Neutron Scattering Studies of Solid Methane

Abstract

<p> The total scattering cross section of solid methane has been measured for temperatures in the range from 4.2°K to 95°K. An anomalous increase in the total scattering cross section was observed at temperatures below 5°K. The effect is ascribed to conversion between nuclear spin species. Such spin conversion had been indicated by specific heat and NMR measurements; the present results give an independent confirmation of this. Calculations of the equilibrium value of <I(I + 1)> at 4.2°K give values lower than those obtained in the NMR work. The specimens used in the present work come to spin equilibrium in times much less than 20 minutes. This is less than the value of 90 minutes for pure methane seen by NMR; however, very small amounts of oxygen impurity in the specimen are known greatly to decrease this time.</p> <p> An abrupt change in the total scattering cross section at the 20°K phase transition is explained by a change in the rotational freedom of the molecules to a more strongly hindered state below the phase transition. In the region above the transition, the Krieger-Nelkin approximation for the total cross section was fitted to the observed temperature dependence of the cross section, treating the effective mass as a free parameter.</p>