20,239 research outputs found
Synchronized voltage contrast display analysis system
An apparatus and method for comparing internal voltage potentials of first and second operating electronic components such as large scale integrated circuits (LSI's) in which voltage differentials are visually identified via an appropriate display means are described. More particularly, in a first embodiment of the invention a first and second scanning electron microscope (SEM) are configured to scan a first and second operating electronic component respectively. The scan pattern of the second SEM is synchronized to that of the first SEM so that both simultaneously scan corresponding portions of the two operating electronic components. Video signals from each SEM corresponding to secondary electron signals generated as a result of a primary electron beam intersecting each operating electronic component in accordance with a predetermined scan pattern are provided to a video mixer and color encoder
Kertesz on Fat Graphs?
The identification of phase transition points, beta_c, with the percolation
thresholds of suitably defined clusters of spins has proved immensely fruitful
in many areas of statistical mechanics. Some time ago Kertesz suggested that
such percolation thresholds for models defined in field might also have
measurable physical consequences for regions of the phase diagram below beta_c,
giving rise to a ``Kertesz line'' running between beta_c and the bond
percolation threshold, beta_p, in the M, beta plane.
Although no thermodynamic singularities were associated with this line it
could still be divined by looking for a change in the behaviour of high-field
series for quantities such as the free energy or magnetisation. Adler and
Stauffer did precisely this with some pre-existing series for the regular
square lattice and simple cubic lattice Ising models and did, indeed, find
evidence for such a change in high-field series around beta_p. Since there is a
general dearth of high-field series there has been no other work along these
lines.
In this paper we use the solution of the Ising model in field on planar
random graphs by Boulatov and Kazakov to carry out a similar exercise for the
Ising model on random graphs (i.e. coupled to 2D quantum gravity). We generate
a high-field series for the Ising model on random graphs and examine
its behaviour for evidence of a Kertesz line
Examples of mathematical modeling tales from the crypt
Mathematical modeling is being increasingly recognized within the biomedical sciences as an important tool that can aid the understanding of biological systems. The heavily regulated cell renewal cycle in the colonic crypt provides a good example of how modeling can be used to find out key features of the system kinetics, and help to explain both the breakdown of homeostasis and the initiation of tumorigenesis. We use the cell population model by Johnston et al. (2007) Proc. Natl. Acad. Sci. USA 104, 4008-4013, to illustrate the power of mathematical modeling by considering two key questions about the cell population dynamics in the colonic crypt. We ask: how can a model describe both homeostasis and unregulated growth in tumorigenesis; and to which parameters in the system is the model most sensitive? In order to address these questions, we discuss what type of modeling approach is most appropriate in the crypt. We use the model to argue why tumorigenesis is observed to occur in stages with long lag phases between periods of rapid growth, and we identify the key parameters
On the proportion of cancer stem cells in a tumour
It is now generally accepted that cancers contain a sub-population, the cancer stem cells (CSCs), which initiate and drive a tumour’s growth. At least until recently it has been widely assumed that only a small proportion of the cells in a tumour are CSCs. Here we use a mathematical model, supported by experimental evidence, to show that such an assumption is unwarranted. We show that CSCs may comprise any possible proportion of the tumour, and that the higher the proportion the more aggressive the tumour is likely to be
Theory of high-energy emission from the pulsar/Be-star system PSR 125963 I: radiation mechanisms and interaction geometry
We study the physical processes of the PSR B1259-63 system containing a 47 ms
pulsar orbiting around a Be star in a highly eccentric orbit. Motivated by the
results of a multiwavelength campaign during the January 1994 periastron
passage of PSR B1259-63, we discuss several issues regarding the mechanism of
high-energy emission. Unpulsed power law emission from the this system was
detected near periastron in the energy range 1-200 keV. We find that the
observed high energy emission from the PSR B1259-63 system is not compatible
with accretion or propeller-powered emission. Shock-powered high-energy
emission produced by the pulsar/outflow interaction is consistent with all high
energy observations. By studying the evolution of the pulsar cavity we
constrain the magnitude and geometry of the mass outflow outflow of the Be
star. The pulsar/outflow interaction is most likely mediated by a collisionless
shock at the internal boundary of the pulsar cavity. The system shows all the
characteristics of a {\it binary plerion} being {\it diffuse} and {\it compact}
near apastron and periastron, respectively. The PSR B1259-63 cavity is subject
to different radiative regimes depending on whether synchrotron or inverse
Compton (IC) cooling dominates the radiation of electron/positron pairs
advected away from the inner boundary of the pulsar cavity. The highly
