3,048 research outputs found
Fission fragment angular distribution measurements of U-235 and U-238 at CERN n_TOF facility
Neutron-induced fission cross sections of U-238 and U-235 are used as standards in the fast neutron region up to 200 MeV. A high accuracy of the standards is relevant to experimentally determine other neutron reaction cross sections. Therefore, the detection efficiency should be corrected by using the angular distribution of the fission fragments (FFAD), which are barely known above 20 MeV. In addition, the angular distribution of the fragments produced in the fission of highly excited and deformed nuclei is an important observable to investigate the nuclear fission process.
In order to measure the FFAD of neutron-induced reactions, a fission detection setup based on parallel-plate avalanche counters (PPACs) has been developed and successfully used at the CERN-n_TOF facility. In this work, we present the preliminary results on the analysis of new U-235(n,f) and U-238(n,f) data in the extended energy range up to 200 MeV compared to the existing experimental data.Postprint (published version
Chemical analysis of NGC 6528: one of the most metal-rich bulge globular cluster
The Bulge Globular Clusters (GCs) are key tracers of this central ancient
component of our Galaxy. It is essential to understand their formation and
evolution to study that of the bulge, as well as their relationship with the
other Galactic GC systems (halo and disk GCs). Our main goals are to obtain
detailed abundances for a sample of seven red giant members of NGC 6528 in
order to characterize its chemical composition and study the relationship of
this GC with the bulge, and with other bulge, halo and disk GCs. Moreover, we
analyze this clusters behavior associated with the Multiple Populations
(MPs) phenomenon. We obtained the stellar parameters and chemical abundances of
light elements (Na, Al), iron-peak elements (V, Cr, Mn, Fe, Co, Ni, Cu),
{\alpha}-elements (O, Mg, Si, Ca, Ti) and heavy elements (Zr, Ba, Eu) in seven
red giant members of NGC 6528 using high resolution spectroscopy from
FLAMES-UVES. We obtained in six stars of our sample a mean iron content of
[Fe/H]=-0.14+/-0.03 dex, in good agreement with other studies. We found no
significant internal iron spread. We detected one candidate variable star,
which was excluded from the mean in iron content, we derived a metallicity in
this star of [Fe/H]=-0.55+/-0.04 dex. Moreover, we found no extended O-Na
anticorrelation but instead only an intrinsic Na spread. In addition, NGC 6528
does not exhibit a Mg-Al anticorrelation, and no significant spread in either
Mg or Al. The {\alpha} and iron-peak elements show good agreement with the
bulge field star trend. The heavy elements are slightly dominated by the
r-process. The chemical analysis suggests an origin and evolution similar to
that of typical old Bulge field stars. Finally, we find remarkable agreement in
the chemical patterns of NGC 6528 and another bulge GC, NGC 6553, suggesting a
similar origin and evolution.Comment: Accepted for publication in A&A. 12 pages, 13 figures, 4 table
NGC 5694: another foster son of the Galactic Halo
We present the results of the analysis of high-resolution spectra obtained
with UVES-FLAMES@VLT for six red giant branch stars in the outer-halo
metal-poor ([Fe/H]I=-1.98 and [Fe/H]II=-1.83) Galactic globular cluster NGC
5694, which has been suggested as a possible incomer by Lee et al. (2006) based
on the anomalous chemical composition of a single cluster giant. We obtain
accurate abundances for a large number of elements and we find that: (a) the
six target stars have the same chemical composition within the uncertainties,
except for Na and Al; (b) the average cluster abundance of \alpha\ elements
(with the only exception of Si) is nearly solar, at odds with typical halo
stars and globular clusters of similar metallicity; (c) Y, Ba, La and Eu
abundances are also significantly lower than in Galactic field stars and star
clusters of similar metallicity. Hence we confirm the Lee et al. classification
of NGC 5694 as a cluster of extra-galactic origin. We provide the first insight
on the Na-O and Mg-Al anti-correlations in this cluster: all the considered
stars have very similar abundance ratios for these elements, except one that
has significantly lower [Na/Fe] and [Al/Fe] ratios, suggesting that some degree
of early self-enrichment has occurred also in this cluster.Comment: Accepted for publication by MNRAS. 15 pages, 8 figures, 4 table
Light-cone quantization of two dimensional field theory in the path integral approach
A quantization condition due to the boundary conditions and the
compatification of the light cone space-time coordinate is identified at
the level of the classical equations for the right-handed fermionic field in
two dimensions. A detailed analysis of the implications of the implementation
of this quantization condition at the quantum level is presented. In the case
of the Thirring model one has selection rules on the excitations as a function
of the coupling and in the case of the Schwinger model a double integer
