686 research outputs found
Multiscale Kinetic Monte-Carlo for Simulating Epitaxial Growth
We present a fast Monte-Carlo algorithm for simulating epitaxial surface
growth, based on the continuous-time Monte-Carlo algorithm of Bortz, Kalos and
Lebowitz. When simulating realistic growth regimes, much computational time is
consumed by the relatively fast dynamics of the adatoms. Continuum and
continuum-discrete hybrid methods have been developed to approach this issue;
however in many situations, the density of adatoms is too low to efficiently
and accurately simulate as a continuum. To solve the problem of fast adatom
dynamics, we allow adatoms to take larger steps, effectively reducing the
number of transitions required. We achieve nearly a factor of ten speed up, for
growth at moderate temperatures and large D/F.Comment: 7 pages, 6 figures; revised text, accepted by PR
The Heavy Photon Search Test Detector
The Heavy Photon Search (HPS), an experiment to search for a hidden sector photon in fixed target electroproduction, is preparing for installation at the Thomas Jefferson National Accelerator Facility (JLab) in the Fall of 2014. As the first stage of this project, the HPS Test Run apparatus was constructed and operated in 2012 to demonstrate the experiment׳s technical feasibility and to confirm that the trigger rates and occupancies are as expected. This paper describes the HPS Test Run apparatus and readout electronics and its performance. In this setting, a heavy photon can be identified as a narrow peak in the e+ e− invariant mass spectrum above the trident background or as a narrow invariant mass peak with a decay vertex displaced from the production target, so charged particle tracking and vertexing are needed for its detection. In the HPS Test Run, charged particles are measured with a compact forward silicon microstrip tracker inside a dipole magnet. Electromagnetic showers are detected in a PbW04 crystal calorimeter situated behind the magnet, and are used to trigger the experiment and identify electrons and positrons. Both detectors are placed close to the beam line and split top-bottom. This arrangement provides sensitivity to low-mass heavy photons, allows clear passage of the unscattered beam, and avoids the spray of degraded electrons coming from the target. The discrimination between prompt and displaced e+ e− pairs requires the first layer of silicon sensors be placed only 10 cm downstream of the target. The expected signal is small, and the trident background huge, so the experiment requires very large statistics. Accordingly, the HPS Test Run utilizes high-rate readout and data acquisition electronics and a fast trigger to exploit the essentially 100% duty cycle of the CEBAF accelerator at JLab
Predicting erythropoietin resistance in hemodialysis patients with type 2 diabetes
<p>Background: Resistance to ESAs (erythropoietin stimulating agents) is highly prevalent in hemodialysis patients with diabetes and associated with an increased mortality. The aim of this study was to identify predictors for ESA resistance and to develop a prediction model for the risk stratification in these patients.</p>
<p>Methods: A post-hoc analysis was conducted of the 4D study, including 1015 patients with type 2 diabetes undergoing hemodialysis. Determinants of ESA resistance were identified by univariate logistic regression analyses. Subsequently, multivariate models were performed with stepwise inclusion of significant predictors from clinical parameters, routine laboratory and specific biomarkers.</p>
<p>Results: In the model restricted to clinical parameters, male sex, shorter dialysis vintage, lower BMI, history of CHF, use of ACE-inhibitors and a higher heart rate were identified as independent predictors of ESA resistance. In regard to routine laboratory markers, lower albumin, lower iron saturation, higher creatinine and higher potassium levels were independently associated with ESA resistance. With respect to specific biomarkers, higher ADMA and CRP levels as well as lower Osteocalcin levels were predictors of ESA resistance.</p>
<p>Conclusions: Easily obtainable clinical parameters and routine laboratory parameters can predict ESA resistance in diabetic hemodialysis patients with good discrimination. Specific biomarkers did not meaningfully further improve the risk prediction of ESA resistance. Routinely assessed data can be used in clinical practice to stratify patients according to the risk of ESA resistance, which may help to assign appropriate treatment strategies.</p>
Characterising white matter in Huntington's disease
Background:
Investigating early white matter (WM) change in Huntington’s Disease (HD) can improve our understanding of the way in which disease spreads from the striatum.
