Model Continuity in Discrete Event Simulation: A Framework for Model-Driven Development of Simulation Models.

Most of the well known modeling and simulation methodologies state the importance of conceptual modeling in simulation studies and they suggest the use of conceptual models during the simulation model development process. However, only a limited number of methodologies refers to howto move from a conceptual model to an executable simulation model. Besides, existing modeling and simulation methodologies do not typically provide a formal method for model transformations between the models in different stages of the development process. Hence, in the current M&S practice, model continuity is usually not fulfilled. In this article, a model driven development framework for modeling and simulation is in order to bridge the gap between different stages of a simulation study and to obtain model continuity. The applicability of the framework is illustrated with a prototype modeling environment and a case study in the discrete event simulation domain

Lutte contre les ravageurs des stocks de céréales et de légumineuses au Sénégal et en Afrique occidentale : synthèse bibliographique

Controlling pests of cereals and legumes in Senegal and West Africa: a review. Post-harvest losses of cereals and legumes are a major problem in Senegal and West Africa. The solutions to eliminate insects, major pests of stored products were mainly chemical. However, due to pollution associated with pesticides use, selection of resistant strains, environmental pollution, poisoning, the search for alternatives is needed. It is reported on different methods of protecting stocks performed alternatively or in combination with pesticides. The major pest species encountered, particularly Prostephanus truncatus (Horn), insect emerging in Senegal, could be controlled by alternative methods including specially the use of insecticide plants. Different aspects related to this alternative way to chemical pesticides are reviewed herein

From Kennedy, to Beyond: Growing Plants in Space

Astronauts cannot have their cake and eat it too, but what about growing a salad and eating it? As NASA continues to push the envelope on Space exploration and inhabitance the need for a fresh food source becomes more vital. The Life Support team at NASA is using a system developed by ORBITEC the VEGGIE, in which astronauts aboard the ISS, and potentially the Moon and Mars, will be capable of growing food. The introduction of plants not only gives astronauts a means of independently supplying food, but also recreation, oxygen replenishment and psychological benefits. The plants were grown in "pillows", the system used for growing plants within the VEGGIE. This test included 4 types of media mixtures that are composed of a clay based media called Arcilite and Fafard #2, which is a peat moss-based media ( <1 mm Arcilite, 1-2 mm of Arcilite, 1:1 <1 mm & 1-2 mm mixture and 1:1 Arcilite & Fafard mixture). Currently, 3 lettuce cultivars are being grown in 4 mixtures of media. Tests were being conducted to see which form of media has the ratio of best growth and least amount of microbes that are harmful. That is essential because a person's body becomes more susceptible to illness when they leave Earth. As a result, test must be conducted on the "pillow" system to assess the levels of microbial activity. The cultivars were tested at different stages during their growing process for microbes. Datum show that the mix of Fafard and Arcilite had the best growth, but also the most microbes. This was due to the fact that Fafard is an organic substance so it contains material necessary for microbes to live. Data suggest that the <1 mm Arcilite has an acceptable amount of growth and a lower level of microbes, because it is non-organic

Efficiency of Traditional Maize Storage and Control Methods in Rural Grain Granaries: a Case Study from Senegal

Maize storage and pest control method as practiced in traditional clay granaries in the Kédougou region in eastern Senegal were evaluated under rural conditions during two successive years. Three storage modes, i.e. maize cobs, winnowed and non-winnowed maize grains, were tested in seven granaries where the insecticidal plants Hyptis spicigera or H. suaveolens were either incorporated in the store structure or deposited as layers intermittently with maize. At the beginning of the storage period, all granaries were artificially infested with 7 pairs Tribolium castaneum and Sitophilus zeamais. No damage, losses or live insects were observed during 7 months of storage when maize cobs were placed between layers of H. spicigera. Compared with the control, incorporation of insecticidal plants within the granary bottom had no significant effect on the damage and loss level irrespective of the storage mode. Non-winnowed maize always suffered less damage and losses than the winnowed variant. In all granaries depredation, insect abundance and moisture content were highest toward the end of storage period between June and July

Spin polarization and magneto-luminescence of confined electron-hole systems

A BCS-like variational wave-function, which is exact in the infinite field limit, is used to study the interplay among Zeeman energies, lateral confinement and particle correlations induced by the Coulomb interactions in strongly pumped neutral quantum dots. Band mixing effects are partially incorporated by means of field-dependent masses and g-factors. The spin polarization and the magneto-luminescence are computed as functions of the number of electron-hole pairs present in the dot and the applied magnetic field.Comment: To appear in Phys. Rev.

Electron spin coherence in semiconductors: Considerations for a spin-based solid state quantum computer architecture

We theoretically consider coherence times for spins in two quantum computer architectures, where the qubit is the spin of an electron bound to a P donor impurity in Si or within a GaAs quantum dot. We show that low temperature decoherence is dominated by spin-spin interactions, through spectral diffusion and dipolar flip-flop mechanisms. These contributions lead to 1-100 $\mu$s calculated spin coherence times for a wide range of parameters, much higher than former estimates based on $T_{2}^{*}$ measurements.Comment: Role of the dipolar interaction clarified; Included discussion on the approximations employed in the spectral diffusion calculation. Final version to appear in Phys. Rev.

Allowed and forbidden transitions in artificial hydrogen and helium atoms

The strength of radiative transitions in atoms is governed by selection rules. Spectroscopic studies of allowed transitions in hydrogen and helium provided crucial evidence for the Bohr's model of an atom. Forbidden transitions, which are actually allowed by higher-order processes or other mechanisms, indicate how well the quantum numbers describe the system. We apply these tests to the quantum states in semiconductor quantum dots (QDs), which are regarded as artificial atoms. Electrons in a QD occupy quantized states in the same manner as electrons in real atoms. However, unlike real atoms, the confinement potential of the QD is anisotropic, and the electrons can easily couple with phonons of the material. Understanding the selection rules for such QDs is an important issue for the manipulation of quantum states. Here we investigate allowed and forbidden transitions for phonon emission in one- and two-electron QDs (artificial hydrogen and helium atoms) by electrical pump-and-probe experiments, and find that the total spin is an excellent quantum number in artificial atoms. This is attractive for potential applications to spin based information storage.Comment: slightly longer version of Nature 419, 278 (2002

Theory of nuclear induced spectral diffusion: Spin decoherence of phosphorus donors in Si and GaAs quantum dots

We propose a model for spectral diffusion of localized spins in semiconductors due to the dipolar fluctuations of lattice nuclear spins. Each nuclear spin flip-flop is assumed to be independent, the rate for this process being calculated by a method of moments. Our calculated spin decoherence time $T_{M}=0.64$ ms for donor electron spins in Si:P is a factor of two longer than spin echo decay measurements. For $^{31}$P nuclear spins we show that spectral diffusion is well into the motional narrowing regime. The calculation for GaAs quantum dots gives $T_{M}=10-50$ $\mu$s depending on the quantum dot size. Our theory indicates that nuclear induced spectral diffusion should not be a serious problem in developing spin-based semiconductor quantum computer architectures.Comment: 15 pages, 9 figures. Accepted for publication in Phys. Rev.