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382 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Hyperpolarized (129)Xe T (1) in oxygenated and deoxygenated blood

The viability of the new technique of hyperpolarized (129)Xe MRI (HypX-MRI) for imaging organs other than the lungs depends on whether the spin-lattice relaxation time, T(1), of (129)Xe is sufficiently long in the blood. In previous experiments by the authors, the T(1) was found to be strongly dependent upon the oxygenation of the blood, with T(1) increasing from about 3 s in deoxygenated samples to about 10 s in oxygenated samples. Contrarily, Tseng et al. (J. Magn. Reson. 1997; 126: 79-86) reported extremely long T(1) values deduced from an indirect experiment in which hyperpolarized (129)Xe was used to create a 'blood-foam'. They found that oxygenation decreased T(1). Pivotal to their experiment is the continual and rapid exchange of hyperpolarized (129)Xe between the gas phase (within blood-foam bubbles) and the dissolved phase (in the skin of the bubbles); this necessitated a complicated analysis to extract the T(1) of (129)Xe in blood. In the present study, the experimental design minimizes gas exchange after the initial bolus of hyperpolarized (129)Xe has been bubbled through the sample. This study confirms that oxygenation increases the T(1) of (129)Xe in blood, from about 4 s in freshly drawn venous blood, to about 13 s in blood oxygenated to arterial levels, and also shifts the red blood cell resonance to higher frequency. Copyright 2000 John Wiley & Sons, Ltd. Abbreviations used BOLD blood oxygen level dependent NOE nuclear overhouses effect PO(2) oxygen partial pressure RBC red blood cells RF radio frequency SNR signal-to-noise ratio.

NASA Discipline Life Support Systems↗
NASA NTRS2009

Traveling-Wave Maser for 32 GHz

The figure depicts a traveling-wave ruby maser that has been designed (though not yet implemented in hardware) to serve as a low-noise amplifier for reception of weak radio signals in the frequency band of 31.8 to 32.3 GHz. The design offers significant improvements over previous designs of 32-GHz traveling-wave masers. In addition, relative to prior designs of 32-GHz amplifiers based on high-electron-mobility transistors, this design affords higher immunity to radio-frequency interference and lower equivalent input noise temperature. In addition to the basic frequency-band and low-noise requirements, the initial design problem included a requirement for capability of operation in a closed-cycle helium refrigerator at a temperature .4 K and a requirement that the design be mechanically simplified, relative to prior designs, in order to minimize the cost of fabrication and assembly. Previous attempts to build 32- GHz traveling-wave masers involved the use of metallic slow-wave structures comprising coupled transverse electromagnetic (TEM)-mode resonators that were subject to very tight tolerances and, hence, were expensive to fabricate and assemble. Impedance matching for coupling signals into and out of these earlier masers was very difficult. A key feature of the design is a slow-wave structure, the metallic portions of which would be mechanically relatively simple in that, unlike in prior slow-wave structures, there would be no internal metal steps, irises, or posts. The metallic portions of the slow-wave structure would consist only of two rectangular metal waveguide arms. The arms would contain sections filled with the active material (ruby) alternating with evanescent-wave sections. This structure would be transparent in both the signal-frequency band (the aforementioned range of 31.8 to 32.3 GHz) and the pump-frequency band (65.75 to 66.75 GHz), and would impose large slowing factors in both frequency bands. Resonant ferrite isolators would be placed in the evanescent-wave sections to provide reverse loss needed to suppress reverse propagation of power at the signal frequency. This design is expected to afford a large gain-bandwidth product at the signal frequency and efficient coupling of the pump power into the paramagnetic spin resonances of the ruby sections. The more efficiently the pump power could be thus coupled, the more efficiently it could be utilized and the heat load on the refrigerator correspondingly reduced.

Shell, James↗
NASA NTRS1995

Propagation of nuclear burning fronts on accreting neutron stars: X-ray bursts and sub-hertz noise

We identify a new regime of time dependent helium burning for high accretion rate neutron stars and suggest that this burning is the origin of the low-level luminosity variations (on timescales of 10-10(exp 4) s, designated the 'very low-frequency noise'(VLFN) by van der Klis and collaborators) always detected in the brightest accreting X-ray sources. Only two nuclear burning regimes were previously recognized. At accretion rates in excess of the Eddington limit (dot-M approximately greater than (1-3) x 10(exp -8) solar mass/yr), the accreted matter fuses steadily. At very low dot-M, the star's entire surface is rapidly (approximately less than 10 s) burned by a fast propagating convective burning front at regular intervals, giving quasi-periodic Type I X-ray bursts. We show that for the observationally interesting range of 5 x 10(exp -10) solar mass/yr approximately less than dot-M approximately less than 10(exp -8) solar mass/yr, parts of the stellar surface burn slowly. At these accretion rates, a local thermonuclear instability starts a fire which propagates horizontally at v approximately 300 cm/s. The fire propagates around the flammable surface in roughly the same time it takes to accrete enough fuel for the next instability (approximately 10(exp 3)-10(exp 4), so that only a few fires are burning at once, giving rise to large luminosity flares. Nuclear burning is always time dependent for sub-Eddington local accretion rates: a local patch undergoes a recurrent cycle, accumulation fuel for hours until it becomes thermally unstable or is 'ignited' by a nearby burning region. The global pattern of burning and the resulting luminosity are thus very dependent on how fast nuclear fires spread around the star. The nuclear burning luminosity is not uniform over the stellar surface and so may provide a handle on measuring, or constraining, the spin periods of these neutron stars.

Bildsten, Lars↗
NASA NTRS2004

New Dynamic Spin Rig Capabilities Used to Determine Rotating Blade Dynamics

The Dynamic Spin Rig Facility at the NASA Glenn Research Center is used to determine the structural response of rotating engine components without the effects of aerodynamic loading. Recently, this rig's capabilities were enhanced through the replacement of grease-lubricated ball bearings with magnetic bearings. Magnetic bearings offer a number of advantages--the most important here being that they not only fully support the rotor system, but excite it as well. Three magnetic bearings support the rotor and provide five axes of controlled motion: an x- and y-axis translation at each of two radial bearings and a z-axis translation in the vertical or axial direction. Sinusoidal excitation (most commonly used) can be imparted on the rotor through the radial magnetic bearings in either a fixed or rotating frame of reference. This excitation is added directly to the magnetic bearing control output. Since the rotor is fully levitated, large translations and rotations of the rotor system can be achieved. Some of the capabilities of this excitation system were determined and reported. The accelerations obtained at the tip of a titanium flat plate test article versus the swept sine excitation sent to both radial bearings in phase and perpendicular to the plane containing the two blades are shown. Recent tests required the excitation of fundamental bending and torsional blade resonances at rotor speeds up to 10,000 rpm. Successful fixed synchronous rotation of the excitation signal provided the best detectable blade resonant vibrations at excitation frequencies up to 1100 Hz for the particular blades of interest. A noncontacting laser measurement system was used to collect blade-tip motions. From these data, the amplitude and frequency of the motion could be determined as well as the blade damping properties. Damping could be determined using two methods: (1) free decay and (2) curve fitting the vibration amplitude as a function of frequency in and around the resonance of interest and using the half-power method. The free decay of a composite blade vibrating at its first bending resonance while rotating at 3000 rp is shown. This new system is currently being used to support the Efficient Low-Noise Fan project at Glenn. The damping properties of prototype hollow composite blades specially designed to reduce fan noise are currently being determined.

Provenza, Andrew J.↗