Objectives:
Here, we provide a detailed characterisation of pathology-related WM change in HD. We first examined WM microstructure using diffusion-weighted imaging, then investigated both underlying biological properties of WM and products of WM damage including iron, myelin plus neurofilament light (NfL), a biofluid marker of axonal degeneration – in parallel with the mutant huntingtin protein (mHTT).
Methods:
We examined WM change in HD gene-carriers from the HD-CSF cohort, baseline visit. We used standard diffusion MRI to measure metrics including fractional anisotropy (FA), a marker of WM integrity, and diffusivity; a novel diffusion model (NODDI) to measure axonal density and organisation; T1 and T2 weighted structural MRI images to derive proxy iron content and myelin-contrast measures; and biofluid concentrations of NfL (in CSF and plasma) and mHTT (in CSF).
Results:
HD gene-carriers displayed reduced FA and increased diffusivity compared to controls, both of which were also associated with disease progression, CSF and mHTT levels. HD gene-carriers also displayed proxy measures of reduced myelin-contrast and iron in the striatum.
Conclusion:
Collectively, these findings present a more complete characterisation of HD-related microstructural brain changes. Correlation between reduced FA, increased axonal orientation and biofluid markers suggest that axonal breakdown is associated with increased WM degeneration, while higher quantitative T2 signal and lower myelin-contrast may indicate a process of demyelination limited to the striatum
A new highly segmented start counter for the CLAS detector
The design, construction and performance of a highly segmented Start Counter are described. The Start Counter is an integral part of the trigger used in photon beam running with CLAS in Hall B at the Thomas Jefferson National Accelerator Facility (TJNAF). The Start Counter is constructed of 24 2.2-mm-thick single-ended scintillation paddles, forming a hermetic hexagon around the target region. This device measures the interaction time of the incoming photon in the target by detecting the outgoing particles. The counter provides complex trigger topologies, shows good efficiency and achieved a time resolution of 350 ps
Psychological Well-Being of Older Adults With Cognitive Deterioration During Quarantine: Preliminary Results From the GeroCovid Initiative
Objectives: The spread of COVID-19 has undeniably unsettled the social, psychological and emotional life of the entire world population. Particular attention should be paid to older adults with dementia, given their vulnerability to emotional stressors. The aim of this retrospective study is to evaluate the impact of the first wave quarantine related to Covid-19 on psychological and affective well-being of older adults with mild/major neurocognitive disorders and of their caregivers.Methods: Data on participants' assessment before the quarantine (PREQ) were retrospectively collected. Patients with Mild Cognitive Impairment (MCI) or dementia were recruited from different Centers for Cognitive Decline and Dementia in Italy. During the quarantine, psychological and affective well-being were evaluated by phone through the administrations of scales measuring anxiety and depression (DASS), perceived stress (PSS), coping strategies (COPE) and the caregivers' burden (CBI). The scales' results were compared across participants' PREQ cognitive level (Mini Mental State Examination, MMSE ≥25, 23–24, and ≤ 22) with multiple linear regression models.Results: The sample included 168 patients (64% women) with a mean age of 79 ± 7 years. After adjusting for potential confounders, more severe cognitive impairment was independently associated with higher DASS and PSS score, and poorer coping strategies (p < 0.05). Cognitive functioning was also inversely associated with CBI.Conclusions: The impact of the quarantine on the psycho-affective well-being of individuals with MCI and dementia and on caregivers' burden varies according to the PREQ cognitive functioning with more severely impaired patients having worse outcomes
The Heavy Photon Search test detector
