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Search Results: 1 - 10 of 189888 matches for " G. Nebbia "
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Comunidades Halófilas de la costa de la Bahía Blanca (Argentina): Caracterización, mapeo y cambios durante los últimos cincuenta a?os
Nebbia,Ana Julia; Zalba,Sergio M.;
Bolet?-n de la Sociedad Argentina de Bot??nica , 2007,
Abstract: we classified and mapped vegetation units along the coast of bahía blanca (argentina), where we discriminated eight habitat types: intertidal flats without vegetation and with high density of crab burrows ( cangrejales ), temporary ponds and puddles, intertidal steppes occupied by stands of sarcocornia perennis or by spartina alterniflora marshes, low allenfolfea patagonica shrubs, low atriplex undulata shrubs, high cyclolepis genistoides shrubs and halophytic grasslands with distichlis spicata . we compared our results with those from a phytosociological survey conducted in the area in 1949 and 1950. we observed a relative constancy in the composition and structure of plant communities, except for the percentage of exotic species being greater than historical ones for the three shrublands types defined, which can respond to human activities in the area. as a conclusion we propose that, despite being in a more pristine status than other ecosystems in the region, coastal communities show symptoms of anthropogenic disturbances that justify the adoption of conservation strategies.
Comunidades Halófilas de la costa de la Bahía Blanca (Argentina): Caracterización, mapeo y cambios durante los últimos cincuenta a os Halophytic communities of the coast of Bahía Blanca (Argentina): characterization, mapping and changes in the last fifty years
Ana Julia Nebbia,Sergio M. Zalba
Boletín de la Sociedad Argentina de Botánica , 2007,
Abstract: Realizamos una clasificación y mapeo de las unidades de vegetación presentes en la franja costera de Bahía Blanca (Argentina), donde identificamos ocho tipos de ambientes naturales: cangrejales, charcos temporarios, zonas intermareales ocupadas por marismas de Sarcocornia perennis o espartillares de Spartina alterniflora , arbustales bajos de Allenrolfea patagonica , arbustales bajos de Atriplex undulata, arbustales altos de Cyclolepis genistoides y pastizales. Comparamos los valores obtenidos a partir de nuestros censos florísticos con datos provenientes de estudios fitosociológicos desarrollados en el área entre 1949 y 1950. Observamos que la composición y estructura de las comunidades se mantuvo relativamente constante, excepto en el caso del porcentaje de especies exóticas, donde se observa que los valores actuales son superiores en el caso de los tres ambientes de arbustal, lo que podría deberse a efectos de actividades humanas. Como conclusión se propone que, si bien este ambiente se encuentra en relativo buen estado de conservación, comparado con otros de la región, muestra signos de deterioro que justifican la adopción de medidas de conservación. We classified and mapped vegetation units along the coast of Bahía Blanca (Argentina), where we discriminated eight habitat types: intertidal flats without vegetation and with high density of crab burrows ( cangrejales ), temporary ponds and puddles, intertidal steppes occupied by stands of Sarcocornia perennis or by Spartina alterniflora marshes, low Allenfolfea patagonica shrubs, low Atriplex undulata shrubs, high Cyclolepis genistoides shrubs and halophytic grasslands with Distichlis spicata . We compared our results with those from a phytosociological survey conducted in the area in 1949 and 1950. We observed a relative constancy in the composition and structure of plant communities, except for the percentage of exotic species being greater than historical ones for the three shrublands types defined, which can respond to human activities in the area. As a conclusion we propose that, despite being in a more pristine status than other ecosystems in the region, coastal communities show symptoms of anthropogenic disturbances that justify the adoption of conservation strategies.
