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Measurement of Steady-State, Space-Dependent, Thermal-Neutron Spectra in Beryllium Assemblies with Strong Transverse Leakage

J. A. Lake, J. M. Kallfelz

Nuclear Science and Engineering

Volume 53 / Number 1 / January 1974 / Pages 27-46


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Thermal-neutron energy spectra have been measured as a function of trans-verse buckling and of space position along the longitudinal axes of 25.4- × 25.4-and 35.6- × 35.6-cm beryllium assemblies by the slow-chopper, time-of-flight technique using the thermal column of the Georgia Tech Research Reactor as the steady-state neutron source. For neutrons moving in the (forward) positive Z direction, we find no evidence of the establishment of, or the tendency toward, discrete asymptotic decay conditions from the strongly space-dependent spectra in either assembly. This is a direct experimental verification of the disappearance of the discrete set of eigenvalues and eigenfunctions in the diffusion-length problem with transverse leakage. These results are in at least qualitative agreement with the transport theory predictions of Williams, but in disagreement with the diffusion theory results of Ahmed, Kothari, and Kumar which predict that a true discrete mode should exist in beryllium assemblies as small as 30 × 30 cm.

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