16,858 research outputs found
Conceptual techniques for reducing parasitic current gain of lateral pnp transistors
Two techniques have been conceptually proposed as possible means of reducing parasitic beta in lateral p-n-p transistors. One method uses a degenerate substrate and high concentration P /plus/ guard-ring diffusion, another places the base contact at the center of an annular ring structure
Many-body Rabi oscillations of Rydberg excitation in small mesoscopic samples
We investigate the collective aspects of Rydberg excitation in ultracold
mesoscopic systems. Strong interactions between Rydberg atoms influence the
excitation process and impose correlations between excited atoms. The
manifestations of the collective behavior of Rydberg excitation are the
many-body Rabi oscillations, spatial correlations between atoms as well as the
fluctuations of the number of excited atoms. We study these phenomena in detail
by numerically solving the many-body Schr\"edinger equation.Comment: 8 pages, 5 figure
Nucler star formation in NGC 6240
We have made use of archival HST BVIJH photometry to constrain the nature of
the three discrete sources, A1, A2 and B1, identified in the double nucleus of
NGC 6240. STARBURST99 models have been fitted to the observed colours, under
the assumption, first, that these sources can be treated as star clusters (i.e.
single, instantaneous episodes of star formation), and subsequently as
star-forming regions (i.e. characterised by continuous star formation). For
both scenarios, we estimate ages as young as 4 million years, integrated masses
ranging between 7x10^6 Msun (B1) and 10^9 Msun (A1) and a rate of 1 supernova
per year, which, together with the stellar winds, sustains a galactic wind of
44 Msun/yr. In the case of continuous star formation, a star-formation rate has
been derived for A1 as high as 270 Msun/yr, similar to what is observed for
warm Ultraluminous Infrared Galaxies (ULIRGs) with a double nucleus. The A1
source is characterised by a mass density of about 1200 Msun/pc^3 which
resembles the CO molecular mass density measured in cold ULIRGs and the stellar
density determined in ``elliptical core'' galaxies. This, together with the
recent discovery of a supermassive binary black hole in the double nucleus of
NGC 6240, might indicate that the ongoing merger could shape the galaxy into a
core elliptical.Comment: 14 pages, 5 postscript figures, accepted by A&
Assessment in Scotland
Assessment practice will follow and reinforce the curriculum and promote high quality learning and teaching approaches. Assessment of children's and young people's progress and achievement during their broad general education to the end of S3 will be based on teachers' assessment of their knowledge and understanding, skills, attributes and capabilities, as described in the experiences and outcomes across the curriculum
Structural and dynamic analysis of the Apollo AFT heat shield Final report, Sep. 12, 1964 - Mar. 1, 1966
Structural and dynamic analysis of Apollo aft heat shiel
Solar Flare X-ray Source Motion as a Response to Electron Spectral Hardening
Context: Solar flare hard X-rays (HXRs) are thought to be produced by
nonthermal coronal electrons stopping in the chromosphere, or remaining trapped
in the corona. The collisional thick target model (CTTM) predicts that sources
produced by harder power-law injection spectra should appear further down the
legs or footpoints of a flare loop. Therefore, hardening of the injected
power-law electron spectrum during flare onset should be concurrent with a
descending hard X-ray source.
Aims: To test this implication of the CTTM by comparing its predicted HXR
source locations with those derived from observations of a solar flare which
exhibits a nonthermally-dominated spectrum before the peak in HXRs, known as an
early impulsive event.
Methods: HXR images and spectra of an early impulsive C-class flare were
obtained using the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI).
Images were reconstructed to produce HXR source height evolutions for three
energy bands. Spatially-integrated spectral analysis was performed to isolate
nonthermal emission, and to determine the power-law index of the electron
injection spectrum. The observed height-time evolutions were then fit with
CTTM-based simulated heights for each energy.
Results: A good match between model and observed source heights was reached,
requiring a density model that agreed well with previous studies of flare loop
densities.
Conclusions: The CTTM has been used to produce a descent of model HXR source
heights that compares well with observations of this event. Based on this
interpretation, downward motion of nonthermal sources should indeed occur in
any flare where there is spectral hardening in the electron distribution during
a flare. However, this would often be masked by thermal emission associated
with flare plasma pre-heating.Comment: 8 pages, 5 figure
Fano Lineshapes Revisited: Symmetric Photoionization Peaks from Pure Continuum Excitation
In a photoionization spectrum in which there is no excitation of the discrete
states, but only the underlying continuum, we have observed resonances which
appear as symmetric peaks, not the commonly expected window resonances.
Furthermore, since the excitation to the unperturbed continuum vanishes, the
cross section expected from Fano's configuration interaction theory is
identically zero. This shortcoming is removed by the explicit introduction of
the phase shifted continuum, which demonstrates that the shape of a resonance,
by itself, provides no information about the relative excitation amplitudes to
the discrete state and the continuum.Comment: 4 pages, 3 figure
- …