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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Ratemeter

An instantaneous reading tachometer in which reoccurring events to be measured in rate, trigger a threestate timing generator in which the first two states are of fixed duration and the third state is of variable duration is described. An electrical decay circuit is set to a reference level by the second state and the third state causes this reference level to decay until the reoccurrence of an event. This triggers a new first state which in turn triggers a sample and hold circuit to hold the decayed level. The decayed level is amplified and provided as an output indicative of the instantaneous rate of occurence of the last two successive events.

Currie, J. R.↗
NASA NTRS1972

Observation of solar particle fluxes over extended solar longitudes.

Detailed particle observations from various Pioneer spacecraft located at different heliolongitudes during the complex solar flare events of Mar. 30 to Apr. 10, 1969, have been utilized to investigate the energy dependence of azimuthal gradients of cosmic ray particles and its effect on the decay of the flare intensity. For an observer located to the east of the centroid of the population, the azimuthal corotation term and the convection term will be additive, resulting in a short decay time constant. An observer located to the west of the centroid of the population will experience a much longer decay time constant, the corotation term partially or completely compensating the loss of particles due to convection. At very low energies, the azimuthal corotation term may even be more than the convection term, thus resulting in a rise in intensity instead of decay during the later part of the event.

Bukata, R. P.↗
NASA NTRS1973

Mach stem structure in exothermic systems.

Incident waves having grossly similar characteristics were obtained by passing a shock wave over a ramp mounted on the divergent part of a convergent-divergent nozzle. The range of triple point parameters in both the naturally occurring detonative Mach stems and those generated in the ramp experiments compared favorably, but some differences were observed in the triple point trajectories and the decay rates of the lead waves. Whereas the lead waves in detonations have been found to decay at rates between those of planar and cylindrical blast waves, the decay observed in the ramp experiments was more like that of planar blast waves. The dominant factor in determining a triple point trajectory was found to be the orientation of the lead wave at the triple point. Comparison of reactive and nonreactive cases indicated a faster rate of decay of the lead wave and a steeper triple point trajectory for the reactive cases than for the nonreactive ones. Also, a sensitivity of the trajectory to initial pressure was observed in the former, although none was present in the nonreactive cases.

Liaugminas, R.↗
NASA NTRS1976

Formation of the 0.511 MeV line in solar flares

The slowing down and annihilation of positrons and the formation of positronium in a solar flare plasma are investigated to determine how the width of the 0.511-MeV line and its strength relative to the three-photon continuum from positronium decay depend on the temperature and density of the medium in which the positron comes to rest. The calculations are limited to the cases of annihilation in a completely ionized plasma, in a partially ionized plasma with an electron/neutron density ratio of 1.0 or 0.1, and in an atomic gas with a very small ion density. Thermally averaged rate coefficients are obtained for the free annihilation of positrons and for positronium formation through radiative recombination in the fully ionized plasma. Positronium formation rates and the resultant energy distributions of the positronium atoms in the partially ionized medium are determined by numerically solving the Fokker-Planck equation in a medium where the ambient free electrons have a Maxwell-Boltzmann distribution of finite temperature but the density of the medium is sufficiently low that positronium atoms decay without further collisions following their formation. A Monte Carlo calculation is performed for the positron energy loss, positronium formation through charge exchange, and positronium breakup in the weakly ionized medium. The energy distributions of decaying positronium atoms and the relative number of triplet to singlet positronium decays are evaluated for ion concentrations and densities characteristic of the solar photosphere.

Crannell, C. J.↗
NASA NTRS1975

Krypton and xenon in Apollo 14 samples - Fission and neutron capture effects in gas-rich samples

Gas-rich Apollo 14 breccias and trench soil are examined for fission xenon from the decay of the extinct isotopes Pu-244 and I-129, and some samples have been found to have an excess fission component which apparently was incorporated after decay elsewhere and was not produced by in situ decay. Two samples have excess Xe-129 resulting from the decay of I-129. The excess is correlated at low temperatures with excess Xe-128 resulting from neutron capture on I-127. This neutron capture effect is accompanied by related low-temperature excesses of Kr-80 and Kr-82 from neutron capture on the bromine isotopes. Surface correlated concentrations of iodine and bromine are calculated from the neutron capture excesses.

Drozd, R.↗
NASA NTRS1983

Steady flow approximations to the helium r-process

A steady flow approximation to the r-process is presented and used for numerical experiments with physical quantities to determine the sensitivity of the process to variations in those quantities. The effect of neutron capture cross sections along the capture path and of recently available improved beta decay rates on the r-process are discussed. The peaks in the observed r-process yield curve near mass numbers 80 and 130 are roughly characterized by a neutron number density of 10 to the 20th per cu/cm; the mean beta decay rates are about 10/s, and the freezing time is comparable to or less than 0.1 s. The peak near mass number 195 is roughly characterized by a neutron number density of 10 to the 21st/cm, the mean beta decay rates are about 100/s, and the freezing time is comparable to or less than 0.01 s. The flow path of the steady state r-process is sensitively dependent upon the neutron capture cross sections in the flow network and on the values of the beta decay rates.

