88 research outputs found
Reaction mechanisms in the scattering of 8 Li on 208 Pb around the Coulomb barrier
We present an analysis of the reaction
8
Li
+
208
Pb
,
which has been recently measured at energies around the Coulomb barrier. The study is focused on the elastic and one-neutron removal channels. The elastic angular distribution has been analyzed by means of the optical model and coupled-channels method. The measured
7
Li
yield has been assumed to come from the one-neutron transfer reaction
208
Pb
(
8
Li
,
7
Li
)
209
Pb
and the breakup reaction
8
Li
+
208
Pb
→
n
+
7
Li
+
208
Pb
.
For the former, an analysis in terms of the distorted wave Born approximation and coupled-channels Born approximation (CCBA) methods has been performed. For the breakup reaction, a generalization of the CCBA method has been used, in which the final states are described as
n
+
208
Pb
continuum states. Transfer and breakup are then treated consistently. The summed angular distribution for the two processes reproduces the shape and absolute magnitude of the measured
7
Li
distribution. We conclude that most of the measured
7
Li
yield comes from the one-neutron transfer channel. We also show that the elastic scattering is notably affected by the strong coupling to transfer channels.Fundaçao para a Ciência e a Tecnología (FCT) de Portugal. POCTI/36282/99Ministerio de Ciencia y Tecnología de España. FPA2002-04181-C04-0
Production of highly-polarized positrons using polarized electrons at MeV energies
The Polarized Electrons for Polarized Positrons experiment at the injector of
the Continuous Electron Beam Accelerator Facility has demonstrated for the
first time the efficient transfer of polarization from electrons to positrons
produced by the polarized bremsstrahlung radiation induced by a polarized
electron beam in a high- target. Positron polarization up to 82\% have been
measured for an initial electron beam momentum of 8.19~MeV/, limited only by
the electron beam polarization. This technique extends polarized positron
capabilities from GeV to MeV electron beams, and opens access to polarized
positron beam physics to a wide community.Comment: 5 pages, 4 figure
Bottom-up assembly of metallic germanium
Extending chip performance beyond current limits of miniaturisation requires new materials and functionalities that integrate well with the silicon platform. Germanium fits these requirements and has been proposed as a high-mobility channel material, a light emitting medium in silicon-integrated lasers, and a plasmonic conductor for bio-sensing. Common to these diverse applications is the need for homogeneous, high electron densities in three-dimensions (3D). Here we use a bottom-up approach to demonstrate the 3D assembly of atomically sharp doping profiles in germanium by a repeated stacking of two-dimensional (2D) high-density phosphorus layers. This produces high-density (1019 to 1020 cm-3) low-resistivity (10-4Ω ∙ cm) metallic germanium of precisely defined thickness, beyond the capabilities of diffusion-based doping technologies. We demonstrate that free electrons from distinct 2D dopant layers coalesce into a homogeneous 3D conductor using anisotropic quantum interference measurements, atom probe tomography, and density functional theory
Breakup reaction models for two- and three-cluster projectiles
Breakup reactions are one of the main tools for the study of exotic nuclei,
and in particular of their continuum. In order to get valuable information from
measurements, a precise reaction model coupled to a fair description of the
projectile is needed. We assume that the projectile initially possesses a
cluster structure, which is revealed by the dissociation process. This
structure is described by a few-body Hamiltonian involving effective forces
between the clusters. Within this assumption, we review various reaction
models. In semiclassical models, the projectile-target relative motion is
described by a classical trajectory and the reaction properties are deduced by
solving a time-dependent Schroedinger equation. We then describe the principle
and variants of the eikonal approximation: the dynamical eikonal approximation,
the standard eikonal approximation, and a corrected version avoiding Coulomb
divergence. Finally, we present the continuum-discretized coupled-channel
method (CDCC), in which the Schroedinger equation is solved with the projectile
continuum approximated by square-integrable states. These models are first
illustrated by applications to two-cluster projectiles for studies of nuclei
far from stability and of reactions useful in astrophysics. Recent extensions
to three-cluster projectiles, like two-neutron halo nuclei, are then presented
and discussed. We end this review with some views of the future in
breakup-reaction theory.Comment: Will constitute a chapter of "Clusters in Nuclei - Vol.2." to be
published as a volume of "Lecture Notes in Physics" (Springer
Dendritic cell deficiencies persist seven months after SARS-CoV-2 infection
Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV)-2 infection induces an exacerbated inflammation driven by innate immunity components. Dendritic cells (DCs) play a key role in the defense against viral infections, for instance plasmacytoid DCs (pDCs), have the capacity to produce vast amounts of interferon-alpha (IFN-α). In COVID-19 there is a deficit in DC numbers and IFN-α production, which has been associated with disease severity. In this work, we described that in addition to the DC deficiency, several DC activation and homing markers were altered in acute COVID-19 patients, which were associated with multiple inflammatory markers. Remarkably, previously hospitalized and nonhospitalized patients remained with decreased numbers of CD1c+ myeloid DCs and pDCs seven months after SARS-CoV-2 infection. Moreover, the expression of DC markers such as CD86 and CD4 were only restored in previously nonhospitalized patients, while no restoration of integrin β7 and indoleamine 2,3-dyoxigenase (IDO) levels were observed. These findings contribute to a better understanding of the immunological sequelae of COVID-19
Search for three-nucleon short-range correlations in light nuclei
We present new data probing short-range correlations (SRCs) in nuclei through the measurement of electron scattering off high-momentum nucleons in nuclei. The inclusive ^{4}He/^{3}He cross section ratio is observed to be both x and Q^{2} independent for 1.52, our data support the hypothesis that a previous claim of three-nucleon correlation dominance was an artifact caused by the limited resolution of the measurement. While 3N-SRCs appear to have an important contribution, our data show that isolating 3N-SRCs is significantly more complicated than for 2N-SRCs.United States. Department of Energy (Contract DE-AC05-06OR23177)United States. Department of Energy (Contract DE-AC02-06CH11357)United States. Department of Energy (Contract DE-FG02-96ER40950
Near-infrared photoluminescence enhancement in Ge/CdS and Ge/ZnS core/shell nanocrystals: Utilizing IV/II-VI semiconductor epitaxy
Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character via strain or doping. Colloidal Ge nanocrystals are particularly interesting in the development of near-infrared materials for applications in bioimaging, telecommunications and energy conversion. Epitaxial growth of a passivating shell is a common strategy employed in the synthesis of highly luminescent II-VI, III-V and IV-VI semiconductor quantum dots. Here, we use relatively unexplored IV/II-VI epitaxy as a way to enhance the photoluminescence and improve the optical stability of colloidal Ge nanocrystals. Selected on the basis of their relatively small lattice mismatch compared with crystalline Ge, we explore the growth of epitaxial CdS and ZnS shells using the successive ion layer adsorption and reaction method. Powder X-ray diffraction and electron microscopy techniques, including energy dispersive X-ray spectroscopy and selected area electron diffraction, clearly show the controllable growth of as many as 20 epitaxial monolayers of CdS atop Ge cores. In contrast, Ge etching and/or replacement by ZnS result in relatively small Ge/ZnS nanocrystals. The presence of an epitaxial II-VI shell greatly enhances the near-infrared photoluminescence and improves the photoluminescence stability of Ge. Ge/II-VI nanocrystals are reproducibly 1-3 orders of magnitude brighter than the brightest Ge cores. Ge/4.9CdS core/shells show the highest photoluminescence quantum yield and longest radiative recombination lifetime. Thiol ligand exchange easily results in near-infrared active, water-soluble Ge/II-VI nanocrystals. We expect this synthetic IV/II-VI epitaxial approach will lead to further studies into the optoelectronic behavior and practical applications of Si and Ge-based nanomaterials
Probing the Repulsive Core of the Nucleon-Nucleon Interaction via the 4He(e,e'pN) Triple-Coincidence Reaction
We studied simultaneously the 4He(e,e'p), 4He(e,e'pp), and 4He(e,e'pn)
reactions at Q^2=2 [GeV/c]2 and x_B>1, for a (e,e'p) missing-momentum range of
400 to 830 MeV/c. The knocked-out proton was detected in coincidence with a
proton or neutron recoiling almost back to back to the missing momentum,
leaving the residual A=2 system at low excitation energy. These data were used
to identify two-nucleon short-range correlated pairs and to deduce their
isospin structure as a function of missing momentum in a region where the
nucleon-nucleon force is expected to change from predominantly tensor to
repulsive. Neutron-proton pairs dominate the high-momentum tail of the nucleon
momentum distributions, but their abundance is reduced as the nucleon momentum
increases beyond ~500 MeV/c. The extracted fraction of proton-proton pairs is
small and almost independent of the missing momentum in the range we studied.
Our data are compared with ab-initio calculations of two-nucleon momentum
distributions in 4He.Comment: 6 pages, 2 figure
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