1,396 research outputs found
Comparing non-perturbative models of the breakup of neutron-halo nuclei
Breakup reactions of loosely-bound nuclei are often used to extract structure
and/or astrophysical information. Here we compare three non-perturbative
reaction theories often used when analyzing breakup experiments, namely the
continuum discretized coupled channel model, the time-dependent approach
relying on a semiclassical approximation, and the dynamical eikonal
approximation. Our test case consists of the breakup of 15C on Pb at 68
MeV/nucleon and 20 MeV/nucleon.Comment: 8 pages, 6 figures, accepted for publication in Phys. Rev.
Deducing spectroscopic factors from wave-function asymptotics
In a coupled-channel model, we explore the effects of coupling between
configurations on the radial behavior of the wave function and, in particular,
on the spectroscopic factor (SF) and the asymptotic normalization coefficient
(ANC). We evaluate the extraction of a SF from the ratio of the ANC of the
coupled-channel model to that of a single-particle approximation of the wave
function. We perform this study within a core + n collective model, which
includes two states of the core that connect by a rotational coupling. To get
additional insights, we also use a simplified model that takes a delta function
for the coupling potential. Calculations are performed for 11Be. Fair agreement
is obtained between the SF inferred from the single-particle approximation and
the one obtained within the coupled-channel models. Significant discrepancies
are observed only for large coupling strength and/or large admixture, that is,
a small SF. This suggests that reliable SFs can be deduced from the
wave-function asymptotics when the structure is dominated by one configuration,
that is, for a large SF.Comment: Title correcte
Asymptotic normalization of mirror states and the effect of couplings
Assuming that the ratio between asymptotic normalization coefficients of
mirror states is model independent, charge symmetry can be used to indirectly
extract astrophysically relevant proton capture reactions on proton-rich nuclei
based on information on stable isotopes. The assumption has been tested for
light nuclei within the microscopic cluster model. In this work we explore the
Hamiltonian independence of the ratio between asymptotic normalization
coefficients of mirror states when deformation and core excitation is
introduced in the system. For this purpose we consider a phenomenological rotor
+ N model where the valence nucleon is subject to a deformed mean field and the
core is allowed to excite. We apply the model to 8Li/8B, 13C/13N, 17O/17F,
23Ne/23Al, and 27Mg/27P. Our results show that for most studied cases, the
ratio between asymptotic normalization coefficients of mirror states is
independent of the strength and multipolarity of the couplings induced. The
exception is for cases in which there is an s-wave coupled to the ground state
of the core, the proton system is loosely bound, and the states have large
admixture with other configurations. We discuss the implications of our results
for novae.Comment: 8 pages, 2 figures, submitted to PR
Mechanisms of direct reactions with halo nuclei
Halo nuclei are exotic nuclei which exhibit a strongly clusterised structure:
they can be seen as one or two valence nucleons loosely bound to a core. Being
observed at the ridge of the valley of stability, halo nuclei are studied
mostly through reactions. In this contribution the reaction models most
commonly used to analyse experimental data are reviewed and compared to one
another. A reaction observable built on the ratio of two angular distributions
is then presented. This ratio enables removing most of the sensitivity to the
reaction mechanism, which emphasises the effects of nuclear structure on the
reaction.Comment: Invited talk given by Pierre Capel at the "10th International
Conference on Clustering Aspects of Nuclear Structure and Dynamics"
(Cluster12), Debrecen, Hungary, 24-28 September 2012. To appear in the
Cluster12 Proceedings in the Open Access Journal of Physics: Conference
Series (JPCS). (5 pages, 3 figures
Peripherality of breakup reactions
The sensitivity of elastic breakup to the interior of the projectile wave
function is analyzed. Breakup calculations of loosely bound nuclei (8B and
11Be) are performed with two different descriptions of the projectile. The
descriptions differ strongly in the interior of the wave function, but exhibit
identical asymptotic properties, namely the same asymptotic normalization
coefficient, and phase shifts. Breakup calculations are performed at
intermediate energies (40-70 MeV/nucleon) on lead and carbon targets as well as
at low energy (26 MeV) on a nickel target. No dependence on the projectile
description is observed. This result confirms that breakup reactions are
peripheral in the sense that they probe only the external part of the wave
function. These measurements are thus not directly sensitive to the total
normalization of the wave function, i.e. spectroscopic factor.Comment: Reviewed version accepted for publication in Phys. Rev. C; 1 new
section (Sec. III E), 2 new figures (Figs. 3 and 5
Recent developments in the eikonal description of the breakup of exotic nuclei
The study of exotic nuclear structures, such as halo nuclei, is usually
performed through nuclear reactions. An accurate reaction model coupled to a
realistic description of the projectile is needed to correctly interpret
experimental data. In this contribution, we briefly summarise the assumptions
made within the modelling of reactions involving halo nuclei. We describe
briefly the Continuum-Discretised Coupled Channel method (CDCC) and the
Dynamical Eikonal Approximation (DEA) in particular and present a comparison
between them for the breakup of 15C on Pb at 68AMeV. We show the problem faced
by the models based on the eikonal approximation at low energy and detail a
correction that enables their extension down to lower beam energies. A new
reaction observable is also presented. It consists of the ratio between angular
distributions for two different processes, such as elastic scattering and
breakup. This ratio is completely independent of the reaction mechanism and
hence is more sensitive to the projectile structure than usual reaction
observables, which makes it a very powerful tool to study exotic structures far
from stability.Comment: Contribution to the proceedings of the XXI International School on
