H. R. Weller - UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale

H. R. Weller
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Name
H. R. Weller
Affiliation
UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale
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Physics - Materials Science (16)
 
Nuclear Experiment (12)
 
Physics - Instrumentation and Detectors (5)
 
Nuclear Theory (4)
 
Mathematics - Numerical Analysis (4)
 
Astrophysics (3)
 
Solar and Stellar Astrophysics (2)
 
Instrumentation and Methods for Astrophysics (1)
 
High Energy Physics - Experiment (1)

Publications Authored By H. R. Weller

Including terrain in atmospheric models gives rise to mesh distortions near the lower boundary that can degrade accuracy and challenge the stability of transport schemes. Multidimensional transport schemes avoid splitting errors on distorted, arbitrary meshes, and method-of-lines schemes have low computational cost because they perform reconstructions at fixed points. This paper presents a multidimensional method-of-lines finite volume transport scheme, "cubicFit", which is designed to be numerically stable on arbitrary meshes. Read More

Dimensionally split advection schemes are attractive for atmospheric modelling due to their efficiency and accuracy in each spatial dimension. Accurate long time-steps can be achieved without significant cost using the flux-form semi-Lagrangian technique. The dimensionally split scheme used here is constructed from the one-dimensional Piecewise Parabolic Method and extended to two dimensions using COSMIC splitting. Read More

A Monge-Amp\`ere (MA) equation arises when seeking an optimally transported mesh that equidistributes a given monitor function in Cartesian space. This MA equation is a fully nonlinear PDE, with a source term that is a function of the gradient of the solution. This nonlinear source term is an additional computational challenge that has received little attention from MA applications in other fields. Read More

An equation of Monge-Amp\`ere type has, for the first time, been solved numerically on the surface of the sphere in order to generate optimally transported (OT) meshes, equidistributed with respect to a monitor function. Optimal transport generates meshes that keep the same connectivity as the original mesh, making them suitable for r-adaptive simulations, in which the equations of motion can be solved in a moving frame of reference in order to avoid mapping the solution between old and new meshes and to avoid load balancing problems on parallel computers. The semi-implicit solution of the Monge-Amp\`ere type equation involves a new linearisation of the Hessian term, and exponential maps are used to map from old to new meshes on the sphere. Read More

We report new measurements of the doubly-polarized photodisintegration of $^3$He at an incident photon energy of 16.5 MeV, carried out at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility located at Triangle Universities Nuclear Laboratory (TUNL). The spin-dependent double-differential cross sections and the contribution from the three--body channel to the Gerasimov-Drell-Hearn (GDH) integrand were extracted and compared with the state-of-the-art three--body calculations. Read More

We present a facile and safe ligand exchange method for readily synthesized CuInSe2 (CIS) and CuIn(1-x)Ga(x)Se2 (CIGS) nanocrystals (NCs) from oleylamine to 1-ethyl-5-thiotetrazole which preserves the colloidal stability of the chalcopyrite structure. 1-ethyl-5-thiotetrazole as thermally degradable ligand is adapted for the first time for trigonal pyramidal CIS NCs (18 nm), elongated CIS NCs (9 nm) and CIGS NCs (6 nm). The exchanged NC solutions are spin-coated onto Si/SiO2 substrates with predefined gold electrodes to yield ordered NC thin films. Read More

Cross sections for \nuc{6}{Li}($\gamma$,$\gamma$)\nuc{6}{Li} have been measured at the High Intensity Gamma-Ray Source (\HIGS) and the sensitivity of these cross sections to the nucleon isoscalar polarizabilities was studied. Data were collected using a quasi-monoenergetic 86 MeV photon beam at photon scattering angles of 40$^{\circ}$--160$^{\circ}$. These results are an extension of a previous measurement at a lower energy. Read More

Elastic scattering of photons from 12C has been investigated using quasi-monoenergetic tagged photons with energies in the range 65 - 115 MeV at laboratory angles of 60 deg, 120 deg, and 150 deg at the Tagged-Photon Facility at the MAX IV Laboratory in Lund, Sweden. A phenomenological model was employed to provide an estimate of the sensitivity of the 12C(g,g)12C cross section to the bound-nucleon polarizabilities. Read More

