850 research outputs found
Comparison of Transfer-to-Continuum and Eikonal Models of Projectile Fragmentation Reactions
Spectroscopic properties of nuclei are accessible with projectile
fragmentation reactions, but approximations made in the reaction theory can
limit the accuracy of the determinations. We examine here two models that have
rather different approximations for the nucleon wave function, the target
interaction, and the treatment of the finite duration of the reaction. The
nucleon-target interaction is treated differently in the eikonal and the
transfer-to-continuum model, but the differences are more significant for light
targets. We propose a new parameterization with that in mind. We also propose a
new formula to calculate the amplitude that combines the better treatment of
the wave function in the eikonal model with the better treatment of the target
interaction in the transfer-to-continuum model.Comment: 21 pages, latex file including 3 tables. 5 figures. Submitted to
Phys. Rev.
Initial State Dependence of the Breakup of Weakly Bound Carbon Isotopes
The one-neutron nuclear breakup from the Carbon isotopes C and
C, is calculated as an example of application of the theory of transfer
to the continuum reactions in the formulation which includes spin coupling.
The effect of the energy sharing between the parallel and transverse neutron
momentum distributions is taken into account thus resulting in a theory which
is more general than sudden eikonal approaches. Both effects are necessary to
understand properly the breakup from not too weakly bound orbitals.
Breakup which leaves the core into an excited state below particle threshold is
also considered. The core-target interaction is treated in the smooth cut-off
approximation. By comparing to presently available experimental data we show
how to make some hypothesis on the quantum numbers and occupancy of the neutron
initial state. Possible ambiguities in the interpretation of inclusive cross
sections are discussed.Comment: 22 RevTeX pages,3 ps figures. Phys. Rev. C, accepte
Imaginary part of the C 9 − Be 9 single-folded optical potential
In a recent publication we have argued that using two very successful n\text{\ensuremath{-}}^{9}\mathrm{Be} optical potentials [A. Bonaccorso and R. J. Charity, Phys. Rev. C 89, 024619 (2014)] and microscopic projectile densities, it is possible to build a single-folded (light-) nucleus- imaginary optical potential which is more accurate than a double-folded optical potential. By comparing to experimental reaction cross sections, we showed for , and projectiles, that a very good agreement between theory and data could be obtained with such a ``bare'' potential, at all but the lowest energies where a small semimicroscopic surface term was added to the single-folded potential to take into account projectile breakup. In this paper we extend this study to the case of projectiles and assess the sensitivity to the projectile density used. We then obtained the modulus of the nucleus-nucleus matrix and parametrize it in terms of a strong-absorption radius and finally extracted the phenomenological energy dependence of this radius. This approach could be the basis for a systematic study of optical potentials for light exotic nuclei scattering on light targets and/or parametrizations of the matrix. Furthermore our study will serve to make a quantitative assessment of the description of the core-target part of knockout reactions, in particular their localization in terms of impact parameters
Proton vs. neutron halo breakup
In this paper we show how effective parameters such as effective binding
energies can be defined for a proton in the combined nuclear-Coulomb potential,
including also the target potential, in the case in which the proton is bound
in a nucleus which is partner of a nuclear reaction. Using such effective
parameters the proton behaves similarly to a neutron. In this way some
unexpected results obtained from dynamical calculations for reactions initiated
by very weakly bound proton halo nuclei can be interpreted. Namely the fact
that stripping dominates the nuclear breakup cross section which in turn
dominates over the Coulomb breakup even when the target is heavy at medium to
high incident energies. Our interpretation helps also clarifying why the
existence and characteristics of a proton halo extracted from different types
of data have sometimes appeared contradictory.Comment: 7 Latex pages, 3 table, 3 ps figures, to appear in Phys. Rev.
Extended sudden approximation model for high-energy nucleon removal reactions
A model based on the sudden approximation has been developed to describe high
energy single nucleon removal reactions. Within this approach, which takes as
its starting point the formalism of Hansen \cite{Anne2}, the nucleon-removal
cross section and the full 3-dimensional momentum distributions of the core
fragments including absorption, diffraction, Coulomb and nuclear-Coulomb
interference amplitudes, have been calculated. The Coulomb breakup has been
treated to all orders for the dipole interaction. The model has been compared
to experimental data for a range of light, neutron-rich psd-shell nuclei. Good
agreement was found for both the inclusive cross sections and momentum
distributions. In the case of C, comparison is also made with the
results of calculations using the transfer-to-the-continuum model. The
calculated 3-dimensional momentum distributions exhibit longitudinal and
transverse momentum components that are strongly coupled by the reaction for
s-wave states, whilst no such effect is apparent for d-waves. Incomplete
detection of transverse momenta arising fromlimited experimental acceptances
thus leads to a narrowing of the longitudinal distributions for nuclei with
significant s-wave valence neutron configurations, as confirmed by the data.