Cameron, A. G. W.↗
NASA NTRS1983

Stiffness reductions during tensile fatigue testing of graphite/epoxy angle-ply laminates

The monitoring of modulus decay during fatigue testing as an indicator of damage accumulation and incipient failure has frequently been postulated. The present study was designed to generate tensile fatigue data under carefully controlled test conditions. A computerized data acquisition system was used to permit the measurement of dynamic modulus without interrupting the fatigue cycling. Two different 8-ply laminate configurations, viz, (+ or - 45)2s and (+ or - 67.5)2s, of a T300/5208 graphite/epoxy composite were tested. The (+ or 45)2s laminate did exhibit some modulus decay, although there was no well-defined correlation with applied stress level or number of cycles. The (+ or - 67.5)2s laminate did not exhibit any measurable modulus decay. Secondary effects observed included a slight recovery of the modulus decay after a test interruption, and a significant viscoelastic (creep) response of the (+ or - 45)2s laminate during fatigue testing.

Odom, E. M.↗
NASA NTRS1985

The stability of dissipative magnetohydrodynamic shear flow in a parallel magnetic field

The linear stability properties of dissipative field-aligned shear flow are described analytically. The results are used to calculate the decay bounds of linearized perturbations occurring in unbounded planes of Couette flow in a parallel magnetic field. It is shown that the perturbations associated with small-amplitude localized disturbances may take the form of rolls along the shear, and exhibit ordinary potential decay, while misaligned perturbations exhibit enhanced decay in the presence of dissipation. A decay criterion is established for MHD shear flow in an accretion disk on the basis of the analytical results.

Lerner, J.↗
NASA NTRS1987

Secondary stream and excitation effects on two-dimensional nozzle plume characteristics

In order to design two-dimensional nozzle/ejector systems for future high performance aircraft, the basic engine exhaust plume velocity and temperature decay as effected by the secondary stream (ejector) and decay augmentation means must be assessed. Included in the assessment of the plume decay characteristics are the effects of nozzle aspect ratio and nozzle/ejector flow conditions. Nozzle/ejector plume decay can be enhanced by suitable excitation of the plume shear layers. Correlation of these factors are developed in a manner similar to those previously developed for conic and dual-flow nozzle plumes.

Vonglahn, Uwe H.↗
NASA NTRS1987

Statistical aspects of solar flares

A survey of the statistical properties of 850 H alpha solar flares during 1975 is presented. Comparison of the results found here with those reported elsewhere for different epochs is accomplished. Distributions of rise time, decay time, and duration are given, as are the mean, mode, median, and 90th percentile values. Proportions by selected groupings are also determined. For flares in general, mean values for rise time, decay time, and duration are 5.2 + or - 0.4 min, and 18.1 + or 1.1 min, respectively. Subflares, accounting for nearly 90 percent of the flares, had mean values lower than those found for flares of H alpha importance greater than 1, and the differences are statistically significant. Likewise, flares of bright and normal relative brightness have mean values of decay time and duration that are significantly longer than those computed for faint flares, and mass-motion related flares are significantly longer than non-mass-motion related flares. Seventy-three percent of the mass-motion related flares are categorized as being a two-ribbon flare and/or being accompanied by a high-speed dark filament. Slow rise time flares (rise time greater than 5 min) have a mean value for duration that is significantly longer than that computed for fast rise time flares, and long-lived duration flares (duration greater than 18 min) have a mean value for rise time that is significantly longer than that computed for short-lived duration flares, suggesting a positive linear relationship between rise time and duration for flares. Monthly occurrence rates for flares in general and by group are found to be linearly related in a positive sense to monthly sunspot number. Statistical testing reveals the association between sunspot number and numbers of flares to be significant at the 95 percent level of confidence, and the t statistic for slope is significant at greater than 99 percent level of confidence. Dependent upon the specific fit, between 58 percent and 94 percent of the variation can be accounted for with the linear fits. A statistically significant Northern Hemisphere flare excess (P less than 1 percent) was found, as was a Western Hemisphere excess (P approx 3 percent). Subflares were more prolific within 45 deg of central meridian (P less than 1 percent), while flares of H alpha importance or = 1 were more prolific near the limbs greater than 45 deg from central meridian; P approx 2 percent). Two-ribbon flares were more frequent within 45 deg of central meridian (P less than 1 percent). Slow rise time flares occurred more frequently in the western hemisphere (P approx 2 percent), as did short-lived duration flares (P approx 9 percent), but fast rise time flares were not preferentially distributed (in terms of east-west or limb-disk). Long-lived duration flares occurred more often within 45 deg 0 central meridian (P approx 7 percent). Mean durations for subflares and flares of H alpha importance or + 1, found within 45 deg of central meridian, are 14 percent and 70 percent, respectively, longer than those found for flares closer to the limb. As compared to flares occurring near cycle maximum, the flares of 1975 (near solar minimum) have mean values of rise time, decay time, and duration that are significantly shorter. A flare near solar maximum, on average, is about 1.6 times longer than one occurring near solar minimum.

