Theory of cooling by flow through narrow pores

We consider the possibility of adding a stage to a dilution refrigerator to provide additional cooling by ``filtering out'' hot atoms. Three methods are considered: 1) Effusion, where holes having diameters larger than a mean-free path allow atoms to pass through easily; 2) Particle waveguide-like motion using very narrow channels that greatly restrict the quantum states of the atoms in a channel. 3) Wall-limited diffusion through channels, in which the wall scattering is disordered so that local density equilibrium is established in a channel. We assume that channel dimension are smaller than the mean-free path for atom-atom interactions. The particle waveguide and the wall-limited diffusion methods using channels on order of the de Broglie wavelength give cooling. Recent advances in nano-filters give this method some hope of being practical.Comment: 10 pages, 3 figures. Corrected typos and made some minor wording change

Nuclear magnetism of pure helium-3 and helium-3--helium-4 mixtures in aerogel

We have carried out a series of NMR experiments on \sp3He systems filling the pores of silica aerogel with 95% porosity. The systems studied included \sp3He-\sp4He mixtures in a wide range of concentrations as well as pure \sp3He. All experiments were conducted in an 8 Tesla field and for temperatures between 5 and 320 mK. This resulted in strong spin polarization at the lowest temperatures (up to approximately 80%) for the localized layer observed for a pure \sp3He sample. For pure \sp3He, the magnetic behavior is dominated by the localized spins, which are found to constitute 6% of the sample. The coupled system of localized plus liquid spins displays rapid transverse magnetic relaxation with an anomalous temperature dependence. The longitudinal magnetization is observed to have, at the lowest temperatures, an anomalously long relaxation constant which cannot be readily associated with either the liquid or the localized spins. For mixtures with intermediate concentrations our measurements provide a preliminary indication of the phase diagram of mixtures filling this porous material

Brute-Force Hyperpolarization for NMR and MRI

Hyperpolarization (HP) of nuclear spins is critical for ultrasensitive nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). We demonstrate an approach for >1500-fold enhancement of key small-molecule metabolites: 1-¹³C-pyruvic acid, 1-¹³C-sodium lactate, and 1-¹³C-acetic acid. The ¹³C solution NMR signal of pyruvic acid was enhanced 1600-fold at <i>B</i> = 1 T and 40 °C by pre-polarizing at 14 T and ∼2.3 K. This “brute-force” approach uses only field and temperature to generate HP. The noted 1 T observation field is appropriate for benchtop NMR and near the typical 1.5 T of MRI, whereas high-field observation scales enhancement as 1/<i>B</i>. Our brute-force process ejects the frozen, solid sample from the low-<i>T</i>, high-<i>B</i> polarizer, passing it through low field (<i>B</i> < 100 G) to facilitate “thermal mixing”. That equilibrates ¹H and ¹³C in hundreds of milliseconds, providing ¹³C HP from ¹H Boltzmann polarization attained at high <i>B</i>/<i>T</i>. The ejected sample arrives at a room-temperature, permanent magnet array, where rapid dissolution with 40 °C water yields HP solute. Transfer to a 1 T NMR system yields ¹³C signals with enhancements at 80% of ideal for noted polarizing conditions. High-resolution NMR of the same product at 9.4 T had consistent enhancement plus resolution of ¹³C shifts and <i>J</i>-couplings for pyruvic acid and its hydrate. Comparable HP was achieved with frozen aqueous lactate, plus notable enhancement of acetic acid, demonstrating broader applicability for small-molecule NMR and metabolic MRI. Brute-force avoids co-solvated free-radicals and microwaves that are essential to competing methods. Here, unadulterated samples obviate concerns about downstream purity and also exhibit slow solid-state spin relaxation, favorable for transporting HP samples