Asymmetries in the longitudinal momentum distributions attributed to
diffractive dissociation are also explored.Comment: 16 figures, submitted to Phys. Rev.
Evanescent-wave coupled right angled buried waveguide: Applications in carbon nanotube mode-locking
In this paper we present a simple but powerful subgraph sampling primitive
that is applicable in a variety of computational models including dynamic graph
streams (where the input graph is defined by a sequence of edge/hyperedge
insertions and deletions) and distributed systems such as MapReduce. In the
case of dynamic graph streams, we use this primitive to prove the following
results:
-- Matching: First, there exists an space algorithm that
returns an exact maximum matching on the assumption the cardinality is at most
. The best previous algorithm used space where is the
number of vertices in the graph and we prove our result is optimal up to
logarithmic factors. Our algorithm has update time. Second,
there exists an space algorithm that returns an
-approximation for matchings of arbitrary size. (Assadi et al. (2015)
showed that this was optimal and independently and concurrently established the
same upper bound.) We generalize both results for weighted matching. Third,
there exists an space algorithm that returns a constant
approximation in graphs with bounded arboricity.
-- Vertex Cover and Hitting Set: There exists an space
algorithm that solves the minimum hitting set problem where is the
cardinality of the input sets and is an upper bound on the size of the
minimum hitting set. We prove this is optimal up to logarithmic factors. Our
algorithm has update time. The case corresponds to minimum
vertex cover.
Finally, we consider a larger family of parameterized problems (including
-matching, disjoint paths, vertex coloring among others) for which our
subgraph sampling primitive yields fast, small-space dynamic graph stream
algorithms. We then show lower bounds for natural problems outside this family
Core excitation in Coulomb breakup reactions
Within the pure Coulomb breakup mechanism, we investigate the one-neutron
removal reaction of the type A(a,b)X with Be and C
projectiles on a heavy target nucleus Pb at the beam energy of 60
MeV/nucleon. Our intention is to examine the prospective of using these
reactions to study the structure of neutron rich nuclei. Integrated partial
cross sections and momentum distributions for the ground as well as excited
bound states of core nuclei are calculated within the finite range distorted
wave Born approximation as well as within the adiabatic model of the Coulomb
breakup. Our results are compared with those obtained in the studies of the
reactions on a light target where the breakup proceeds via the pure nuclear
mechanism. We find that the transitions to excited states of the core are quite
weak in the Coulomb dominated process as compared to the pure nuclear breakup.Comment: Revtex format, five postscript figures included, to appear in Phys.
Rev.
Intrinsic response time of graphene photodetectors
Graphene-based photodetectors are promising new devices for high-speed
optoelectronic applications. However, despite recent efforts, it is not clear
what determines the ultimate speed limit of these devices. Here, we present
measurements of the intrinsic response time of metal-graphene-metal
photodetectors with monolayer graphene using an optical correlation technique
with ultrashort laser pulses. We obtain a response time of 2.1 ps that is
mainly given by the short lifetime of the photogenerated carriers. This time
translates into a bandwidth of ~262 GHz. Moreover, we investigate the
dependence of the response time on gate voltage and illumination laser power
Continuous dynamic response along a pre-existing structural discontinuity induced by the 2001 eruption at Mt. Etna
The intrusive process of the 2001 Mt. Etna eruption was accompanied by marked ground deformation and relevant seismic activity recorded between 12 and 17 of July (INGV-CT, 2001). At the same time, extensometer data evidenced the re-activation of a dry surface failure zone on the high south-eastern sector of Mt. Etna; this fracture system, formed in 1989, has been related to the propagation of a shallow blade-like dike along a NNESSW discontinuity (Bonaccorso and Davis, 1993; Bianco et al., 1998). The NNW-SSE discontinuity represents a complex low cohesion structure in which deformation may concentrate. Displacement measurements recorded on the surface fracture and the constraints obtained from seismicity show that the intrusion phase of the 2001 eruption has forced the NNE-SSW structure to move continuously with prevalent left-lateral displacement from a depth of 2–2.5 km b. s. l. to the surface with a compositive slip of about 3–5 centimeters.Published56/ (2004)ope
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