Wilson, Robert M.↗
NASA NTRS1987

Determination of lifetime and surface recombination velocity of p-n junction solar cells and diodes by observing transients

The unified view of transient methods for the determination of recombination lifetime tau and back surface recombination velocity S presented here for silicon solar cells and diodes attempts to define limitations of existing methods and to evolve improvements. The presence of sizable junction capacitance for silicon devices under forward voltage invalidates the use of conventional open-circuit voltage decay (OCVD) and reverse recovery. This led Green (1983) to his method of compensated open-circuit voltage decay, in which the addition of an external resistor shunting the solar cell partially corrects for the presence of the junction capacitance. Setting this resistance to zero produces an electrical short-circuit current-decay method, which has the advantage of enabling determination of both tau and S. In an alternate approach, one may insert the functional dependence of the junction capacitance on forward voltage. This new method, denoted by the acronym OCVDCAP, enables the determination of tau with apparently greater accuracy than that obtained by previous methods utilizing voltage transients. But OCVDCAP has in common with the previous methods that it determines tau only and has practical utility only for determining tau of long-base devices. This means that it is useful only for thick base regions. In principle, however, it has an advantage over short-circuit current decay: it requires only pressure contacts, not ohmic contacts, and therefore may be used to determine tau after key processing steps in manufacturing.

Lindholm, Fredrik A.↗
NASA NTRS1987

Nucleogenic noble gas components in the Cape York iron meteorite

The paper reports data on neutron capture products of the secondary cosmic ray component, the inferred proton and neutron fluences, and the identification of double beta decay of Se-82 in heavily shielded samples of the Cape York iron meteorite. One purpose of this study is to develop a new chronometer for cosmic ray exposure, based on the nuclides I-129 (16 My half-life) and Xe-129 from low energy cosmic ray reactions on Te. The abundance ratio of these two nuclides permits the determination of an (effective) exposure age of 93 + or - 16 My, which represents the first exposure age datum of Cape York. The very small concentrations of spallogenic Ar-38 = 6.5 x 10 to the -10th cu cm STP/g in the metal and troilite (per g Fe) document the heavily shielded locations of the sample. An excess of Xe-129 in the troilite is shown to be entirely due to the decay of cosmic-ray-produced I-129. On the other hand, an inclusion in the troilite reveals the presence of Xe-129 from extinct I-129 and documents its about 4.5 Gy formation age. Mono-isotopic excess of Kr-82 is identified as due to beta-beta-decay of Se-82 with an inferred half-life of 1.0 x 10 to the 20th y. This represents the first beta-beta-decay product observed in a meteorite.

Murty, S. V. S.↗
NASA NTRS1987

Secondary stream and excitation effects on two-dimensional nozzle plume characteristics

In order to design two-dimensional nozzle/ejector systems for future high performance aircraft, the basic engine exhaust plume velocity and temperature decay as effected by the secondary stream (ejector) and decay augmentation means must be assessed. Included in the assessment of the plume decay characteristics are the effects of nozzle aspect ratio and nozzle/ejector flow conditions. Nozzle/ejector plume decay can be enhanced by suitable excitation of the plume shear layers. Correlations of these factors are developed in a manner similar to those previously developed for conic and dual-flow nozzle plumes.

Von Glahn, Uwe H.↗
NASA NTRS1988

Critical fluid light scattering

The objective is to measure the decay rates of critical density fluctuations in a simple fluid (xenon) very near its liquid-vapor critical point using laser light scattering and photon correlation spectroscopy. Such experiments were severely limited on Earth by the presence of gravity which causes large density gradients in the sample when the compressibility diverges approaching the critical point. The goal is to measure fluctuation decay rates at least two decades closer to the critical point than is possible on earth, with a resolution of 3 microK. This will require loading the sample to 0.1 percent of the critical density and taking data as close as 100 microK to the critical temperature. The minimum mission time of 100 hours will allow a complete range of temperature points to be covered, limited by the thermal response of the sample. Other technical problems have to be addressed such as multiple scattering and the effect of wetting layers. The experiment entails measurement of the scattering intensity fluctuation decay rate at two angles for each temperature and simultaneously recording the scattering intensities and sample turbidity (from the transmission). The analyzed intensity and turbidity data gives the correlation length at each temperature and locates the critical temperature. The fluctuation decay rate data from these measurements will provide a severe test of the generalized hydrodynamic theories of transport coefficients in the critical regions. When compared to equivalent data from binary liquid critical mixtures they will test the universality of critical dynamics.

