594 research outputs found
Pressure dependence of the sound velocity in a 2D lattice of Hertz-Mindlin balls: a mean field description
We study the dependence on the external pressure of the velocities
of long wavelength sound waves in a confined 2D h.c.p. lattice of 3D
elastic frictional balls interacting via one-sided Hertz-Mindlin contact
forces, whose diameters exhibit mild dispersion. The presence of an underlying
long range order enables us to build an effective medium description which
incorporates the radial fluctuations of the contact forces acting on a single
site. Due to the non linearity of Hertz elasticity, self-consistency results in
a highly non-linear differential equation for the "equation of state" linking
the effective stiffness of the array with the applied pressure, from which
sound velocities are then obtained. The results are in excellent agreement with
existing experimental results and simulations in the high and intermediate
pressure regimes. It emerges from the analysis that the departure of
from the ideal Hertz behavior must be attributed primarily to the
fluctuations of the stress field, rather than to the pressure dependence of the
number of contacts
Why Effective Medium Theory Fails in Granular Materials
Experimentally it is known that the bulk modulus, K, and shear modulus, \mu,
of a granular assembly of elastic spheres increase with pressure, p, faster
than the p^1/3 law predicted by effective medium theory (EMT) based on
Hertz-Mindlin contact forces. To understand the origin of these discrepancies,
we perform numerical simulations of granular aggregates under compression. We
show that EMT can describe the moduli pressure dependence if one includes the
increasing number of grain-grain contacts with p. Most important, the affine
assumption (which underlies EMT), is found to be valid for K(p) but breakdown
seriously for \mu(p). This explains why the experimental and numerical values
of \mu(p) are much smaller than the EMT predictions.Comment: 4 pages, 5 figures, http://polymer.bu.edu/~hmaks
Diagnostic accuracy of DXA compared to conventional spine radiographs for the detection of vertebral fractures in children
Objectives
In children, radiography is performed to diagnose vertebral fractures and dual energy x-ray absorptiometry (DXA) to
assess bone density. In adults, DXA assesses both. We aimed to establish whether
DXA can replace spine radiographs in assessment of paediatric vertebral fractures.
Methods
Prospectively, lateral spine radiographs and lateral spine DXA of 250 children
performed on the same day were independently scored by three radiologists using
the simplified algorithm based qualitative technique and blinded to results of the
other modality. Consensus radiograph read and second read of 100 random images
were performed. Diagnostic accuracy, inter/intraobserver and intermodality
agreements, patient/carer experience and radiation dose were assessed.
Results
Average sensitivity and specificity (95% confidence interval) in diagnosing one or
more vertebral fractures requiring treatment was 70% (58%-82%) and 97% (94%-
100%) respectively for DXA and 74% (55%-93%) and 96% (95%-98%) for
radiographs. Fleiss’ kappa for interobserver and average kappa for intraobserver
reliability were 0.371 and 0.631 respectively for DXA and 0.418 and 0.621 for
radiographs. Average effective dose was 41.9µSv for DXA and 232.7µSv for
radiographs. Image quality was similar.
Conclusion
Given comparable image quality and non-inferior diagnostic accuracy, lateral spine
DXA should replace conventional radiographs for assessment of vertebral fractures
in children
HS 1700+6416: the first high redshift non lensed NAL-QSO showing variable high velocity outflows
We present a detailed analysis of the X-ray emission of HS 1700+6416, a high
redshift (z=2.7348), luminous quasar, classified as a Narrow Absorption Line
(NAL) quasar on the basis of its SDSS spectrum. The source has been observed 9
times by Chandra and once by XMM from 2000 to 2007. Long term variability is
clearly detected, between the observations, in the 2-10 keV flux varying by a
factor of three (~3-9x10^-14 erg s^-1 cm^-2) and in the amount of neutral
absorption (Nh < 10^22 cm^-2 in 2000 and 2002 and Nh=4.4+-1.2x10^22 cm^-2 in
2007). Most interestingly, one broad absorption feature is clearly detected at
10.3+-0.7 keV (rest frame) in the 2000 Chandra observation, while two similar
features, at 8.9+-0.4 and at 12.5+-0.7 keV, are visible when the 8 contiguous
Chandra observations of 2007 are stacked together. In the XMM observation of
2002, strongly affected by background flares, there is a hint for a similar
feature at 8.0+-0.3 keV. We interpreted these features as absorption lines from
a high velocity, highly ionized (i.e. Fe XXV, FeXXVI) outflowing gas. In this
scenario, the outflow velocities inferred are in the range v=0.12-0.59c. To
reproduce the observed features, the gas must have high column density
(Nh>3x10^23 cm^-2), high ionization parameter (log(xi)>3.3 erg cm s^-1) and a
large range of velocities (Delta V~10^4 km s^-1). This Absorption Line QSO is
the fourth high-z quasar displaying X-ray signatures of variable, high velocity
outflows, and among these, is the only one non-lensed. A rough estimate of the
minimum kinetic energy carried by the wind of up to 18% L(bol), based on a
biconical geometry of the wind, implies that the amount of energy injected in
the outflow environment is large enough to produce effective mechanical
feedback.Comment: 10 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Application of NICE guideline NG12 to the initial assessment of patients with lower gastrointestinal symptoms:not FIT for purpose?
