2,646 research outputs found
Electron capture to continuum in collisions of bare projectiles with Ne targets
Abstract. We have investigated the cusp resulting from electron capture to the continuum of 1.25-5 MeV m u- ' fully stripped hydrogen and oxygen as a function of the collision energy and the detector angular resolution B o. It is revealed that the characteristic cusp shape parameters depend strongly on the experimental resolution. Our experimental data are "pared with the second-order Born theory and the impulse approximation. Both theories mnhrm the 8, dependence of the shape parameters and gjve a reasonable descrip-lion of the cusp asymmetry. However, theory tends to overestimate the absolute cross sections, in particular in the case of oxygen. 1
Excitation of the electric pygmy dipole resonance by inelastic electron scattering
To complete earlier studies of the properties of the electric pygmy dipole
resonance (PDR) obtained in various nuclear reactions, the excitation of the
1 states in Ce by scattering for momentum transfers
~fm is calculated within the plane-wave and distorted-wave
Born approximations. The excited states of the nucleus are described within the
Quasiparticle Random Phase Approximation (QRPA), but also within the
Quasiparticle-Phonon Model (QPM) by accounting for the coupling to complex
configurations. It is demonstrated that the excitation mechanism of the PDR
states in reactions is predominantly of transversal nature for
scattering angles . Being thus mediated by the
convection and spin nuclear currents, the like the
reaction, may provide additional information to the one obtained from Coulomb-
and hadronic excitations of the PDR in , , and
heavy-ion scattering reactions. The calculations predict that the
cross sections for the strongest individual PDR states are in general about
three orders of magnitude smaller as compared to the one of the lowest
state for the studied kinematics, but that they may become dominant at extreme
backward angles.Comment: Prepared for the special issue of EPJA on the topic "Giant, Pygmy,
Pairing Resonances and related topics" dedicated to the memory of Pier
Francesco Bortigno
QED corrections to elastic electron-nucleus scattering beyond the first-order Born approximation
A potential for the vertex and self-energy correction is derived from the
first-order Born theory. The inclusion of this potential in the Dirac equation,
together with the Uehling potential for vacuum polarization, allows for a
nonperturbative treatment of these QED effects within the phase-shift analysis.
Investigating the 12C and 208Pb targets, a considerable deviation of the
respective cross-section change from the Born results is found, which becomes
larger with increasing momentum transfer. Estimates for the correction to the
beam-normal spin asymmetry are also provided. For the 12C nucleus, dispersion
effects are considered as well.Comment: 9 pages, 10 figure
Fingering convection and cloudless models for cool brown dwarf atmospheres
This work aims to improve the current understanding of the atmospheres of
brown dwarfs, especially cold ones with spectral type T and Y, whose modeling
is a current challenge. Silicate and iron clouds are believed to disappear at
the photosphere at the L/T transition, but cloudless models fail to reproduce
correctly the spectra of T dwarfs, advocating for the addition of more physics,
e.g. other types of clouds or internal energy transport mechanisms. We use a
one-dimensional (1D) radiative/convective equilibrium code ATMO to investigate
this issue. This code includes both equilibrium and out-of-equilibrium
chemistry and solves consistently the PT structure. Included opacity sources
are H2-H2, H2-He, H2O, CO, CO2, CH4, NH3, K, Na, and TiO, VO if they are
present in the atmosphere. We show that the spectra of Y dwarfs can be
accurately reproduced with a cloudless model if vertical mixing and NH3
quenching are taken into account. T dwarf spectra still have some reddening in
e.g. J - H compared to cloudless models. This reddening can be reproduced by
slightly reducing the temperature gradient in the atmosphere. We propose that
this reduction of the stabilizing temperature gradient in these layers, leading
to cooler structures, is due to the onset of fingering convection, triggered by
the destabilizing impact of condensation of very thin dust.Comment: Accepted in ApJ
- …