We propose a new type of surfactants, namely, tetrazole derivatives which can be controllably removed from the nanoparticle surface. Tetrazoles are a peculiar class of heterocyclic compounds. The presence of four nitrogen atoms in the tetrazole ring determines their interesting physical and chemical properties. Read More

The existing Nuclear Resonance Fluorescence (NRF) setup at the HI{\gamma}S facility at the Triangle Universities Nuclear Laboratory at Duke University has been extended in order to perform {\gamma}-{\gamma} coincidence experiments. The new setup combines large volume LaBr3:Ce detectors and high resolution HPGe detectors in a very close geometry to offer high efficiency, high energy resolution as well as high count rate capabilities at the same time. The combination of a highly efficient {\gamma}-ray spectroscopy setup with the mono-energetic high-intensity photon beam of HI{\gamma}S provides a worldwide unique experimental facility to investigate the {\gamma}-decay pattern of dipole excitations in atomic nuclei. Read More

The first measurement of the three-body photodisintegration of longitudinally-polarized ^3He with a circularly-polarized \gamma-ray beam was carried out at the High Intensity \gamma-ray Source (HI\gamma S) facility located at Triangle Universities Nuclear Laboratory (TUNL). The spin-dependent double-differential cross sections and the contributions from the three-body photodisintegration to the ^3He GDH integrand are presented and compared with state-of-the-art three-body calculations at the incident photon energies of 12.8 and 14. Read More

2013Mar
Affiliations: 1UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 2UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 3UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 4UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 5UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 6UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 7UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 8UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 9UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 10UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 11UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 12UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 13UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale, 14UConn, TUNL, NCCU, PTB, Weizmann, UCL, Yale

The second 2+ state of 12C, predicted over fifty years ago as an excitation of the Hoyle state, has been unambiguously identified using the 12C(g,a_0)8Be reaction. The alpha particles produced by the photodisintegration of 12C were detected using an Optical Time Projection Chamber (O-TPC). Data were collected at beam energies between 9. Read More

The attachment of semiconducting nanoparticles to carbon nanotubes is one of the most challenging subjects in nanotechnology. Successful high coverage attachment and control over the charge transfer mechanism and photo-current generation opens a wide field of new applications such as highly effective solar cells and fibre-enhanced polymers. In this work we study the charge transfer in individual double-wall carbon nanotubes highly covered with uniform ZnO nanoparticles. Read More

We present the full thermoelectric characterization of nanostructured bulk PbTe and PbTe-PbSe samples fabricated from colloidal core-shell nanoparticles followed by spark plasma sintering. An unusually large thermopower is found in both materials, and the possibility of energy filtering as opposed to grain boundary scattering as an explanation is discussed. A decreased Debye temperature and an increased molar specific heat are in accordance with recent predictions for nanostructured materials. Read More

We report the growth of an unstable shell-like gold structure around dihexagonal pyramidal CdSe nanocrystals in organic solution and the structural transformation to spherical domains by two means: i) electron beam irradiation (in situ) and (ii) addition of a strong reducing agent during synthesis. By varying the conditions of gold deposition, such as ligands present or the geometry of the CdSe nanocrystals, we were able to tune the gold domain size between 1.4 nm to 3. Read More

Controlling anisotropy is a key concept to generate complex functionality in advanced materials. For this, oriented attachment of nanocrystal building blocks, a self assembly of particles into larger single crystalline objects, is one of the most promising approaches in nanotechnology. We report here the 2D oriented attachment of PbS nanocrystals into ultra-thin single crystal sheets with dimensions on the micrometer scale. Read More

We report a solution-processed, ligand supported synthesis of 15-20 nm thick Sb(2-x)Bi(x)Te3 nanoplatelets. After complete ligand removal by a facile NH3-based etching procedure, the platelets are spark plasma sintered to a p-type nanostructured bulk material with preserved crystal grain sizes. Due to this nanostructure, the total thermal conductivity is reduced by 60 % in combination with a reduction in electric conductivity of as low as 20 % as compared to the bulk material demonstrating the feasibility of the phonon-glass electron-crystal concept. Read More

