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Decay of Thermalized Neutron Fields in Graphite

C. H. Reed, C. N. Henry, A. A. Usner

Nuclear Science and Engineering

Volume 30 / Number 3 / December 1967 / Pages 362-373


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Asymptotic decay constants for pulse-induced “thermalized” neutron fields have been measured for graphite cubical assemblies having geometric bucklings varying from 9.30 × 10–4 cm–2 to 13.44 × 10–3 cm–2. A value of 700 μ sec was observed to be a sufficient time after the neutron pulse to identify and evaluate fundamental-mode decay in the smallest system included in the above interval of buckling. Values of the infinite-medium neutron lifetime –1 “Fick’slaw” diffusion coefficient D0, as well as the so-called “diffusion-cooling” coefficient C, were obtained from least-squares fits to the experimental α/ρ vs B2/ρ2 data and are mutually consistent and stable over a large interval of B2 and in good agreement with theory. The existence of a well-defined negative FB6 term has been verified. An “effective” higher-mode decay of (3570 ± 80)sec–1, independent of system buckling, was obtained and is consistent with the concept of a continuum lying above a critical limit for fundamental-mode decay. An apparent critical limit (vt)min has been identified in the interval 2392 sec–1 < (vt)min < 2648 sec–1 which corresponds to the interval of buckling 13.44 × 10–3 cm–2 to 16.53 × 10–3 cm–2.

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