Measurement of total cross-section and spin conversion in solid methane

Development of Cold Neutron Scattering Kernels for Advanced Moderators

“…8) and applied to the evaluation of neutron cross sections for that system [9,10]. Those formulations were then used to analyze neutron scattering and transmission experiments [11,12,13]. In our study we oriented our effort to the development of a simple model which should preserve the main dynamical features (as well as a proper description of spin correlation effects) of the system and therefore be able to make reliable predictions in terms of neutron fluxes emerging from such cold moderator material.…”

Measurements of the neutron brightness from a phase II solid methane moderator at the LENS neutron source

“…Transitions between levels of different symmetry, which require a change in the spin state of the protons, cannot be induced by phonon interactions alone but must be mediated by spin-dependent interactions such as the weak dipole-dipole interaction among protons. As a consequence, the spin system shows very slow relaxation to thermal equilibrium after a sudden change of the lattice temperature [7]. Although the high temperature approximation is appropriate for methane at room temperature, the ratio doesn't change even after the melting point of solid methane (∼ 90.6K).…”

“…is a good approximation for long wavelength neutrons [7]. Since each rotational state is related to a distinct total nuclear spin I, σ tot is proportional to their occupation number g i in Eq.…”

“…In all cases, the conversion rate of two rotational systems are different. For temperature above ∼4K, the occupation number of the hindered molecules are always near their high temperature limit and consequently the conversion behavior is dominated by the free rotation mode [7]. Neutrons were generated in the Be target from 9 Be (p, xn) nuclear reaction with protons.…”

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

“…Transitions between levels of different symmetry cannot be induced by phonon interaction alone, but are mediated by the weak dipole-dipole interaction among protons. As a consequence, the spin system shows slow thermal relaxation after a sudden change of the lattice temperature[17].III. NEUTRON CROSS SECTION MODEL OF SOLID METHANE A methane molecule in a crystal shows three different degrees of freedom in the solid phase.…”