Antimicrobial resistance, class 1 and 2 integrons and gene cassettes in avian Escherichia coli
Patrizia Nebbia,Clara Tramuta,Mauro Giammarino,Cecilia Ambrogi
Italian Journal of Animal Science , 2010, DOI: 10.4081/ijas.2008.391
Abstract: Seventy-four Escherichia coli isolates were collected from domestic, synanthropic free living birds as well as wild and exotic birds, all living in captivity. Antimicrobial susceptibility was tested against a panel of 9 antibiotics, and presence of integrons (Class 1 and Class 2) and gene cassettes was analysed by PCR and sequencing, respectively. Twenty-eight isolates proved positive for Class 1 integrons and 19 for Class 2. Gene cassette arrangements were determined in 23 integron-positive isolates, which harboured one (aadA1) two (dfrA1-aadA1) or three (dfrA7-dfrA1-aadA1, dfrA1-sat1-aadA1) cassettes in their variable region. E. coli multiresistance to antimicrobials was observed in all groups examined, in particular domestic and synanthropic birds showed resistance to at least 4 antibiotics. A large number of isolates from domestic and synantropic birds proved to be Class 1 integron- positive, but unexpectedly, we observed many Class 2 integrons, usually considered less frequent.
Experimental determination of the symmetry energy of a low density nuclear gas
S. Kowalski,J. B. Natowitz,S. Shlomo,R. Wada,K. Hagel,J. Wang,T. Materna,Z. Chen,Y. G. Ma,L. Qin,A. S. Botvina,D. Fabris,M. Lunardon,S. Moretto,G. Nebbia,S. Pesente,V. Rizzi,G. Viesti,M. Cinausero,G. Prete,T. Keutgen,Y. El Masri,Z. Majka,A. Ono
Physics , 2006, DOI: 10.1103/PhysRevC.75.014601
Abstract: Experimental analyses of moderate temperature nuclear gases produced in the violent collisions of 35 MeV/nucleon$^{64}$Zn projectiles with $^{92}$Mo and $^{197}$Au target nuclei reveal a large degree of alpha particle clustering at low densities. For these gases, temperature and density dependent symmetry energy coefficients have been derived from isoscaling analyses of the yields of nuclei with A $\leq 4$. At densities of 0.01 to 0.05 times the ground state density of symmetric nuclear matter, the temperature and density dependent symmetry energies are 10.7 to 13.5 MeV. These values are much larger than those obtained in mean field calculations. They are in quite good agreement with results of a recently proposed Virial Equation of State calculation.
Laboratory Tests of Low Density Astrophysical Equations of State
L. Qin,K. Hagel,R. Wada,J. B. Natowitz,S. Shlomo,A. Bonasera,G. Roepke,S. Typel,Z. Chen,M. Huang,J. Wang,H. Zheng,S. Kowalski,M. Barbui,M. R. D. Rodrigues,K. Schmidt,D. Fabris,M. Lunardon,S. Moretto,G. Nebbia,S. Pesente,V. Rizzi,G. Viesti,M. Cinausero,G. Prete,T. Keutgen,Y. El Masri,Z. Majka
Physics , 2011, DOI: 10.1103/PhysRevLett.108.172701
Abstract: Clustering in low density nuclear matter has been investigated using the NIMROD multi-detector at Texas A&M University. Thermal coalescence modes were employed to extract densities, $\rho$, and temperatures, $T$, for evolving systems formed in collisions of 47 $A$ MeV $^{40}$Ar + $^{112}$Sn,$^{124}$Sn and $^{64}$Zn + $^{112}$Sn, $^{124}$Sn. The yields of $d$, $t$, $^{3}$He, and $^{4}$He have been determined at $\rho$ = 0.002 to 0.032 nucleons/fm$^{3}$ and $T$= 5 to 10 MeV. The experimentally derived equilibrium constants for $\alpha$ particle production are compared with those predicted by a number of astrophysical equations of state. The data provide important new constraints on the model calculations.
