386 research outputs found
Excitation and decay of projectile-like fragments formed in dissipative peripheral collisions at intermediate energies
Projectile-like fragments (PLF:15<=Z<=46) formed in peripheral and
mid-peripheral collisions of 114Cd projectiles with 92Mo nuclei at E/A=50 MeV
have been detected at very forward angles, 2.1 deg.<=theta_lab<=4.2 deg.
Calorimetric analysis of the charged particles observed in coincidence with the
PLF reveals that the excitation of the primary PLF is strongly related to its
velocity damping. Furthermore, for a given V_PLF*, its excitation is not
related to its size, Z_PLF*. For the largest velocity damping, the excitation
energy attained is large, approximately commensurate with a system at the
limiting temperatureComment: 5 pages, 6 figure
Neutron to proton ratios of quasiprojectile and midrapidity emission in the Zn + Zn reaction at 45 MeV/nucleon
Simultaneous measurement of both neutrons and charged particles emitted in
the reaction Zn + Zn at 45 MeV/nucleon allows comparison of the
neutron to proton ratio at midrapidity with that at projectile rapidity. The
evolution of N/Z in both rapidity regimes with increasing centrality is
examined. For the completely re-constructed midrapidity material one finds that
the neutron-to-proton ratio is above that of the overall Zn + Zn
system. In contrast, the re-constructed ratio for the quasiprojectile is below
that of the overall system. This difference provides the most complete evidence
to date of neutron enrichment of midrapidity nuclear matter at the expense of
the quasiprojectile
Interplay of initial deformation and Coulomb proximity on nuclear decay
Alpha particles emitted from an excited projectile-like fragment (PLF*)
formed in a peripheral collision of two intermediate-energy heavy ions exhibit
a strong preference for emission towards the target-like fragment (TLF). The
interplay of the initial deformation of the PLF* caused by the reaction,
Coulomb proximity, and the rotation of the PLF* results in the observed
anisotropic angular distribution. Changes in the shape of the angular
distribution with excitation energy are interpreted as being the result of
forming more elongated initial geometries in the more peripheral collisions.Comment: 4 figure
Fragment Isospin as a Probe of Heavy-Ion Collisions
Isotope ratios of fragments produced at mid-rapidity in peripheral and
central collisions of 114Cd ions with 92Mo and 98Mo target nuclei at E/A = 50
MeV are compared. Neutron-rich isotopes are preferentially produced in central
collisions as compared to peripheral collisions. The influence of the size (A),
density, N/Z, E*/A, and Eflow/A of the emitting source on the measured isotope
ratios was explored by comparison with a statistical model (SMM). The
mid-rapidity region associated with peripheral collisions does not appear to be
neutron-enriched relative to central collisions.Comment: 12 pages including figure
Fragment Production in Non-central Collisions of Intermediate Energy Heavy Ions
The defining characteristics of fragment emission resulting from the
non-central collision of 114Cd ions with 92Mo target nuclei at E/A = 50 MeV are
presented. Charge correlations and average relative velocities for mid-velocity
fragment emission exhibit significant differences when compared to standard
statistical decay. These differences associated with similar velocity
dissipation are indicative of the influence of the entrance channel dynamics on
the fragment production process
Evaluating and improving the Community Land Model's sensitivity to land cover
Modeling studies have shown the importance of biogeophysical effects of deforestation on local climate conditions but have also highlighted the lack of agreement across different models. Recently, remote-sensing observations have been used to assess the contrast in albedo, evapotranspiration (ET), and land surface temperature (LST) between forest and nearby open land on a global scale. These observations provide an unprecedented opportunity to evaluate the ability of land surface models to simulate the biogeophysical effects of forests. Here, we evaluate the representation of the difference of forest minus open land (i.e., grassland and cropland) in albedo, ET, and LST in the Community Land Model version 4.5 (CLM4.5) using various remote-sensing and in situ data sources. To extract the local sensitivity to land cover, we analyze plant functional type level output from global CLM4.5 simulations, using a model configuration that attributes a separate soil column to each plant functional type. Using the separated soil column configuration, CLM4.5 is able to realistically reproduce the biogeophysical contrast between forest and open land in terms of albedo, daily mean LST, and daily maximum LST, while the effect on daily minimum LST is not well captured by the model. Furthermore, we identify that the ET contrast between forests and open land is underestimated in CLM4.5 compared to observation-based products and even reversed in sign for some regions, even when considering uncertainties in these products. We then show that these biases can be partly alleviated by modifying several model parameters, such as the root distribution, the formulation of plant water uptake, the light limitation of photosynthesis, and the maximum rate of carboxylation. Furthermore, the ET contrast between forest and open land needs to be better constrained by observations to foster convergence amongst different land surface models on the biogeophysical effects of forests. Overall, this study demonstrates the potential of comparing subgrid model output to local observations to improve current land surface models' ability to simulate land cover change effects, which is a promising approach to reduce uncertainties in future assessments of land use impacts on climate
Porins and small-molecule translocation across the outer membrane of Gram-negative bacteria
Gram-negative bacteria and their complex cell envelope, which comprises an outer membrane and an inner membrane, are an important and attractive system for studying the translocation of small molecules across biological membranes. In the outer membrane of Enterobacteriaceae, trimeric porins control the cellular uptake of small molecules, including nutrients and antibacterial agents. The relatively slow porin-mediated passive uptake across the outer membrane and active efflux via efflux pumps in the inner membrane creates a permeability barrier. The synergistic action of outer membrane permeability, efflux pump activities and enzymatic degradation efficiently reduces the intracellular concentrations of small molecules and contributes to the emergence of antibiotic resistance. In this Review, we discuss recent advances in our understanding of the molecular and functional roles of general porins in small-molecule translocation in Enterobacteriaceae and consider the crucial contribution of porins in antibiotic resistance
Fragmentation in Peripheral Heavy-Ion Collisions: from Neck Emission to Spectator Decays
Invariant cross sections of intermediate mass fragments in peripheral
collisions of Au on Au at incident energies between 40 and 150 AMeV have been
measured with the 4-pi multi-detector INDRA. The maximum of the fragment
production is located near mid-rapidity at the lower energies and moves
gradually towards the projectile and target rapidities as the energy is
increased. Schematic calculations within an extended Goldhaber model suggest
that the observed cross-section distributions and their evolution with energy
are predominantly the result of the clustering requirement for the emerging
fragments and of their Coulomb repulsion from the projectile and target
residues. The quantitative comparison with transverse energy spectra and
fragment charge distributions emphasizes the role of hard scattered nucleons in
the fragmentation process.Comment: 5 pages, 5 eps figures, RevTeX4, submitted to Phys. Lett.
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