241 research outputs found
Irreversible nucleation in molecular beam epitaxy: From theory to experiments
Recently, the nucleation rate on top of a terrace during the irreversible
growth of a crystal surface by MBE has been determined exactly. In this paper
we go beyond the standard model usually employed to study the nucleation
process, and we analyze the qualitative and quantitative consequences of two
important additional physical ingredients: the nonuniformity of the
Ehrlich-Schwoebel barrier at the step-edge, because of the existence of kinks,
and the steering effects, due to the interaction between the atoms of the flux
and the substrate. We apply our results to typical experiments of second layer
nucleation.Comment: 11 pages. Table I corrected and one appendix added. To be published
in Phys. Rev. B (scheduled issue: 15 February 2003
Globally Anisotropic High Porosity Silica Aerogels
We discuss two methods by which high porosity silica aerogels can be
engineered to exhibit global anisotropy. First, anisotropy can be introduced
with axial strain. In addition, intrinsic anisotropy can result during growth
and drying stages and, suitably controlled, it can be correlated with
preferential radial shrinkage in cylindrical samples. We have performed small
angle X-ray scattering (SAXS) to characterize these two types of anisotropy. We
show that global anisotropy originating from either strain or shrinkage leads
to optical birefringence and that optical cross-polarization studies are a
useful characterization of the uniformity of the imposed global anisotropy.Comment: 18 pages, 14 figures, submitted to Journal of Non-Crystalline Solid
Development of late blight resistant potatoes by cisgenic stacking
Background Phytophthora infestans, causing late blight in potato, remains one of the most devastating pathogens in potato production and late blight resistance is a top priority in potato breeding. The introduction of multiple resistance (R) genes with different spectra from crossable species into potato varieties is required. Cisgenesis is a promising approach that introduces native genes from the crops own gene pool using GM technology, thereby retaining favourable characteristics of established varieties. Results We pursued a cisgenesis approach to introduce two broad spectrum potato late blight R genes, Rpi-sto1 and Rpi-vnt1.1 from the crossable species Solanum stoloniferum and Solanum venturii, respectively, into three different potato varieties. First, single R gene-containing transgenic plants were produced for all varieties to be used as references for the resistance levels and spectra to be expected in the respective genetic backgrounds. Next, a construct containing both cisgenic late blight R genes (Rpi-vnt1.1 and Rpi-sto1), but lacking the bacterial kanamycin resistance selection marker (NPTII) was transformed to the three selected potato varieties using Agrobacterium-mediated transformation. Gene transfer events were selected by PCR among regenerated shoots. Through further analyses involving morphological evaluations in the greenhouse, responsiveness to Avr genes and late blight resistance in detached leaf assays, the selection was narrowed down to eight independent events. These cisgenic events were selected because they showed broad spectrum late blight resistance due to the activity of both introduced R genes. The marker-free transformation was compared to kanamycin resistance assisted transformation in terms of T-DNA and vector backbone integration frequency. Also, differences in regeneration time and genotype dependency were evaluated. Conclusions We developed a marker-free transformation pipeline to select potato plants functionally expressing a stack of late blight R genes. Marker-free transformation is less genotype dependent and less prone to vector backbone integration as compared to marker-assisted transformation. Thereby, this study provides an important tool for the successful deployment of R genes in agriculture and contributes to the production of potentially durable late blight resistant potatoes
Measurement of the B0-anti-B0-Oscillation Frequency with Inclusive Dilepton Events
The - oscillation frequency has been measured with a sample of
23 million \B\bar B pairs collected with the BABAR detector at the PEP-II
asymmetric B Factory at SLAC. In this sample, we select events in which both B
mesons decay semileptonically and use the charge of the leptons to identify the
flavor of each B meson. A simultaneous fit to the decay time difference
distributions for opposite- and same-sign dilepton events gives ps.Comment: 7 pages, 1 figure, submitted to Physical Review Letter
An Ultra-Low Background PMT for Liquid Xenon Detectors
Results are presented from radioactivity screening of two models of
photomultiplier tubes designed for use in current and future liquid xenon
experiments. The Hamamatsu 5.6 cm diameter R8778 PMT, used in the LUX dark
matter experiment, has yielded a positive detection of four common radioactive
isotopes: 238U, 232Th, 40K, and 60Co. Screening of LUX materials has rendered
backgrounds from other detector materials subdominant to the R8778
contribution. A prototype Hamamatsu 7.6 cm diameter R11410 MOD PMT has also
been screened, with benchmark isotope counts measured at <0.4 238U / <0.3 232Th
/ <8.3 40K / 2.0+-0.2 60Co mBq/PMT. This represents a large reduction, equal to
a change of \times 1/24 238U / \times 1/9 232Th / \times 1/8 40K per PMT,
between R8778 and R11410 MOD, concurrent with a doubling of the photocathode
surface area (4.5 cm to 6.4 cm diameter). 60Co measurements are comparable
between the PMTs, but can be significantly reduced in future R11410 MOD units