Re-emerging syphilis: a detrended correspondence analysis of the behaviour of HIV positive and negative gay men
BACKGROUND: Recent syphilis outbreaks in the UK have raised serious concerns about the sexual health of the population. Moreover, syphilis appears more likely to facilitate HIV transmission than any other sexually transmitted infection (STI). METHODS: The sexual and other risk behaviour of a sample of HIV positive and negative gay men with and without syphilis was subjected to a detrended correspondence analysis (DCA). RESULTS: A DCA plot was used to illustrate similarity of individuals in terms of their behaviours, regardless of their infection status. The majority of those with syphilis (78%; 18/23) fell into a high-risk group with more partners, and use of anonymous sex venues and drugs during sex. However, 16% of uninfected controls (8/49) and 62% of HIV positive individuals without syphilis (8/13) also fell into this high-risk group. CONCLUSIONS: Using a statistical technique that is novel for this type of investigation, we demonstrate behavioural overlaps between syphilis-infected individuals in an ongoing UK outbreak and uninfected HIV positive and negative controls. Given the high-risk behaviour of a significant proportion of uninfected individuals, ongoing transmission of syphilis and HIV in this population seems likely
Response to novel objects and foraging tasks by common marmoset (Callithrix Jacchus) female Pairs
Many studies have shown that environmental enrichment can significantly improve the psychological well-being of captive primates, increasing the occurrence of explorative behavior and thus reducing boredom. The response of primates to enrichment devices may be affected by many factors such as species, sex, age, personality and social context. Environmental enrichment is particularly important for social primates living in unnatural social groupings (i.e. same-sex pairs or singly housed animals), who have very few, or no, benefits from the presence of social companions in addition to all the problems related to captivity (e.g. increased inactivity). This study analyses the effects of enrichment devices (i.e. novel objects and foraging tasks) on the behavior of common marmoset (Callithrix jacchus) female pairs, a species that usually lives in family groups. It aims to determine which aspects of an enrichment device are more likely to elicit explorative behaviors, and how aggressive and stress-related behaviors are affected by its presence. Overall, the marmosets explored foraging tasks significantly longer than novel objects. The type of object, which varied in size, shape and aural responsiveness (i.e. they made a noise when the monkey touched them), did not affect the response of the monkeys, but they explored objects that were placed higher in the enclosure more than those placed lower down.Younger monkeys were more attracted to the enrichment devices than the older ones. Finally, stress-related behavior (i.e. scratching) significantly decreased when the monkeys were presented with the objects; aggressive behavior as unaffected. This study supports the importance of environmental enrichment for captive primates and shows that in marmosets its effectiveness strongly depends upon the height of the device in the enclosure and the presence of hidden food. The findings can be explained ifone considers the foraging behavior of wild common marmosets. Broader applications for the research findings are suggested in relation to enrichment
Infrared Excess Sources: Compton Thick QSOs, low luminosity Seyferts or starbursts?
We explore the nature of Infrared Excess sources (IRX), which are proposed as
candidates for luminous L_X(2-10keV)>1e43erg/s Compton Thick (N_H>2e24cm^{-2}z\approx1z\approx2$ IRX population are not Compton
Thick QSOs but low luminosity [L_X(2-10keV)<1e43erg/s], possibly Compton Thin,
AGN or dusty starbursts. It is shown that the decomposition of the AGN and
starburst contribution to the mid-IR is essential for interpreting the nature
of this population, as star-formation may dominate this wavelength regime.Comment: Accepted by MNRA
Reproductive failure, possible maternal infanticide, and cannibalism in wild moustached tamarins, Saguinus mystax
Maternal infanticide in wild non-human primates has only been reported twice. In this paper, we report a possible new case of infanticide and cannibalism within a series of four successive reproductive failures in wild moustached tamarins, Saguinus mystax. Necropsy and genetic analyses of the corpses enabled us to rule out any pathology, and to determine paternity. The mother was seen biting and then eating the head of its own infant during a period when another female was pregnant and gave birth just 1 month later. Before that, the perpetrator had given birth to twins three times successfully when four to five adult and subadult males were present in the group. Although we do not know for certain that the infant was alive when the mother started biting it, our field observations preceding the event suggest it probably was. The possible infanticide case and the two cases of births and early death of the infants occurred while only two to three adult males were present in the group. This could be the second case of maternal infanticide reported in the genus Saguinus and the similar circumstances suggest a common pattern. We discuss these events in the light of the different functional explanations of infanticide and conclude that parental manipulation was the most likely: the mother could have terminated the investment in offspring that had low chances of survival in a group with low availability of helpers
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