465 research outputs found
DWBA analysis of the 13C(6Li,d)17O reaction at 10 MeV/nucleon and its astrophysical implications
The value of the alpha spectroscopic factor (S_alpha) of the 6.356 MeV 1/2+
state of 17O is believed to have significant astrophysical implications due to
the importance of the 13C(alpha,n)16O reaction as a possible source of neutron
production for the s process. To further study this effect, an accurate
measurement of the 13C(6Li,d)17O reaction at E_lab = 60 MeV has been performed
recently by Kubono et al., who found a new value for the spectroscopic factor
of the 6.356 MeV 1/2+ state of 17O based on a distorted wave Born approximation
(DWBA) analysis of these data. This new value, S_alpha approximately = 0.011,
is surprisingly much smaller than those used previously in astrophysical
calculations (S_alpha approximately = 0.3-0.7) and thus poses a serious
question as to the role of the 13C(alpha,n)16O reaction as a source of neutron
production. In this work we perform a detailed analysis of the same
13C(6Li,d)17O data within the DWBA as well as the coupled reaction channel
(CRC) formalism. Our analysis yields an S_alpha value of over an order of
magnitude larger than that of Kubono et al. for the 6.356 MeV 1/2+ state of
17O.Comment: 17 pages, 4 figures, minor changes, accepted by Nuclear Physics
Comparison of Solar Fine Structure Observed Simultaneously in Ly-{\alpha} and Mg II h
The Chromospheric Lyman Alpha Spectropolarimeter (CLASP) observed the Sun in
H I Lyman-{\alpha} during a suborbital rocket flight on September 3, 2015. The
Interface Region Imaging Telescope (IRIS) coordinated with the CLASP
observations and recorded nearly simultaneous and co-spatial observations in
the Mg II h&k lines. The Mg II h and Ly-{\alpha} lines are important
transitions, energetically and diagnostically, in the chromosphere. The
canonical solar atmosphere model predicts that these lines form in close
proximity to each other and so we expect that the line profiles will exhibit
similar variability. In this analysis, we present these coordinated
observations and discuss how the two profiles compare over a region of quiet
sun at viewing angles that approach the limb. In addition to the observations,
we synthesize both line profiles using a 3D radiation-MHD simulation. In the
observations, we find that the peak width and the peak intensities are well
correlated between the lines. For the simulation, we do not find the same
relationship. We have attempted to mitigate the instrumental differences
between IRIS and CLASP and to reproduce the instrumental factors in the
synthetic profiles. The model indicates that formation heights of the lines
differ in a somewhat regular fashion related to magnetic geometry. This
variation explains to some degree the lack of correlation, observed and
synthesized, between Mg II and Ly-{\alpha}. Our analysis will aid in the
definition of future observatories that aim to link dynamics in the
chromosphere and transition region.Comment: Accepted by Ap
The Chromospheric Telescope
We introduce the Chromospheric Telescope (ChroTel) at the Observatorio del
Teide in Izana on Tenerife as a new multi-wavelength imaging telescope for
full-disk synoptic observations of the solar chromosphere. We describe the
design of the instrument and summarize its performance during the first one and
a half years of operation. We present a method to derive line-of-sight velocity
maps of the full solar disk from filtergrams taken in and near the He I
infrared line at 10830 \AA.Comment: 12 pages, 9 figure
Filamentary pulse self-compression: The impact of the cell windows
Self-compression of multi-millijoule laser pulses during filamentary propagation is usually explained by the interplay of self-focusing and defocusing effects, causing a substantial concentration of energy on the axis of the propagating optical pulse. Recently, it has been argued that cell windows may play a decisive role in the self-compression mechanism. As such windows have to be used for media other than air their presence is often unavoidable, yet they present a sudden non-adiabatic change in dispersion and nonlinearity that should lead to a destruction of the temporal and spatial integrity of the light bullets generated in the self-compression mechanism. We now experimentally prove that there is in fact a self-healing mechanism that helps to overcome the potentially destructive consequences of the cell windows. We show in two carefully conducted experiments that the cell window position decisively influences activation or inhibition of the self-healing mechanism. A comparison with a windowless cell shows that presence of this mechanism is an important prerequisite for the exploitation of self-compression effects in windowed cells filled with inert gases
