12,008 research outputs found
Factors modifying the response of large animals to low-intensity radiation exposure
In assessing the biological response to space radiation, two of the most important modifying factors are dose protraction and dose distribution to the body. Studies are reported in which sheep and swine were used to compare the hematology and lethality response resulting from radiation exposure encountered in a variety of forms, including acute (high dose-rate), chronic (low dose-rate), combinations of acute and chronic, and whether received as a continuous or as fractionated exposure. While sheep and swine are basically similar in response to acute radiation, their sensitivity to chronic irradiation is markedly different. Sheep remain relatively sensitive as the radiation exposure is protracted while swine are more resistant and capable of surviving extremely large doses of chronic irradiation. This response to chronic irradiation correlated well with changes in radiosensitivity and recovery following an acute, sublethal exposure
A new solid-state logarithmic radiometer
Combination of temperature-compensated logarithmic amplifiers and p-i-n photodiodes operating in zero-bias mode provides lightweight radiometer for detecting spectral intensities encompassing more than three decades over a range of at least 300 to 800 nanometers at low power levels
A practical experience with independent verification and validation
One approach to reducing software cost and increasing reliability is the use of an independent verification and validation (IV & V) methodology. The Software Engineering Laboratory (SEL) applied the IV & V methodology to two medium-size flight dynamics software development projects. Then, to measure the effectiveness of the IV & V approach, the SEL compared these two projects with two similar past projects, using measures like productivity, reliability, and maintain ablilty. Results indicated that the use of the IV & V methodology did not help the overall process nor improve the product in these cases
Criteria for software modularization
A central issue in programming practice involves determining the appropriate size and information content of a software module. This study attempted to determine the effectiveness of two widely used criteria for software modularization, strength and size, in reducing fault rate and development cost. Data from 453 FORTRAN modules developed by professional programmers were analyzed. The results indicated that module strength is a good criterion with respect to fault rate, whereas arbitrary module size limitations inhibit programmer productivity. This analysis is a first step toward defining empirically based standards for software modularization
The Anisotropy in the Cosmic Microwave Background At Degree Angular Scales
We detect anisotropy in the cosmic microwave background (CMB) at degree
angular scales and confirm a previous detection reported by Wollack et al.
(1993). The root-mean-squared amplitude of the fluctuations is K. This may be expressed as the square root of the angular power spectrum
in a band of multipoles between . We find K. The measured spectral
index of the fluctuations is consistent with zero, the value expected for the
CMB. The spectral index corresponding to Galactic free-free emission, the most
likely foreground contaminant, is rejected at approximately .
The analysis is based on three independent data sets. The first, taken in
1993, spans the 26 - 36 GHz frequency range with three frequency bands; the
second was taken with the same radiometer as the first but during an
independent observing campaign in 1994; and the third, also take in 1994, spans
the 36-46 GHz range in three bands. For each telescope position and radiometer
channel, the drifts in the instrument offset are K/day over a period
of one month. The dependence of the inferred anisotropy on the calibration and
data editing is addressed.Comment: 16 pages, 2 figures. Saskatoon 1993/1994 combined analysi
Control of rotordynamic instability in a typical gas turbine's power system
The effect of rotor internal friction on the system's stability was studied when operated above the first critical speed. This internal friction is commonly caused by sliding press fits or sliding splines. Under conditions of high speed and low bearing damping, these systems will occassionally whirl at a frequency less than the shaft's rotational speed. This subsynchronous precession is a self excited phenomenon and stress reversals are created. This phenomenon was observed during engine testing. The reduction of spline friction and/or the inclusion of squeeze film damping have controlled the instability. Case history and the detail design of the squeeze film dampers is discussed
Numerical modeling of dynamic powder compaction using the Kawakita equation of state
Dynamic powder compaction is analyzed using the assumption that the powder behaves, while it is being compacted, like a hydrodynamic fluid in which deviatoric stress and heat conduction effects can be ignored throughout the process. This enables techniques of computational fluid dynamics such the equilibrium flux method to be used as a modeling tool. The equation of state of the powder under compression is assumed to be a modified version of the Kawakita loading curve. Computer simulations using this model are performed for conditions matching as closely as possible with those from experiments by Page and Killen [Powder Metall. 30, 233 (1987)]. The numerical and experimental results are compared and a surprising degree of qualitative agreement is observed
Magneto-elastic oscillations of neutron stars: exploring different magnetic field configurations
We study magneto-elastic oscillations of highly magnetized neutron stars
(magnetars) which have been proposed as an explanation for the quasi-periodic
oscillations (QPOs) appearing in the decaying tail of the giant flares of soft
gamma-ray repeaters (SGRs). We extend previous studies by investigating various
magnetic field configurations, computing the Alfv\'en spectrum in each case and
performing magneto-elastic simulations for a selected number of models. By
identifying the observed frequencies of 28 Hz (SGR 1900+14) and 30 Hz (SGR
1806-20) with the fundamental Alfv\'en QPOs, we estimate the required surface
magnetic field strength. For the magnetic field configurations investigated
(dipole-like poloidal, mixed toroidal-poloidal with a dipole-like poloidal
component and a toroidal field confined to the region of field lines closing
inside the star, and for poloidal fields with an additional quadrupole-like
component) the estimated dipole spin-down magnetic fields are between 8x10^14 G
and 4x10^15 G, in broad agreement with spin-down estimates for the SGR sources
producing giant flares. A number of these models exhibit a rich Alfv\'en
continuum revealing new turning points which can produce QPOs. This allows one
to explain most of the observed QPO frequencies as associated with
magneto-elastic QPOs. In particular, we construct a possible configuration with
two turning points in the spectrum which can explain all observed QPOs of SGR
1900+14. Finally, we find that magnetic field configurations which are entirely
confined in the crust (if the core is assumed to be a type I superconductor)
are not favoured, due to difficulties in explaining the lowest observed QPO
frequencies (f<30 Hz).Comment: 21 pages, 16 figures, 6 tables, matched to version accepted by MNRAS
with extended comparison/discussion to previous wor
Measuring software technology
Results are reported from a series of investigations into the effectiveness of various methods and tools used in a software production environment. The basis for the analysis is a project data base, built through extensive data collection and process instrumentation. The project profiles become an organizational memory, serving as a reference point for an active program of measurement and experimentation on software technology
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