Inelastic scattering and spectral measurements of advanced cold moderator media

Conrad, Harald; Kuhs, Werner F.; Nünighoff, K.; Pohl, Ch.; Prager, M.; Schweika, W.

doi:10.1016/j.physb.2004.03.173

Cited by 12 publications

(14 citation statements)

References 3 publications

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“…Three kinds of local vibrations with 1.8, 2.8 and 10 meV occur in the THF hydrate. 30 The acoustic phonons having energies equal to or higher than the energy of the local vibrations are scattered strongly by these vibrations. Because of the strong scattering of the phonons by the guest molecules, the motion of the acoustic phonons over the crystal becomes diffusive.…”

Section: Discussionmentioning

confidence: 99%

Thermal conductivity of tetrahydrofuran hydrate

Кривчиков

Manzheliı̆

Korolyuk

et al. 2005

Phys. Chem. Chem. Phys.

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The thermal conductivity of tetrahydrofuran hydrate has been measured in the temperature region 2-220 K by the steady-state potentiometric method. The temperature dependence of the thermal conductivity exhibits behavior typical of amorphous substances. It is shown that above 100 K the mean free path of the phonons is considerably smaller than the lattice parameter and is no longer dependent on temperature.

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Section: Discussionmentioning

confidence: 99%

Thermal conductivity of tetrahydrofuran hydrate

Кривчиков

Manzheliı̆

Korolyuk

et al. 2005

Phys. Chem. Chem. Phys.

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“…7 shows a linear increase of the linewidths with temperature above T B 50 K. Below this temperature the shifting and broadening tunneling lines have not yet merged into the elastic line and thus cause an apparently increased linewidth. In a similar way methane clathrate shows clear features of the quantum character of rotation up to T B 70 K. 22 Thus, the spectra can only be interpreted classically at sample temperatures larger than the activation energies. Therefore the reorienting methyl group seems not to feel the corrugation of the potential surface.…”

Section: Classical Reorientationmentioning

confidence: 96%

Adsorption sites and rotational tunneling of methyl groups in cubic I methyl fluoride water clathrate

Prager

Baumert

Press

et al. 2005

Phys. Chem. Chem. Phys.

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Neutron spectroscopy in the meV and meV regime and quasielastic scattering is applied to characterize the dynamics of methyl groups of methyl fluoride guest molecules in cubic I CH 3 F-water clathrate. Only above T B 60 K quasielastic spectra are unaffected by quantum effects. They are well described by two Lorentzians representing the CH 3 F species in the small and large cages of the structure. The intensities show that both cages are completely filled. The linear broadenings with temperature follow the model of rotational diffusion. Two clearly separated tunneling bands were observed at T ¼ 4.2 K and are also assigned to the two types of water cages. Disorder of the environment (H-bonds) is reflected in the shape of the bands. For the less hindered species housing the large cages the tunneling band can be quantitatively converted into a potential distribution function within the model of single particle rotation. Transitions to excited rotational states show the dominance of a sixfold potential term V 6 ¼ 13 meV modified by a weak threefold term distributed around a characteristic value V 3 ¼ 0.9 meV. The potential distribution of V 3 influences the barrier for classical reorientation only weakly in agreement with the results from quasielastic data. Adsorption sites with the guest molecules oriented towards a hydrogen bond along one of twelve local twofold axes of the cage are proposed. Such sites are consistent with the sixfold rotational potential and earlier results from methyl iodide clathrate. Rotation-translation coupling as an alternative dynamical process is excluded.

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“…Otherwise there is also the viable option to couple the O 2 -hydrate to a premoderator made of an advanced cold moderator medium, such as solid methane, methane clathrate or mesitylene, all offering better thermalization at low temperature than liquid hydrogen or deuterium [51].…”

Section: Discussionmentioning

confidence: 99%

Cooling neutrons using non-dispersive magnetic excitations

Zimmer

2014

Preprint

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A new method is proposed for cooling neutrons by inelastic magnetic scattering in weakly absorbing, cold paramagnetic systems. Kinetic neutron energy is removed in constant decrements determined by the Zeeman energy of paramagnetic atoms or ions in an external magnetic field, or by zero-field level splittings in magnetic molecules. Analytical solutions of the stationary neutron transport equation are given using inelastic neutron scattering cross sections derived in an appendix. They neglect any inelastic process except the paramagnetic scattering and hence still underestimate very-cold neutron densities. Molecular oxygen with its triplet ground state appears particularly promising, notably as a host in fully deuterated O 2 -clathrate hydrate, or more exotically, in dry O 2 -4 He van der Waals clusters. At a neutron temperature about 6 K, for which neutron conversion to ultra-cold neutrons by single-phonon emission in pure superfluid 4 He works best, conversion rates due to paramagnetic scattering in the clathrate are found to be a factor 9 larger. While in conversion the neutron imparts only a single energy quantum to the medium, the multi-step paramagnetic cooling cascade leads to further strong enhancements of very-cold neutron densities, e.g., by a factor 14 (57) for an initial neutron temperature of 30 K (100 K), for the moderator held at about 1.3 K. Due to a favorable Bragg cutoff of the O 2 -clathrate the cascade-cooling can take effect in a moderator with linear extensions smaller than a meter. The paramagnetic cooling mechanism may offer benefits in novel intense sources of very cold neutrons and for enhancing production of ultra-cold neutrons.

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Inelastic scattering and spectral measurements of advanced cold moderator media

Cited by 12 publications

References 3 publications

Thermal conductivity of tetrahydrofuran hydrate

Thermal conductivity of tetrahydrofuran hydrate

Adsorption sites and rotational tunneling of methyl groups in cubic I methyl fluoride water clathrate

Cooling neutrons using non-dispersive magnetic excitations

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