Thermal Neutron Spectra Measured by Time-of-Flight Method

• Published in: Journal of nuclear science and technology Vol. 34; no. 8; pp. 723 - 733
• Main Author: HIRAGA, Fujio
• Format: Journal Article
• Language: English
• Published: Tokyo Taylor & Francis 01.08.1997; Atomic Energy Society of Japan; Taylor & Francis Ltd
• Subjects: Exact sciences and technology; flight time spectra; graphite moderators; neutron current; Neutron physics; neutron temperature; Nuclear engineering and nuclear power studies; Nuclear physics; Physics; pulsed neutron sources; simulation; spatial distribution; spatial effect; spatial harmonics; time-energy-dependent diffusion equation; time-of-flight method; Neutron flux; Thermal neutrons; Neutron sources; Graphite; Time-of-flight method; Numerical simulation; Neutron moderator; Nuclear physics
• DOI: 10.1080/18811248.1997.9733736
• ISSN: 0022-3131

Spatial effect-the variations in space of transient neutron flux noted in pulsed neutron experiments, and which is ascribable to the occurrence in moderator of spatial harmonics-is studied on the measured flight time spectra of neutrons reentering the atmosphere from parametrically varied depths of a graphite moderator. Measurements were made using moderator slabs of two different axial lengths, to examine the influence of differences in moderator size on the spatial effect. The master equation representing the scalar neutron flux as function of time and energy is solved by a semi-analytic method that takes account of spatial harmonics. Simulation of the solution thus obtained of the master equation proved to reproduce the measured flight time spectra with good accuracy. It was indicated that the spatial effect on the thermal neutron spectrum is stronger with a larger than with a smaller moderator slab. The neutron temperature in graphite moderator was calculated by formula fitted in Maxwellian distribution to the peaks of the thermal neutron spectra. It was indicated that in a graphite moderator of 120 cm axial length, spatial harmonics caused a variation of more than 40 K in neutron temperature between depths in moderator differing by 30 cm.