18,804 research outputs found
Clinical and economic evaluation of laparoscopic surgery compared with medical management for gastro-oesophageal reflux disease : 5-year follow-up of multicentre randomised trial (the REFLUX trial)
Peer reviewedPublisher PD
"Making Safety Happen" Through Probabilistic Risk Assessment at NASA
NASA is using Probabilistic Risk Assessment (PRA) as one of the tools in its Safety & Mission Assurance (S&MA) tool belt to identify and quantify risks associated with human spaceflight. This paper discusses some of the challenges and benefits associated with developing and using PRA for NASA human space programs. Some programs have entered operation prior to developing a PRA, while some have implemented PRA from the start of the program. It has been observed that the earlier a design change is made in the concept or design phase, the less impact it has on cost and schedule. Not finding risks until the operation phase yields much costlier design changes and major delays, which can result in discussions of just accepting the risk. Risk contributors identified by PRA are not just associated with hardware failures. They include but are not limited to crew fatality due to medical causes, the environment the vehicle and crew are exposed to, the software being used, and the reliability of the crew performing required actions. Some programs have entered operation prior to developing a PRA, and while PRA can still provide a benefit for operations and future design trades, the benefit of implementing PRA from the start of the program provides the added benefit of informing design and reducing risk early in program development. Currently, NASAs International Space Station (ISS) program is in its 20th year of on-orbit operations around the Earth and has several new programs in the design phase preparing to enter the operation phase all of which have active (or living) PRAs. These programs incorporate PRA as part of their Risk-Informed, Decision-Making (RIDM) process. For new NASA human spaceflight programs discussion begins with mission concept, establishing requirements, forming the PRA team, and continues through the design cycles into the operational phase. Several examples of PRA related applications and observed lessons are included
Evolution of speckle during spinodal decomposition
Time-dependent properties of the speckled intensity patterns created by
scattering coherent radiation from materials undergoing spinodal decomposition
are investigated by numerical integration of the Cahn-Hilliard-Cook equation.
For binary systems which obey a local conservation law, the characteristic
domain size is known to grow in time as with n=1/3,
where B is a constant. The intensities of individual speckles are found to be
nonstationary, persistent time series. The two-time intensity covariance at
wave vector can be collapsed onto a scaling function , where and . Both analytically and numerically, the covariance
is found to depend on only through in the
small- limit and in the large-
limit, consistent with a simple theory of moving interfaces that applies to any
universality class described by a scalar order parameter. The speckle-intensity
covariance is numerically demonstrated to be equal to the square of the
two-time structure factor of the scattering material, for which an analytic
scaling function is obtained for large In addition, the two-time,
two-point order-parameter correlation function is found to scale as
, even for quite large
distances . The asymptotic power-law exponent for the autocorrelation
function is found to be , violating an upper bound
conjectured by Fisher and Huse.Comment: RevTex: 11 pages + 12 figures, submitted to PR
The Phenomenology of a Top Quark Seesaw Model
The top quark seesaw mechanism offers a method for constructing a composite
Higgs field without the usual difficulties that accompany traditional
technicolor or topcolor theories. The focus of this article is to study the
phenomenology of the new physics required by this mechanism. After establishing
a set of criteria for a plausible top quark seesaw theory, we develop two
models, the first of which has a heavy weak singlet fermion with hypercharge
4/3 while the second has, in addition, a heavy weak singlet hypercharge -2/3
fermion. At low energies, these theories contain one or two Higgs doublets
respectively. We then derive the low energy effective Higgs potential in detail
for the two-doublet theory as well as study the likely experimental signatures
for both theories. A strong constraint on the one-doublet model is the measured
value of the rho parameter which permits the new heavy fermion to have a mass
of about 5-7 TeV, when the Higgs has a mass greater than 300 GeV. In the
two-doublet model, mixing of the new heavy Y=-2/3 fermion and the b quark
affects the prediction for R_b. In order to agree with the current limits on
R_b, the mass of this fermion should be at least 12 TeV. The mass of the heavy
Y=4/3 fermion in the two-doublet model is not as sharply constrained by
experiments and can be as light as 2.5 TeV.Comment: 33 pages, 12 figures, uses harvmac and picte
Antiarrhythmic and proarrhythmic effects of subcutaneous nerve stimulation in ambulatory dogs
Background
High output subcutaneous nerve stimulation (ScNS) remodels the stellate ganglia and suppresses cardiac arrhythmia.
Objective
To test the hypothesis that long duration low output ScNS causes cardiac nerve sprouting, increases plasma norepinephrine concentration and the durations of paroxysmal atrial tachycardia (PAT) in ambulatory dogs.
