13,395 research outputs found
Advanced engine study program
A design and analysis study was conducted to provide advanced engine descriptions and parametric data for space transfer vehicles. The study was based on an advanced oxygen/hydrogen engine in the 7,500 to 50,000 lbf thrust range. Emphasis was placed on defining requirements for high-performance engines capable of achieving reliable and versatile operation in a space environment. Four variations on the expander cycle were compared, and the advantages and disadvantages of each were assessed. Parametric weight, envelope, and performance data were generated over a range of 7,500 to 50,000 lb thrust and a wide range of chamber pressure and nozzle expansion ratio
Sco X-1: The Evolution and Nature of the Twin Compact Radio Lobes
The radio components associated with the LMXB Sco X-1 have been monitored
with extensive VLBI imaging at 1.7 and 5.0 GHz over four years, including a
56-hour continuous VLBI observation in 1999 June. We often detected one strong
and one weak compact radio component, moving in opposite directions from the
radio core. We suggest that the moving components are radio lobes generated by
the disruption of energy flow in a twin-beam from the binary system. The
average lifetime of a lobe-pair, the space motion of the lobes and the measured
energy flow in the beam are discussed in arXiv:astro-ph/0104325. The lobe has a
flux density that is variable over a time-scale of one hour, a measured minimum
size of 1 mas (2.8 au), and is extended perpendicular to its motion. This short
electron radiative lifetime may be caused by synchrotron losses if the lobe
magnetic field is 300 G, or by adiabatic expansion of the electrons as soon as
they are produced at the working surface. The lobes also show periods of slow
expansion and a steepening radio spectrum, perhaps related to the
characteristics of the beam energy flow. The radio morphology for Sco X-1 is
more simple than for most other Galactic jet sources. The lobes of Sco X-1 are
similar to hot-spots found in many extragalactic double sources. Scaling the
phenomena observed in Sco X-1 to extragalactic sources implies hot-spot
variability time-scales of 10^4 yr and hot-spot lifetimes of 10^5 yr. The
recurrent formation of lobes in Sco X-1 probably does not occur for
extragalactic radio sources.Comment: 22 pages of text + 16 figures. ApJ, in pres
Will the Scottish Cancer Target for the year 2000 be met? The use of cancer registration and death records to predict future cancer incidence and mortality in Scotland.
Cancer mortality data reflect disease incidence and the effectiveness of treatment. Incidence data, however, reflect the burden of disease in the population and indicate the need for prevention measures, diagnostic services and cancer treatment facilities. Monitoring of targets mandates that both be considered. The Scottish Cancer Target, established in 1991, proposed that a reduction of 15% in mortality from cancer in the under-65s should be achieved between 1986 and 2000. Each year in Scotland approximately 8300 persons under 65 are diagnosed with cancer and 4500 die from the disease. The most common malignancies, in terms of both incident cases and deaths, in the under-65s, are lung and large bowel cancer in males, and breast, large bowel and lung cancer in females. A decrease of 6% in the number of cancer cases diagnosed in males under 65 is predicted between 1986 and 2000, whereas the number of cases in females in the year 2000 is expected to remain at the 1986 level. In contrast, substantial reductions in mortality are expected for both sexes: 17% and 25% in males and females respectively. Demographic changes will influence the numbers of cancer cases and deaths in the Scottish population in the year 2000. However, long-term trends in the major risk factors, such as smoking, are likely to be the most important determinants of the future cancer burden
Composition, silicate anion structure and morphology of calcium silicate hydrates (C-S-H) synthesized by silica-lime reaction and by the controlled hydration of tricalcium silicate (C3S)
The main product of Portland cement hydration is C-S-H. Despite constituting more than half of the volume of hydrated pastes and having an important role in strength development, very little is known about the factors that determine its morphology. To investigate the relationship between the chemical composition, silicate anion structure and morphology of C-S-H, samples were synthesized via silica-lime reactions and by the hydration of C3S under controlled lime concentrations and with/ without accelerators. The silicate anion structure of the samples was studied by 29Si MAS NMR and the morphology and chemical composition by TEM and SEM. All samples prepared via silica-lime reactions with bulk Ca/Si up to 1.5 were foil-like. The hydration of C3S at fixed lime concentration yielded foil-like C-S-H for [CaO]22mmol/L. A relationship between the silicate anion structure and the morphology of C-S-H was found for the samples fabricated with accelerators
Nonlinear acoustic and microwave absorption in disordered semiconductors
Nonlinear hopping absorption of ultrasound and electromagnetic waves in
amorphous and doped semiconductors is considered. It is shown that even at low
amplitudes of the electric (or acoustic) field the nonlinear corrections to the
relaxational absorption appear anomalously large. The physical reason for such
behavior is that the nonlinear contribution is dominated by a small group of
close impurity pairs having one electron per pair. Since the group is small, it
is strongly influenced by the field. An external magnetic field strongly
influences the absorption by changing the overlap between the pair components'
wave functions. It is important that the influence is substantially different
for the linear and nonlinear contributions. This property provides an
additional tool to extract nonlinear effects.Comment: correction : misspelled name in references correcte
Light scattering study of low-energy vibrational excitations in the metallic glass NiZr using electronic Raman scattering
The Raman response of the metallic glass NiZr is measured as a
function of polarization and temperature and analyzed theoretically.