Fermi Surface and Electron Correlation Effects of Ferromagnetic Iron
The electronic band structure of bulk ferromagnetic iron is explored by
angle-resolved photoemission for electron correlation effects. Fermi surface
cross-sections as well as band maps are contrasted with density functional
calculations. The Fermi vectors and band parameters obtained from photoemission
and their prediction from band theory are analyzed in detail. Generally good
agreement is found for the Fermi surface. A bandwidth reduction for shallow
bands of ~ 30 % is observed. Additional strong quasiparticle renormalization
effects are found near the Fermi level, leading to a considerable mass
enhancement. The role of electronic correlation effects and the electronic
coupling to magnetic excitations is discussed in view of the experimental
results.Comment: 12 pages, 14 figures, 1 tabl
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Electronic sputtering and desorption effects in TOF-SIMS studies using slow highly charged ions like Au{sup 69+}
Secondary ion yields from highly oriented pyrolytic graphite (HOPG) and SiO{sub 2} (native oxide on float zone silicon) targets at impact of slow (v {approx} 0.3 v{sub bohr}) highly charged ions have been measured by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). A direct comparison of collisional and electronic effects in secondary ion production using a beam of charge state equilibrated 300 keV Xe{sup 1+} shows a secondary ion yield increase with incident ion charge of {>=}100
Magnetic fields of opposite polarity in sunspot penumbrae
Context. A significant part of the penumbral magnetic field returns below the
surface in the very deep photosphere. For lines in the visible, a large portion
of this return field can only be detected indirectly by studying its imprints
on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe
a narrow layer in the very deep photosphere, providing the possibility of
directly measuring the orientation of magnetic fields close to the solar
surface.
Aims. We study the topology of the penumbral magnetic field in the lower
photosphere, focusing on regions where it returns below the surface.
Methods. We analyzed 71 spectropolarimetric datasets from Hinode and from the
GREGOR infrared spectrograph. We inferred the quality and polarimetric accuracy
of the infrared data after applying several reduction steps. Techniques of
spectral inversion and forward synthesis were used to test the detection
algorithm. We compared the morphology and the fractional penumbral area covered
by reversed-polarity and three-lobed Stokes V profiles for sunspots at disk
center. We determined the amount of reversed-polarity and three-lobed Stokes V
profiles in visible and infrared data of sunspots at various heliocentric
angles. From the results, we computed center-to-limb variation curves, which
were interpreted in the context of existing penumbral models.
Results. Observations in visible and near-infrared spectral lines yield a
significant difference in the penumbral area covered by magnetic fields of
opposite polarity. In the infrared, the number of reversed-polarity Stokes V
profiles is smaller by a factor of two than in the visible. For three-lobed
Stokes V profiles the numbers differ by up to an order of magnitude.Comment: 11 pages 10 figures plus appendix (2 pages 3 figures). Accepted as
part of the A&A special issue on the GREGOR solar telescop
Multilayer neural networks with extensively many hidden units
The information processing abilities of a multilayer neural network with a
number of hidden units scaling as the input dimension are studied using
statistical mechanics methods. The mapping from the input layer to the hidden
units is performed by general symmetric Boolean functions whereas the hidden
layer is connected to the output by either discrete or continuous couplings.
Introducing an overlap in the space of Boolean functions as order parameter the
storage capacity if found to scale with the logarithm of the number of
implementable Boolean functions. The generalization behaviour is smooth for
continuous couplings and shows a discontinuous transition to perfect
generalization for discrete ones.Comment: 4 pages, 2 figure
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