Gammon, Robert W.↗
NASA NTRS1989

Atmospheric drag model calibrations for spacecraft lifetime prediction

Although solar activity prediction uncertainty normally dominates decay prediction error budget for near-Earth spacecraft, the effect of drag force modeling errors for given levels of solar activity needs to be considered. Two atmospheric density models, the modified Harris-Priester model and the Jacchia-Roberts model, to reproduce the decay histories of the Solar Mesosphere Explorer (SME) and Solar Maximum Mission (SMM) spacecraft in the 490- to 540-kilometer altitude range were analyzed. Historical solar activity data were used in the input to the density computations. For each spacecraft and atmospheric model, a drag scaling adjustment factor was determined for a high-solar-activity year, such that the observed annual decay in the mean semimajor axis was reproduced by an averaged variation-of-parameters (VOP) orbit propagation. The SME (SMM) calibration was performed using calendar year 1983 (1982). The resulting calibration factors differ by 20 to 40 percent from the predictions of the prelaunch ballistic coefficients. The orbit propagations for each spacecraft were extended to the middle of 1988 using the calibrated drag models. For the Jaccia-Roberts density model, the observed decay in the mean semimajor axis of SME (SMM) over the 4.5-year (5.5-year) predictive period was reproduced to within 1.5 (4.4) percent. The corresponding figure for the Harris-Priester model was 8.6 (20.6) percent. Detailed results and conclusions regarding the importance of accurate drag force modeling for lifetime predictions are presented.

Binebrink, A. L.↗
NASA NTRS1990

Alternative catalysts for low-temperature CO-oxidation

MnO sub x, Ag/MnO sub x, Cu/MnO sub x, Pt/MnO sub x, Ru/MnO sub x, Au/CeO sub x, and Au/Fe2O3 were synthesized and tested for CO oxidation activity in low concentrations of stoichiometric CO and O2 at 30 to 75 C. Catalytic activity was measured for periods as long as 18000 minutes. At 75 deg Au/MnO sub x is most active sustaining nearly 100 percent CO conversion for 10000 minutes. It also retains high activity at 50 and 30 C with negligible decay in activity. A direct comparison between an unpretreated 10 percent Au/MnO sub x catalyst and an optimized 19.5 percent Pt/SnO sub 2 (pretreated) catalyst shows that the Au/MnO sub x catalyst exhibits much higher catalytic activity and far superior decay characteristics. Other catalysts including Au/CeO sub x and Au/Fe2O3 also perform well. The Cu/MnO sub x exhibits a high initial activity which decays rapidly. After the decay period the activity remains very stable making Cu/MnO sub x a potential candidate for long-term applications such as CO2 lasers in space.

Gardner, Steven D.↗
NASA NTRS1989

Pulse shape discrimination for background rejection in germanium gamma-ray detectors

A pulse-shape discrimination (PSD) technique is developed to reject the beta-decay background resulting from activation of Ge gamma-ray detectors by cosmic-ray secondaries. These beta decays are a major source of background at 0.2-2 MeV energies in well shielded Ge detector systems. The technique exploits the difference between the detected current pulse shapes of single- and multiple-site energy depositions within the detector: beta decays are primarily single-site events, while photons at these energies typically Compton scatter before being photoelectrically absorbed to produce multiple-site events. Depending upon the amount of background due to sources other than beta decay, PSD can more than double the detector sensitivity.

Feffer, P. T.↗
NASA NTRS1991

Gamma-ray lines from type I supernovae

Analytic calculations are made of the time-dependent fluxes, shapes, median energies, and effective widths of the gamma-ray lines expected from the decay of nucleosynthetic Ni-56, Co-56, Fe-56, and Co-57 in current models of type I SNe. The attenuation and distortion of the line shapes due to Compton scattering and photoelectric absorption in the SN ejecta, line broadening from the expansion, and further distortion by the lookback effect from the differential light-travel time across the SN and by special relativistic effects are all taken into account. It is shown that several gamma-ray lines from such SNe should be measurable by detectors on the Gamma Ray Observatory and by the high-resolution spectrometer on the proposed Nuclear Astrophysics Explorer. The most intense and most detectable lines are the 0.847 and 1.238 MeV lines from Co-56 decay. Five other Co-56 decay lines, as well as the 0.158 MeV line from Ni-56 decay, should also be measurable. The models which can be tested by such observations are pointed out.

Chan, Kai Wing↗