structure of the vacuum is derived in the light-cone frame. Two different
quantized chiral Schwinger models are found, one of them without a
-vacuum structure. A generalization of the quantization condition to
theories with several fermionic fields and to higher dimensions is presented.Comment: revtex, 14 p
Momentum and energy preserving integrators for nonholonomic dynamics
In this paper, we propose a geometric integrator for nonholonomic mechanical
systems. It can be applied to discrete Lagrangian systems specified through a
discrete Lagrangian defined on QxQ, where Q is the configuration manifold, and
a (generally nonintegrable) distribution in TQ. In the proposed method, a
discretization of the constraints is not required. We show that the method
preserves the discrete nonholonomic momentum map, and also that the
nonholonomic constraints are preserved in average. We study in particular the
case where Q has a Lie group structure and the discrete Lagrangian and/or
nonholonomic constraints have various invariance properties, and show that the
method is also energy-preserving in some important cases.Comment: 18 pages, 6 figures; v2: example and figures added, minor correction
to example 2; v3: added section on nonholonomic Stoermer-Verlet metho
Stellar parameters for stars of the CoRoT exoplanet field
Aims:To support the computation and evolutionary interpretation of periods
associated with the rotational modulation, oscillations, and variability of
stars located in the CoRoT fields, we are conducting a spectroscopic survey for
stars located in the fields already observed by the satellite. These
observations allow us to compute physical and chemical parameters for our
stellar sample. Method: Using spectroscopic observations obtained with UVES/VLT
and Hydra/Blanco, and based on standard analysis techniques, we computed
physical and chemical parameters (, , ,
, , , and ) for a large
sample of CoRoT targets. Results: We provide physical and chemical parameters
for a sample comprised of 138 CoRoT targets. Our analysis shows the stars in
our sample are located in different evolutionary stages, ranging from the main
sequence to the red giant branch, and range in spectral type from F to K. The
physical and chemical properties for the stellar sample are in agreement with
typical values reported for FGK stars. However, we report three stars
presenting abnormal lithium behavior in the CoRoT fields. These parameters
allow us to properly characterize the intrinsic properties of the stars in
these fields. Our results reveal important differences in the distributions of
metallicity, , and evolutionary status for stars belonging to
different CoRoT fields, in agreement with results obtained independently from
ground-based photometric surveys. Conclusions: Our spectroscopic catalog, by
providing much-needed spectroscopic information for a large sample of CoRoT
targets, will be of key importance for the successful accomplishment of several
different programs related to the CoRoT mission, thus it will help further
boost the scientific return associated with this space mission.Comment: 43 pages, 17 figures, accepted for publication in A&
Special Geometry of Euclidean Supersymmetry III: the local r-map, instantons and black holes
We define and study projective special para-Kahler manifolds and show that
they appear as target manifolds when reducing five-dimensional vector
multiplets coupled to supergravity with respect to time. The dimensional
reductions with respect to time and space are carried out in a uniform way
using an epsilon-complex notation. We explain the relation of our formalism to
other formalisms of special geometry used in the literature. In the second part
of the paper we investigate instanton solutions and their dimensional lifting
to black holes. We show that the instanton action, which can be defined after
dualising axions into tensor fields, agrees with the ADM mass of the
corresponding black hole. The relation between actions via Wick rotation, Hodge
dualisation and analytic continuation of axions is discussed.Comment: 72 pages, 2 figure
A general framework for nonholonomic mechanics: Nonholonomic Systems on Lie affgebroids
This paper presents a geometric description of Lagrangian and Hamiltonian
systems on Lie affgebroids subject to affine nonholonomic constraints. We
define the notion of nonholonomically constrained system, and characterize
regularity conditions that guarantee that the dynamics of the system can be
obtained as a suitable projection of the unconstrained dynamics. It is shown
that one can define an almost aff-Poisson bracket on the constraint AV-bundle,
which plays a prominent role in the description of nonholonomic dynamics.
Moreover, these developments give a general description of nonholonomic systems
and the unified treatment permits to study nonholonomic systems after or before
reduction in the same framework. Also, it is not necessary to distinguish
between linear or affine constraints and the methods are valid for explicitly
time-dependent systems.Comment: 50 page
- …