Thermal Inelastic Scattering of Cold Neutrons in Polycrystalline Graphite

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Abstract
The temperature variation of the thermal inelastic scattering cross section of cold neutrons in graphite, calculated on the assumption of a Debye frequency spectrum, is in disagreement with experiments. This is not surprising in view of the highly anisotropic nature of its lattice. In this paper we have modified the formulas of the scattering cross section obtained earlier by us, for the case of an anisotropic layer lattice. Using the Krumhansl and Brooks model of graphite with the values of the parameters occurring in their theory as fixed by Keesom and Pearlman on the basis of their specific heat measurements at low temperatures, we have calculated the inelastic scattering cross section for 10 A neutrons as a function of temperature. The calculated values are in reasonably good agreement with the experimental results of the Brookhaven group. For room temperature we have also calculated the variation of the scattering cross section with neutron wavelength, and we find that it is in disagreement with the observed variation. This discrepancy may partly be due to small-angle scattering arising from the finite grain size of carbon.

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