174 research outputs found

    Spectral studies of ocean water using DOAS

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    International audienceMethods enabling the retrieval of oceanic parameter from the space borne instrumentation Scanning Imaging Absorption Spectrometer for Atmospheric ChartographY (SCIAMACHY) using Differential Optical Absorption Spectroscopy (DOAS) are presented. SCIAMACHY onboard ENVISAT measures back scattered solar radiation at a spectral resolution (0.2 to 1.5 nm). The DOAS method was used for the first time to fit modelled Vibrational Raman Scattering (VRS) in liquid water and in situ measured phytoplankton absorption reference spectra to optical depths measured by SCIAMACHY. Spectral structures of VRS and phytoplankton absorption were clearly found in these optical depths. Both fitting approaches lead to consistent results. DOAS fits correlate with estimates of chlorophyll concentrations: low fit factors for VRS retrievals correspond to large chlorophyll concentrations and vice versa; large fit factors for phytoplankton absorption correspond with high chlorophyll concentrations and vice versa. From these results a simple retrieval technique taking advantage of both measurements is shown. First maps of global chlorophyll concentrations were compared to the corresponding MODIS measurements with very promising results. In addition, results from this study will be used to improve atmospheric trace gas DOAS-retrievals from visible wavelengths by including these oceanographic signatures

    Raman coupler for a trapped two-component quantum-degenerate Fermi gas

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    We investigate theoretically the Raman coupling between two internal states of a trapped low-density quantum-degenerate Fermi gas. In general, the trap frequencies associated with the two internal states can be different, leading to the onset of collapses and revivals in the population difference of the two internal states. This behavior can be changed drastically by two-body collisions. In particular, we show that under appropriate conditions they can suppress the dephasing leading to the collapse of the population difference, and restore almost full Rabi oscillations between the two internal states. These results are compared and contrasted to those for a quantum-degenerate bosonic gas.Comment: 7 pages incl. 7 PostScript figures (.eps), LaTeX using RevTeX4, submitted to Phys. Rev. A, modified versio

    Fission Hindrance in hot 216Th: Evaporation Residue Measurements

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    The fusion evaporation-residue cross section for 32S+184W has been measured at beam energies of E_beam = 165, 174, 185, 196, 205, 215, 225, 236, 246,and 257 MeV using the ATLAS Fragment Mass Analyzer. The data are compared with Statistical Model calculations and it is found that a nuclear dissipation strength, which increases with excitation energy, is required to reproduce the excitation function. A comparison with previously published data show that the dissipation strength depends strongly on the shell structure of the nuclear system.Comment: 15 pages 9 figure

    Role of the target orientation angle and orbital angular momentum in the evaporation residue production

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    The influence of the orientation angles of the target nucleus symmetry axis relative to the beam direction on the production of the evaporation residues is investigated for the 48^{48}Ca+154^{154}Sm reaction as a function of the beam energy. At low energies (Ec.m.<E_{\rm c.m.}<137 MeV), the yield of evaporation residues is observed only for collisions with small orientation angles (αT<450\alpha_T<45^0). At large energies (about Ec.m.=E_{\rm c.m.}=140--180 MeV) all the orientation angles αT\alpha_T can contribute to the evaporation residue cross section σER\sigma_{ER} in the 10--100 mb range, and at Ec.m.>E_{c.m.}>180 MeV σER\sigma_{ER} ranges around 0.1--10 mb because the fission barrier for a compound nucleus decreases by increasing its excitation energy and angular momentum.Comment: 20 pages, 10 figures, submitted to JPS

    Light-particle emission from the fissioning nuclei 126Ba, 188Pt and (266,272,278)/110: theoretical predictions and experimental results

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    We present a comparison of our model treating fission dynamics in conjunction with light-particle (n, p, alpha) evaporation with the available experimental data for the nuclei 126Ba, 188Pt and three isotopes of the element Z=110. The dynamics of the symmetric fission process is described through the solution of a classical Langevin equation for a single collective variable characterizing the nuclear deformation along the fission path. A microscopic approach is used to evaluate the emission rates for pre-fission light particles. Entrance-channel effects are taken into account by generating an initial spin distribution of the compound nucleus formed by the fusion of two deformed nuclei with different relative orientations

    Manifestation of transient effects in fission induced by relativistic heavy-ion collisions

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    We examine the manifestation of transient effects in fission by analysing experimental data where fission is induced by peripheral heavy-ion collisions at relativistic energies. Available total nuclear fission cross sections of 238U at 1 A GeV on gold and uranium targets are compared with a nuclear-reaction code, where transient effects in fission are modelled using different approximations to the numerical time-dependent fission-decay width: a new analytical description based on the solution of the Fokker-Planck equation and two widely used but less realistic descriptions, a step function and an exponential-like function. The experimental data are only reproduced when transient effects are considered. The deduced value of the dissipation strength depends strongly on the approximation applied for the time-dependent fission-decay width and is estimated to be of the order of 2x10**21 s**(-1). A careful analysis sheds severe doubts on the use of the exponential-like in-growth function largely used in the past. Finally, we discuss which should be the characteristics of experimental observables to be most sensitive to transient effects in fissionComment: 18 pages, 2 figures, background information on http://www-w2k.gsi.de/kschmidt

    Highly deformed 40^{40}Ca configurations in 28^{28}Si + 12^{12}C

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    The possible occurrence of highly deformed configurations in the 40^{40}Ca di-nuclear system formed in the 28^{28}Si + 12^{12}C reaction is investigated by analyzing the spectra of emitted light charged particles. Both inclusive and exclusive measurements of the heavy fragments (A \geq 10) and their associated light charged particles (protons and α\alpha particles) have been made at the IReS Strasbourg {\sc VIVITRON} Tandem facility at bombarding energies of Elab(28E_{lab} (^{28}Si) = 112 MeV and 180 MeV by using the {\sc ICARE} charged particle multidetector array. The energy spectra, velocity distributions, and both in-plane and out-of-plane angular correlations of light charged particles are compared to statistical-model calculations using a consistent set of parameters with spin-dependent level densities. The analysis suggests the onset of large nuclear deformation in 40^{40}Ca at high spin.Comment: 33 pages, 11 figure
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