Methods
We prospectively randomized 22 dogs (11 males and 11 females) into 5 different output groups for 2 months of ScNS: 0 mA (sham) (N=6), 0.25 mA (N=4), 1.5 mA (N=4), 2.5 mA (N=4) and 3.5 mA (N=4).
Results
As compared with baseline, the changes of the durations of PAT episodes per 48 hours were significantly different among different groups (sham, -5.0±9.5 s; 0.25 mA 95.5±71.0 s; 1.5 mA, -99.3±39.6 s; 2.5 mA, -155.3±87.8 s and 3.5 mA, -76.3±44.8 s, p<0.001). The 3.5 mA group had greater reduction of sinus heart rate than the sham group (-29.8±15.0 bpm vs -14.5±3.0 bpm, p=0.038). Immunohistochemical studies showed that the 0.25 mA group had a significantly increased while 2.5 mA and 3.5 mA stimulation had a significantly reduced growth-associated protein 43 nerve densities in both atria and ventricles. The plasma Norepinephrine concentrations in 0.25 mA group was 5063.0±4366.0 pg/ml, which was significantly higher than other groups of dogs (739.3±946.3, p=0.009). There were no significant differences in the effects of simulation between males and females.
Conclusions
In ambulatory dogs, low output ScNS causes cardiac nerve sprouting, increases plasma norepinephrine concentration and the duration of PAT episodes while high output ScNS is antiarrhythmic
Spacings of Quarkonium Levels with the Same Principal Quantum Number
The spacings between bound-state levels of the Schr\"odinger equation with
the same principal quantum number but orbital angular momenta
differing by unity are found to be nearly equal for a wide range of power
potentials , with . Semiclassical approximations are in accord with this behavior. The
result is applied to estimates of masses for quarkonium levels which have not
yet been observed, including the 2P states and the 1D
states.Comment: 20 pages, latex, 3 uuencoded figures submitted separately (process
using psfig.sty
Application of Confocal Laser Scanning Microscopy to Cytocompatibility Testing of Potential Orthopaedic Materials in Immortalised Osteoblast-Like Cell Lines
Confocal laser scanning microscopy (CLSM) was used in conjunction with in vitro cell culture to investigate cellular interactions with orthopaedic biomaterials. Transfected rat and human osteoblasts were seeded on two potential isoelastic hip prosthesis materials, carbon fibre reinforced polyetheretherketone (PEEK) and epoxy. Titanium 318 alloy was employed as a control. Determination of the material surface contour, an important factor influencing cellular adhesion, proliferation and function, was performed using the industry standard Talysurf® and compared to analogous results obtained using the CLSM. The latter technique consistently gave higher values of material roughness but offers the advantage that it can be used to correlate roughness with cell distribution on the same samples, whereas Talysurf® measurement of roughness requires clean rigid samples. Image analysis and processing, performed on cells after attachment and culture on the materials for 48 hours, provided cell morphology data. Cells cultured on titanium were larger, with a higher percentage of cytoplasm, than those grown on either of the other materials. The macroscopic surface of epoxy resulted in smaller cells with altered morphology, which orientated themselves along carbon fibres. In conclusion, we believe CLSM offers great potential for investigating the cellular interactions of biomaterials involving minimal sample preparation, non-invasive optical sectioning of samples and minimal opportunity for generation of cellular deformation and sample preparation artefacts
Molecular weight effects on chain pull-out fracture of reinforced polymeric interfaces
Using Brownian dynamics, we simulate the fracture of polymer interfaces
reinforced by diblock connector chains. We find that for short chains the
interface fracture toughness depends linearly on the degree of polymerization
of the connector chains, while for longer chains the dependence becomes
. Based on the geometry of initial chain configuration, we propose a
scaling argument that accounts for both short and long chain limits and
crossover between them.Comment: 5 pages, 3 figure
Strength functions, entropies and duality in weakly to strongly interacting fermionic systems
We revisit statistical wavefunction properties of finite systems of
interacting fermions in the light of strength functions and their participation
ratio and information entropy. For weakly interacting fermions in a mean-field
with random two-body interactions of increasing strength , the
strength functions are well known to change, in the regime where level
fluctuations follow Wigner's surmise, from Breit-Wigner to Gaussian form. We
propose an ansatz for the function describing this transition which we use to
investigate the participation ratio and the information entropy during this crossover, thereby extending the known behavior valid in the
Gaussian domain into much of the Breit-Wigner domain. Our method also allows us
to derive the scaling law for the duality point , where
, and in both the weak () and strong
mixing () basis coincide as ,
where is the number of fermions. As an application, the ansatz function for
strength functions is used in describing the Breit-Wigner to Gaussian
transition seen in neutral atoms CeI to SmI with valence electrons changing
from 4 to 8
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