non-thermal nature of the observed X-ray/gamma-ray emission near periastron
establishes the existence of an efficient particle acceleration mechanism
within a timescale shown to be less than s. A synchrotron/IC
model of emission of e\pm-pairs accelerated at the inner shock front of the
pulsar cavity and adiabatically expanding in the MHD flow provides an excellent
explanation of the observed time variableX-ray flux and spectrum from the PSRComment: 68 pages, accepted for publication in the Astrophys. J. on Aug. 26,
199
Cognitive representations of disability behaviours in people with mobility limitations : consistency with theoretical constructs
Disability is conceptualised as behaviour by psychological theory and as a result of bodily impairment by medical models. However, how people with disabilities conceptualise those disabilities is unclear. The purpose of this study was to examine disability representations in people with mobility disabilities. Thirteen people with mobility disabilities completed personal repertory grids (using the method of triads) applied to activities used to measure disabilities. Ten judges with expertise in health psychology then examined the correspondence between the elicited disability constructs and psychological and medical models of disability. Participants with mobility disabilities generated 73 personal constructs ofdisability. These constructs were judged consistent with the content of two psychological models, namely the theory of planned behaviour and social cognitive theory and with the main medical model of disability, the International Classification of Functioning Disability and Health.Individuals with activity limitations conceptualise activities in a manner that is compatible with both psychological and medical models. This ensures adequate communication in contexts where the medical model is relevant, e.g. clinical contexts, as well as in everyday conversation about activities and behaviours. Finally, integrated models of disability may be of value for theory driven interdisciplinary approaches to disability and rehabilitation
Marijuana and Youth
This paper contains the first estimates of the price sensitivity of the prevalence of youth marijuana use. Survey data on marijuana use by high school seniors from the Monitoring the Future Project are combined with data on marijuana prices and potency from the Drug Enforcement Administration Office of Intelligence or Intelligence Division. Our estimates of the price elasticity of annual marijuana participation range from 0.06 to 0.47, while those for thirty day participation range from 0.002 to 0.69. These estimates clearly imply that changes in the real, quality adjusted price of marijuana contributed significantly to the trends in youth marijuana use between 1982 and 1998, particularly during the contraction in use from 1982 to 1992. Similarly, changes in youth perceptions of the harms associated with regular marijuana use had a substantial impact on both the contraction in use during the 1982 though 1992 period and the subsequent expansion in use after 1992. These findings underscore the usefulness of considering price in addition to more traditional determinants in any analysis of marijuana consumption decisions made by youths.
Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches
A semi-classical Monte Carlo model for studying three-dimensional carrier
dynamics in photoconductive switches is presented. The model was used to
simulate the process of photoexcitation in GaAs-based photoconductive antennas
illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed
the power and frequency bandwidth of THz radiation emitted from these devices
as a function of bias voltage, pump pulse duration and pump pulse location. We
show that the mechanisms limiting the THz power emitted from photoconductive
switches fall into two regimes: when illuminated with short duration (<40 fs)
laser pulses the energy distribution of the Gaussian pulses constrains the
emitted power, while for long (>40 fs) pulses, screening is the primary
power-limiting mechanism. A discussion of the dynamics of bias field screening
in the gap region is presented. The emitted terahertz power was found to be
enhanced when the exciting laser pulse was in close proximity to the anode of
the photoconductive emitter, in agreement with experimental results. We show
that this enhancement arises from the electric field distribution within the
emitter combined with a difference in the mobilities of electrons and holes.Comment: 7 pages, 7 figure
Chandra Observation of the Globular Cluster NGC 6440 and the Nature of Cluster X-ray Luminosity Functions
As part of our campaign to determine the nature of the various source
populations of the low-luminosity globular cluster X-ray sources, we have
obtained a Chandra X-ray Observatory ACIS-S3 image of the globular cluster NGC
6440. We detect 24 sources to a limiting luminosity of ~2 times 10^31 erg/s
(0.5-2.5keV) inside the cluster's half-mass radius, all of which lie within ~2
core radii of the cluster center. We also find excess emission in and around
the core which could be due to unresolved point sources. Based upon X-ray
luminosities and colors, we conclude that there are 4-5 likely quiescent
low-mass X-ray binaries and that most of the other sources are cataclysmic
variables. We compare these results to Chandra results from other globular
clusters and find the X-ray luminosity functions differ among the clusters.Comment: 9 pages, 4 figures, accepted by ApJ, minor changes, added table of
clusters' physical parameter
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