The Heavy Photon Search (HPS), an experiment to search for a hidden sector photon in fixed target electroproduction, is preparing for installation at the Thomas Jefferson National Accelerator Facility (JLab) in the Fall of 2014. As the first stage of this project, the HPS Test Run apparatus was constructed and operated in 2012 to demonstrate the experiment׳s technical feasibility and to confirm that the trigger rates and occupancies are as expected. This paper describes the HPS Test Run apparatus and readout electronics and its performance. In this setting, a heavy photon can be identified as a narrow peak in the e+e− invariant mass spectrum above the trident background or as a narrow invariant mass peak with a decay vertex displaced from the production target, so charged particle tracking and vertexing are needed for its detection. In the HPS Test Run, charged particles are measured with a compact forward silicon microstrip tracker inside a dipole magnet. Electromagnetic showers are detected in a PbW04 crystal calorimeter situated behind the magnet, and are used to trigger the experiment and identify electrons and positrons. Both detectors are placed close to the beam line and split top-bottom. This arrangement provides sensitivity to low-mass heavy photons, allows clear passage of the unscattered beam, and avoids the spray of degraded electrons coming from the target. The discrimination between prompt and displaced e+e− pairs requires the first layer of silicon sensors be placed only 10 cm downstream of the target. The expected signal is small, and the trident background huge, so the experiment requires very large statistics. Accordingly, the HPS Test Run utilizes high-rate readout and data acquisition electronics and a fast trigger to exploit the essentially 100% duty cycle of the CEBAF accelerator at JLab
Measurement of the Polarized Structure Function for in the Resonance Region
The polarized longitudinal-transverse structure function
has been measured using the reaction in the
resonance region at and 0.65 GeV. No previous
data exist for this reaction channel. The kinematically
complete experiment was performed at Jefferson Lab with the CEBAF Large
Acceptance Spectrometer (CLAS) using longitudinally polarized electrons at an
energy of 1.515 GeV. A partial wave analysis of the data shows generally better
agreement with recent phenomenological models of pion electroproduction
compared to the previously measured channel. A fit to both
and channels using a unitary isobar model suggests the unitarized
Born terms provide a consistent description of the non-resonant background. The
-channel pion pole term is important in the channel through a
rescattering correction, which could be model-dependent.Comment: 6 pages, LaTex, 5 eps figures: Submitted to PRC/Brief Reports v2:
Updated referenc
Measurement of Deeply Virtual Compton Scattering with a Polarized Proton Target
The longitudinal target-spin asymmetry A_UL for the exclusive
electroproduction of high energy photons was measured for the first time in
p(e,e'p\gamma). The data have been accumulated at Jefferson Lab with the CLAS
spectrometer using 5.7 GeV electrons and a longitudinally polarized NH_3
target. A significant azimuthal angular dependence was observed, resulting from
the interference of the Deeply Virtual Compton Scattering and Bethe-Heitler
processes. The amplitude of the sin(phi) moment is 0.252 +/- 0.042(stat) +/-
0.020(sys). Theoretical calculations are in good agreement with the magnitude
and the kinematic dependence of the target-spin asymmetry, which is sensitive
to the generalized parton distributions H and H-tilde.Comment: Modified text slightly, added reference
Kinetic Monte Carlo Simulation of Strained Heteroepitaxial Growth with Intermixing
An efficient method for the simulation of strained heteroepitaxial growth
with intermixing using kinetic Monte Carlo is presented. The model used is
based on a solid-on-solid bond counting formulation in which elastic effects
are incorporated using a ball and spring model. While idealized, this model
nevertheless captures many aspects of heteroepitaxial growth, including
nucleation, surface diffusion, and long range effects due elastic interaction.
The algorithm combines a fast evaluation of the elastic displacement field with
an efficient implementation of a rejection-reduced kinetic Monte Carlo based on
using upper bounds for the rates. The former is achieved by using a multigrid
method for global updates of the displacement field and an expanding box method
for local updates. The simulations show the importance of intermixing on the
growth of a strained film. Further the method is used to simulate the growth of
self-assembled stacked quantum dots
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