Experimental Determination of In-Medium Cluster Binding Energies and Mott Points in Nuclear Matter
K. Hagel,R. Wada,L. Qin,J. B. Natowitz,S. Shlomo,A. Bonasera,G. R?pke,S. Typel,Z. Chen,M. Huang,J. Wang,H. Zheng,S. Kowalski,C. Bottosso,M. Barbui,M. R. D. Rodrigues,K. Schmidt,D. Fabris,M. Lunardon,S. Moretto,G. Nebbia,S. Pesente,V. Rizzi,G. Viesti,M. Cinausero,G. Prete,T. Keutgen,Y. El Masri,Z. Majka
Physics , 2011, DOI: 10.1103/PhysRevLett.108.062702
Abstract: In medium binding energies and Mott points for $d$, $t$, $^3$He and $\alpha$ clusters in low density nuclear matter have been determined at specific combinations of temperature and density in low density nuclear matter produced in collisions of 47$A$ MeV $^{40}$Ar and $^{64}$Zn projectiles with $^{112}$Sn and $^{124}$Sn target nuclei. The experimentally derived values of the in medium modified binding energies are in good agreement with recent theoretical predictions based upon the implementation of Pauli blocking effects in a quantum statistical approach.
The Nuclear Matter Symmetry Energy at $0.03\leq ρ/ρ_0\leq 0.2$
R. Wada,K. Hagel,L. Qin,J. B. Natowitz,G. R?pke,S. Shlomo,A. Bonasera,S. Typel,Z. Chen,M. Huang,J. Wang,H. Zheng,S. Kowalski,C. Bottosso,M. Barbui,M. R. D. Rodrigues,K. Schmidt,D. Fabris,M. Lunardon,S. Moretto,G. Nebbia,S. Pesente,V. Rizzi,G. Viesti,M. Cinausero,G. Prete,T. Keutgen,Y. El Masri,Z. Majka
Physics , 2011, DOI: 10.1103/PhysRevC.85.064618
Abstract: Measurements of the density dependence of the free symmetry energy in low density clustered matter have been extended using the NIMROD multi-detector at Texas A&M University. Thermal coalescence models were employed to extract densities, $\rho$, and temperatures, $T$, for evolving systems formed in collisions of 47 $A$ MeV $^{40}$Ar + $^{112}$Sn,$^{124}$Sn and $^{64}$Zn + $^{112}$Sn, $^{124}$Sn. Densities of $0.03 \leq \rho/\rho_0 \leq 0.2$ and temperatures in the range 5 to 10 MeV have been sampled. The free symmetry energy coefficients are found to be in good agreement with values calculated using a quantum statistical model. Values of the corresponding symmetry energy coefficient are derived from the data using entropies derived from the model.
Reaction Dynamics and Multifragmentation in Fermi Energy Heavy Ion Reactions
R. Wada,T. Keutgen,K. Hagel,Y. G. Ma,J. Wang,M. Murray,L. Qin,P. Smith,J. B. Natowitz,R. Alfarro,J. Cibor,M. Cinausero,Y. El Masri,D. Fabris,E. Fioretto,A. Keksis,M. Lunardon,A. Makeev,N. Marie,E. Martin,A. Martinez-Davalos,A. Menchaca-Rocha,G. Nebbia,G. Prete,V. Rizzi,A. Ruangma,D. V. Shetty,G. Souliotis,P. Staszel,M. Veselsky,G. Viesti,E. M. Winchester,S. J. Yennello,Z. Majka,A. Ono
Physics , 2003, DOI: 10.1103/PhysRevC.69.044610
Abstract: The reaction systems, 64Zn + 58Ni, 64Zn + 92Mo, 64Zn + 197Au, at 26A, 35A and 47A MeV, have been studied both in experiments with a 4$\pi$ detector array, NIMROD, and with Antisymmetrized Molecular Dynamics model calculations employing effective interactions corresponding to soft and stiff equations of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained. The velocity distributions of $\alpha$ particles and fragments with Z >= 3 show distinct differences in calculations with the soft EOS and the stiff EOS. The velocity distributions of $\alpha$ particle and Intermediate Mass Fragments (IMF's) are best described by the stiff EOS. Neither of the above direct observables nor the strength of the elliptic flow are sensitive to changes in the in-medium nucleon-nucleon (NN) cross sections. A detailed analysis of the central collision events calculated with the stiff EOS revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with 197Au at 47A MeV a significant radial expansion takes place. For reactions with 58Ni and 92Mo at 47A MeV semi-transparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain.
