3 research outputs found
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
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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-<sup>13</sup>C-pyruvic acid, 1-<sup>13</sup>C-sodium
lactate, and 1-<sup>13</sup>C-acetic acid. The <sup>13</sup>C 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 <sup>1</sup>H and <sup>13</sup>C in hundreds of
milliseconds, providing <sup>13</sup>C HP from <sup>1</sup>H 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 <sup>13</sup>C 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 <sup>13</sup>C 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