THE DIFFUSION COOLING OF NEUTRONS IN FINITE MODERATOR

Journal Article · Wed Jan 01 04:00:00 EST 1958 · J. Nuclear Energy

DOI:https://doi.org/10.1016/0891-3919(58)90005-6· OSTI ID:4257703

Nelkin, M

When a pulse of fast neutrons is thermalized in a finite moderating medium, the asymptotic spectrum at long times is slower than an equilibrium Maxwellian distribution, because of preferential leakage of higherenergy neutrons. This phenomenon of ''diffusion cooling'' can be observed by examining the variation of the asymptotic neutron lifetime with the size of the moderating medium. In this paper, the phenomenon is investigated within the context of energy-dependent diffusion theory. A variational expression for the decay constant is formulated and evaluated for trial solutions of Maxwellian form for the neutron spectrum. For small deviations from thermal equilibrium, an expression of the form proposed by von Dardel is obtained. The present analysis relates the diffusion-cooling term in this expression to the energy dependence of the transport mean free path, and to the mean square energy transfer between thermal neutrons and moderator. An analysis of the available experimental results for beryllium, graphite, and water is carried out and yields information on the effects of chemical binding on the energy transfer between neutrons and moderator. The results for graphite are consistent with the model of Krumhansl and Brooks for the specific heat. The results for beryllium, however, are not consistent with a Debye model which fits the specific heat, and it is suggested that the diffusion cooling is actually smaller than the available experimental value. The analysis for water is less sound theoretically, but it indicates a considerable hindering of the molecular rotations in the liquid. (auth)

• Research Organization: General Atomic Div. of General Dynamics Corp., San Diego, Calif.
• NSA Number: NSA-13-007961
• OSTI ID: 4257703
• Journal Information: J. Nuclear Energy, Journal Name: J. Nuclear Energy Vol. Vol: 8
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

BERYLLIUM
BINDING ENERGY
CRYSTALS
DECAY
DIFFUSION
FAST NEUTRONS
GRAPHITE
LIFETIME
LIQUIDS
MEAN FREE PATH
MODERATORS
MOLECULES
NEUTRONS
PHYSICS AND MATHEMATICS
PULSES
SLOWDOWN
SPECIFIC HEAT
SPECTRA
THERMAL NEUTRONS
TRANSPORT THEORY
WATER