Microscopic model for the neutron dynamic structure factor of solid methane in phase II

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Vol. 620; no. 2; pp. 382 - 390
• Main Authors: Shin, Yunchang, Mike Snow, W., Liu, C.Y., Lavelle, C.M., Baxter, David V.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 11.08.2010
• Subjects: Accelerators; Cold neutron; Cross sections; Dynamics; Mathematical models; Methane; Neutron scattering function; Structure factor; Translations; Vibration; Methane; Neutron scattering function; Cold neutron
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/j.nima.2010.03.109

We have constructed a microscopic model for the neutron dynamic structure factor S ( Q , ω ) of solid methane in phase II. We expect this model to apply for neutron energies below 1 eV at pressures near 1 bar and temperatures below 20 K where methane possesses both free rotation and hindered rotation modes of the tetrahedral molecules in the unit cell. The model treats the motions of molecular translations, intra-molecular vibrations and the free and hindered rotations of methane molecule as independent. Total scattering cross-sections calculated from the model agree with the cross-section measurements for incident neutron energies of 0.5 meV–1 eV. The effective density of states is extracted from the model. We also present the quantitative calculation of the separate contributions of the two different rotational modes to the inelastic cross-section for different methane temperatures in phase II.