Nuclear Physics and Applications & the International Symposium on Exotic
Nuclei, dedicated to the 60th Anniversary of the JINR (Dubna) (Varna,
Bulgaria, 6-12 September 2015), 7 pages, 4 figure
Natural flow regime, degree of alteration and environmental ows in the Mula stream (Segura River basin, SE Spain)
Natural ftow regime, degree of alteration and environmental ftows in the Mula stream (Segura River basin, SE Spain) The Mula stream, a tributary located in the southern part of the Segura River basin, possesses habitats and species of European interest for which it has been declared Special Area of Conservation (SAC) within the Natura 2000 network. However, strong agricultural pressures on the superficial and groundwater resources of the Mula stream are threatening its ecological state. A characterisation of the natural flow regime of the stream and its main tributaries was carried out to design environmental flow regimes (EFRs) and make the conservation of the ecosystem compatible with agricultural requirements. Natural hydrographs, based on daily data corresponding to pre-dam periods, showed a high inter- and intra-annual variability typical of Mediterranean streams, with high flows in autumn and spring and low flows in summer. After the construction of the La Cierva reservoir in the Mula stream, the flow regime has been progressively altered in parallel with the expansion of irrigated agriculture in the watershed. The current regime shows a significant reduction in the magnitude of flows and a reversal of the seasonal pattero, with droughts during winter instead of summer months becoming more frequent and long-Iasting. Different EFRs are presented for three conservation scenarios of natural flow regimes in wet, average and dry years. Once the environmental flows were subtracted from the natural ones, the available water resources for agriculture were clearly insufficient in al! studied scenarios, which poses problems for the implementation of such environmental flows in this basin.El Río Mula, un afluente localizado en la parte meridional de la cuenca del Río Segura, posee una elevada riqueza de hábitats de interés comunitario por los que ha sido declarado UC dentro de la Red Natura 2000. Sin embargo, la fuerte presión agrícola sobre sus recursos superficiales y subterráneos amenaza su estado ecológico. En este estudio se ha llevado a cabo una caracterización del régimen natural de caudales del río y sus principales afluentes con el fin de diseñar Regímenes Ambientales de Caudales (RACs) y hacer la conservación del ecosistemafluvial compatible con las demandas agrícolas. Los hidrogramas naturales, obtenidos a partir de registros diarios de aforos correspondientes a periodos previos a la construcción de embalses, mostraron una alta variabilidad inter e intranual típica de los ríos mediterráneos, con altos caudales en otoño y primavera y bajos en verano. Tras la construcción del embalse de La Cierva en el Río Mula el régimen de caudales ha sido alterado progresivamente en paralelo a la expansión de la agricultura de regadío en la cuenca. El régimen actual muestra una reducción significativa en la magnitud de los caudales y una inversión del patrón estacional, es decir, sequías en invierno en lugar de en verano y cada vez más frecuentes y duraderas. Se presentan los diferentes RACs estimados para tres escenarios de conservación de los regímenes naturales de caudal, tanto para años húmedos como para medios y secos. En todos los escenarios estudiados, los recursos hídricos disponibles para la agricultura, previa detracción de los caudales ambientales, son claramente insuficientes, lo que dificulta la implementación de dichos caudales ambientales en esta cuenca
Energy dependence of non-local potentials
Recently a variety of studies have shown the importance of including
non-locality in the description of reactions. The goal of this work is to
revisit the phenomenological approach to determining non-local optical
potentials from elastic scattering. We perform a analysis of neutron
elastic scattering data off Ca, Zr and Pb at energies MeV, assuming a Perey and Buck or Tian, Pang, and Ma non-local
form for the optical potential. We introduce energy and asymmetry dependencies
in the imaginary part of the potential and refit the data to obtain a global
parameterization. Independently of the starting point in the minimization
procedure, an energy dependence in the imaginary depth is required for a good
description of the data across the included energy range. We present two
parameterizations, both of which represent an improvement over the original
potentials for the fitted nuclei as well as for other nuclei not included in
our fit. Our results show that, even when including the standard Gaussian
non-locality in optical potentials, a significant energy dependence is required
to describe elastic-scattering data.Comment: 6 pages, 3 figures, accepted by Phys. Rev. C Rapid Communicatio
Measuring individuals' response quality in self-administered psychological tests : an introduction to Gendre's functional method
The functional method is a new test theory using a new scoring method that assumes complexity in test structure, and thus takes into account every correlation between factors and items. The main specificity of the functional method is to model test scores by multiple regression instead of estimating them by using simplistic sums of points. In order to proceed, the functional method requires the creation of hyperspherical measurement space, in which item responses are expressed by their correlation with orthogonal factors. This method has three main qualities. First, measures are expressed in the absolute metric of correlations; therefore, items, scales and persons are expressed in the same measurement space using the same single metric. Second, factors are systematically orthogonal and without errors, which is optimal in order to predict other outcomes. Such predictions can be performed to estimate how one would answer to other tests, or even to model one's response strategy if it was perfectly coherent. Third, the functional method provides measures of individuals' response validity (i.e., control indices). Herein, we propose a standard procedure in order to identify whether test results are interpretable and to exclude invalid results caused by various response biases based on control indices
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