C.M. Edwards

Precise and efficient in situ ortho — para-hydrogen converter

Abstract Chromic oxide (CrO3) supported by silica gel has been found to provide a highly efficient catalytic surface for ortho-para conversion at low temperatures. The material is also less susceptible to poisoning than the commonly used ferric hydroxide gels.

Local ordering in dilute ortho-para-hydrogen mixtures at low temperatures

We report experimental observations of the local orientational ordering of ortho-H2 molecules in dilute solid ortho-para mixtures for low temperatures. The results indicate a smooth, continuous freezing of the rotational degrees of freedom without any evidence for a distinct discontinuity in the temperature dependence of the local order parameters for the 37–600 mK temperature range.

Vacancy-induced nuclear spin-lattice relaxation in solid H2

Abstract We report new results for the study of vacancy motion in solid hydrogen deduced from measurements of the nuclear spin-lattice relaxation of HD impurities. The temperature dependence below 11K is compared with Ebner and Sungs model for vacancy quantum tunneling.

Pressure dependence of tunneling of isotopic impurities in solid parahydrogen

Abstract Pulsed NMR measurements have been performed at low temperatures on solid hydrogen mixtures containing 1.1%HD and 0.5% ortho-H 2 impurities for pressures between 0.1 and 136 bar. The pressure dependence of the transverse spin relaxation time T 2 is analyzed in terms of a model in which the HD particles move through the parahydrogen lattice by quantum tunneling. The Gruneisen constant for the tunneling frequency τ −1 c , Γ τ c =∂ln τ c ∂ lnV , is found to be −31±3, using the theory of Kubo and Tomita to estimate τ c from T 2 .

Simple reliable low power liquid helium level monitor for continuous operation

Abstract We describe a simple reliable liquid helium level monitor using readily available construction materials and integrated circuit components. The total power dissipation delivered to the helium bath is typically 2 mW, allowing for continuous operation without excessive additional boil-off rates.

Comparison of local ordering of orthohydrogen molecules above and below the percolation threshold in solid ortho-para mixtures

Abstract We report measurements of the local orientational ordering in solid ortho-para hydrogen mixtures for ortho concentrations near the site percolation threshold. The results indicate a weak collective ordering for concentrations above percolation which is absent for low concentrations.

Reliable low-temperature NMR cell for pressure studies

A simple Kel-F coil former, preformed and heat shrunk onto metal posts, was used successfully for high-field NMR studies of solids and fluids at low temperatures (0.035 < T < 30 K) and for pressures of 0 < P < 4 MPa. The seals were vacuum tight to superfluid 4He over the full pressure range.

Multistage separator column for ortho-hydrogen and para-deuterium production

Abstract A simple, seven-stage desorption column of activated alumina with independent thermal control of each stage has been used to provide highly enriched samples of ortho-hydrogen (97%) and para-deuterium (92%).

Quantum Diffusion of HD Impurities in Solid Hydrogen

The NMR linewidth of HD impurities in solid para hydrogen has been measured to be less than the rigid lattice value. Motion due to quantum diffusion, similar to the exchange of atoms in dense hcp 3He, of the HD molecules is believed to be the cause of the narrowing of the linewidth. This motion is severely restricted for ortho-H2 because of their quadrupole-quadrupole interactions with ortho neighbors. Pulsed NMR experiments designed to measure the motional effects on NMR spin echos are discussed. Pressure and temperature effects on the HD motion are also examined.

Tunneling of HD Impurities in Solid Hydrogen

Measurements of the transverse nuclear spin relaxation times T2 of HD impurities in solid hydrogen have been carried out at low temperatures to test for quantum tunneling of the impurities in a para-H2 matrix. The motion of the HD impurities is believed to be similar in nature to the exchange of atoms in dense hcp 3He. We compare the observed density and temperature dependence of T2 with that predicted by the quantum tunneling model.