through further material selection. Assuming PMT activity equal to the measured
90% upper limits, Monte Carlo estimates indicate that replacement of R8778 PMTs
with R11410 MOD PMTs will change LUX PMT electron recoil background
contributions by a factor of \times1/25 after further material selection for
60Co reduction, and nuclear recoil backgrounds by a factor of \times 1/36. The
strong reduction in backgrounds below the measured R8778 levels makes the
R11410 MOD a very competitive technology for use in large-scale liquid xenon
detectors.Comment: v2 updated to include content after reviewer comments (Sep 2012
Regulation of 5-HT Receptors and the Hypothalamic-Pituitary-Adrenal Axis
Disturbances in the serotonin (5-HT) system is the neurobiological abnormality most consistently associated with suicide. Hyperactivity of the hypothalmic-pituitary-adrenal (HPA) axis is also described in suicide victims. The HPA axis is the classical neuroendocrine system that responds to stress and whose final product, corticosteroids, targets components of the limbic system, particularly the hippocampus. We will review resulsts from animal studies that point to the possibility that many of the 5-HT receptor changes observed in suicide brains may be a result of, or may be worsened by, the HPA overactivity that may be present in some suicide victims. The results of these studies can be summarized as follows: (1) chronic unpredictable stress produces high corticosteroid levels in rats; (2) chronic stress also results in changes in specific 5-HT receptors (increases in cortical 5-HT2A and decreases in hipocampal 5-HT1A and 5-HT1B); (3) chronic antidepressant administration prevents many of the 5-HT receptor changes observed after stress; and (4) chronic antidepressant administration reverses the overactivity of the HPA axis. If indeed 5-HT receptors have a partial role in controlling affective states, then their modulation by corticosteroids provides a potential mechanism by which these hormones may regulate mood. These data may also provide a biological understanding of how stressful events may increase the risk for suicide in vulnerable individuals and may help us elucidate the neurobiological underpinnings of treatment resistance.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73437/1/j.1749-6632.1997.tb52357.x.pd
LUXSim: A Component-Centric Approach to Low-Background Simulations
Geant4 has been used throughout the nuclear and high-energy physics community
to simulate energy depositions in various detectors and materials. These
simulations have mostly been run with a source beam outside the detector. In
the case of low-background physics, however, a primary concern is the effect on
the detector from radioactivity inherent in the detector parts themselves. From
this standpoint, there is no single source or beam, but rather a collection of
sources with potentially complicated spatial extent. LUXSim is a simulation
framework used by the LUX collaboration that takes a component-centric approach
to event generation and recording. A new set of classes allows for multiple
radioactive sources to be set within any number of components at run time, with
the entire collection of sources handled within a single simulation run.
Various levels of information can also be recorded from the individual
components, with these record levels also being set at runtime. This
flexibility in both source generation and information recording is possible
without the need to recompile, reducing the complexity of code management and
the proliferation of versions. Within the code itself, casting geometry objects
within this new set of classes rather than as the default Geant4 classes
automatically extends this flexibility to every individual component. No
additional work is required on the part of the developer, reducing development
time and increasing confidence in the results. We describe the guiding
principles behind LUXSim, detail some of its unique classes and methods, and
give examples of usage.
* Corresponding author, [email protected]: 45 pages, 15 figure
Measurement of the CP-Violating Asymmetry Amplitude sin2
We present results on time-dependent CP-violating asymmetries in neutral B decays to several CP eigenstates. The measurements use a data sample of about 88 million Y(4S) --> B Bbar decays collected between 1999 and 2002 with the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We study events in which one neutral B meson is fully reconstructed in a final state containing a charmonium meson and the other B meson is determined to be either a B0 or B0bar from its decay products. The amplitude of the CP-violating asymmetry, which in the Standard Model is proportional to sin2beta, is derived from the decay-time distributions in such events. We measure sin2beta = 0.741 +/- 0.067 (stat) +/- 0.033 (syst) and |lambda| = 0.948 +/- 0.051 (stat) +/- 0.017 (syst). The magnitude of lambda is consistent with unity, in agreement with the Standard Model expectation of no direct CP violation in these modes
A search for the decay
We search for the rare flavor-changing neutral-current decay in a data sample of 82 fb collected with the {\sl BABAR}
detector at the PEP-II B-factory. Signal events are selected by examining the
properties of the system recoiling against either a reconstructed hadronic or
semileptonic charged-B decay. Using these two independent samples we obtain a
combined limit of
at the 90% confidence level. In addition, by selecting for pions rather than
kaons, we obtain a limit of using only the hadronic B reconstruction method.Comment: 7 pages, 8 postscript figures, submitted to Phys. Rev. Let
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