Unexpectedly, the intensity in the range up to 300\wn increases upon cooling,
which is counterintuitive when the response originates from vibrations alone as
in insulators. The increase finds a natural explanation if the conduction
electrons are assumed to scatter on localized vibrations with a scattering
probability proportional to the Debye-Waller factor. None of our assumptions is
material specific, and the results are expected to be relevant for disordered
systems in general.Comment: 5 pages, 3 figure
Low-Temperature Dephasing in Disordered Conductors: the Effect of ``1/f'' Fluctuations
Electronic quantum effects in disordered conductors are controlled by the
dephasing rate of conduction electrons. This rate is expected to vanish with
the temperature. We consider the very intriguing recently reported apparent
saturation of this dephasing rate in several systems at very low temperatures.
We show that the ``standard model'' of a conductor with static defects can {\em
not} have such an effect. However, allowing some dynamics of the defects may
produce it.Comment: 6page
A Case for Renewed Activity in the Giant Radio Galaxy J0116-473
We present ATCA radio observations of the giant radio galaxy J0116-473 at 12
and 22 cm wavelengths in total intensity and polarization. The images clearly
reveal a bright inner-double structure within more extended edge-brightened
lobe emission. The lack of hotspots at the ends of the outer lobes, the strong
core and the inner-double structure with its edge-brightened morphology lead us
to suggest that this giant radio galaxy is undergoing a renewed nuclear
activity: J0116-473 appears to be a striking example of a radio galaxy where a
young double source is evolving within older lobe material. We also report the
detection of a Mpc-long linear feature which is oriented perpendicular to the
radio axis and has a high fractional polarization.Comment: 25 pages, 10 figures, appeared in 2002 ApJ, 565, 25
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Model risk: illuminating the black box
This paper presents latest thinking from the Institute and Faculty of Actuaries’ Model Risk Working Party and follows on from their Phase I work, Model Risk: Daring to Open the Black Box. This is a more practical paper and presents the contributors’ experiences of model risk gained from a wide range of financial and non-financial organisations with suggestions for good practice and proven methods to reduce model risk. After a recap of the Phase I work, examples of model risk communication are given covering communication: to the Board; to the regulator; and to external stakeholders. We present a practical framework for model risk management and quantification with examples of the key actors, processes and cultural challenge. Lessons learned are then presented from other industries that make extensive use of models and include the weather forecasting, software and aerospace industries. Finally, a series of case studies in practical model risk management and mitigation are presented from the contributors’ own experiences covering primarily financial services
Transport properties and point contact spectra of Ni_xNb_{1-x} metallic glasses
Bulk resistivity and point contact spectra of Ni_xNb_{1-x} metallic glasses
have been investigated as functions of temperature (0.3-300K) and magnetic
field (0-12T). Metallic glasses in this family undergo a superconducting phase
transition determined by the Nb concentration. When superconductivity was
suppressed by a strong magnetic field, both the bulk sample R(T) and the point
contact differential resistance curves of Ni_xNb_{1-x} showed logarithmic
behavior at low energies, which is explained by a strong electron - "two level
system" coupling. We studied the temperature, magnetic field and contact
resistance dependence of Ni_{44}Nb_{56} point-contact spectra in the
superconducting state and found telegraph-like fluctuations superimposed on
superconducting characteristics. These R(V) characteristics are extremely
sensitive detectors for slow relaxing "two level system" motion.Comment: 4 pages, 5 figure
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