Critical Behavior in Light Nuclear Systems: Experimental Aspects
Y. G. Ma,J. B. Natowitz,R. Wada,K. Hagel,J. Wang,T. Keutgen,Z. Majka,M. Murray,L. Qin,P. Smith,R. Alfaro,J. Cibor,M. Cinausero,Y. El Masri,D. Fabris,E. Fioretto,A. Keksis,M. Lunardon,A. Makeev,N. Marie,E. Martin,A. Martinez-Davalos,A. Menchaca-Rocha,G. Nebbia,G. Prete,V. Rizzi,A. Ruangma,D. V. Shetty,G. Souliotis,P. Staszel,M. Veselsky,G. Viesti,E. M. Winchester,S. J. Yennello
Physics , 2004, DOI: 10.1103/PhysRevC.71.054606
Abstract: An extensive experimental survey of the features of the disassembly of a small quasi-projectile system with $A \sim$ 36, produced in the reactions of 47 MeV/nucleon $^{40}$Ar + $^{27}$Al, $^{48}$Ti and $^{58}$Ni, has been carried out. Nuclei in the excitation energy range of 1-9 MeV/u have been investigated employing a new method to reconstruct the quasi-projectile source. At an excitation energy $\sim$ 5.6 MeV/nucleon many observables indicate the presence of maximal fluctuations in the de-excitation processes. The fragment topological structure shows that the rank sorted fragments obey Zipf's law at the point of largest fluctuations providing another indication of a liquid gas phase transition. The caloric curve for this system shows a monotonic increase of temperature with excitation energy and no apparent plateau. The temperature at the point of maximal fluctuations is $8.3 \pm 0.5$ MeV. Taking this temperature as the critical temperature and employing the caloric curve information we have extracted the critical exponents $\beta$, $\gamma$ and $\sigma$ from the data. Their values are also consistent with the values of the universality class of the liquid gas phase transition. Taken together, this body of evidence strongly suggests a phase change in an equilibrated mesoscopic system at, or extremely close to, the critical point.
Evidence of Critical Behavior in the Disassembly of Nuclei with A ~ 36
Y. G. Ma,R. Wada,K. Hagel,J. Wang,T. Keutgen,Z. Majka,M. Murray,L. Qin,P. Smith,J. B. Natowitz,R. Alfaro,J. Cibor,M. Cinausero,Y. El Masri,D. Fabris,E. Fioretto,A. Keksis,M. Lunardon,A. Makeev,N. Marie,E. Martin,A. Martinez-Davalos,A. Menchaca-Rocha,G. Nebbia,G. Prete,V. Rizzi,A. Ruangma,D. V. Shetty,G. Souliotis,P. Staszel,M. Veselsky,G. Viesti,E. M. Winchester,S. J. Yennello
Physics , 2003, DOI: 10.1103/PhysRevC.69.031604
Abstract: A wide variety of observables indicate that maximal fluctuations in the disassembly of hot nuclei with A ~ 36 occur at an excitation energy of 5.6 +- 0.5 MeV/u and temperature of 8.3 +- 0.5 MeV. Associated with this point of maximal fluctuations are a number of quantitative indicators of apparent critical behavior. The associated caloric curve does not appear to show a flattening such as that seen for heavier systems. This suggests that, in contrast to similar signals seen for liquid-gas transitions in heavier nuclei, the observed behavior in these very light nuclei is associated with a transition much closer to the critical point.
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