The formation of monodisperse, tunable sized, alloyed nanoparticles of Ni, Co, or Fe with Pt and pure Pt nanoparticles attached to carbon nanotubes has been investigated. Following homogeneous nucleation, nanoparticles attach directly to non-functionalized singlewall and multiwall carbon nanotubes during nanoparticle synthesis as a function of ligand nature and the nanoparticle work function. These ligands do not only provide a way to tune the chemical composition, size and shape of the nanoparticles but also control a strong reversible interaction with carbon nanotubes and permit controlling the nanoparticle coverage. Read More

2011Jan
Affiliations: 1UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 2UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 3UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 4UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 5UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 6UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 7UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 8UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 9UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 10UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 11UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 12UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 13UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 14UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 15UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 16UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 17UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration, 18UConn-Yale-TUNL-Weizmann-PTB-UCL Collaboration

We report on the construction, tests, calibrations and commissioning of an Optical Readout Time Projection Chamber (O-TPC) detector operating with a CO2(80%) + N2(20%) gas mixture at 100 and 150 Torr. It was designed to measure the cross sections of several key nuclear reactions involved in stellar evolution. In particular, a study of the rate of formation of oxygen and carbon during the process of helium burning will be performed by exposing the chamber gas to intense nearly mono-energetic gamma-ray beams at the High Intensity Gamma Source (HIgS) facility. Read More

Indium phosphide (InP) nanostructures have been synthesized by means of colloidal chemistry. Under appropriate conditions needle-shaped nanostructures composed of an In head and an InP tail with lengths up to several micrometers could be generated in a one-pot synthesis. The growth is interpreted in terms of simultaneous decomposition of the In precursor and in situ generation of In and InP nanostructures. Read More

We report a novel synthesis for near monodisperse, sub-10-nm Bi2Te3 nanoparticles. At first, a new reduction route to bismuth nanoparticles is described which are applied as starting materials in the formation of rhombohedral Bi2Te3 nanoparticles. After ligand removal by a novel hydrazine hydrate etching procedure, the nanoparticle powder is spark plasma sintered to a pellet with preserved crystal grain sizes. Read More

Monolayers of colloidally synthesized cobalt-platinum nanoparticles of different diameters characterized by TEM (transmission electron microscopy) were deposited on structured silicon oxide substrates and characterized by SEM (scanning electron microscopy), GISAXS (grazing incidence x-ray scattering), and electric transport measurements. The highly ordered nanoparticle films show a thermally activated electron hopping between spatially adjacent particles at room temperature and Coulomb blockade at low temperatures. We present a novel approach to experimentally determine the particles charging energies giving values of 6. Read More

Insights to the mechanism of CdSe nanoparticle attachment to carbon nanotubes following the hot injection method are discussed. It was observed that the presence of water improves the nanotube coverage while Cl containing media are responsible for the shape transformation of the nanoparticles and further attachment to the carbon lattice. The experiments also show that the mechanism taking place involves the right balance of several factors, namely, low passivated nanoparticle surface, particles with well-defined crystallographic facets, and interaction with an organics-free sp2 carbon lattice. Read More

A review of photo-pion experiments on the nucleon in the near threshold region is presented. Comparisons of the results are made with the predictions of the low energy theorems of QCD calculated using chiral perturbation theory (ChPT) which is based on the spontaneous breaking of chiral symmetry as well as its explicit breaking due to the finite quark masses. As a result of the vanishing of the threshold amplitudes in the chiral limit, the experiments are difficult since the cross sections are small. Read More

The crystallographic structure of CdSe nanoparticles attached to carbon nanotubes has been elucidated by means of high resolution transmission electron microscopy and high angle annular dark field scanning transmission electron microscopy tomography. CdSe rod-like nanoparticles, grown in solution together with carbon nanotubes, undergo a morphological transformation and become attached to the carbon surface. Electron tomography reveals that the nanoparticles are hexagonal-based with the (001) planes epitaxially matched to the outer graphene layer. Read More

The Langmuir-Blodgett technique was utilized and optimized to produce closed monolayers of cobalt-platinum nanoparticles over vast areas. It is shown that sample preparation, "dipping angle", and subphase type have a strong impact on the quality of the produced films. The amount of ligands on the nanoparticles surface must be minimized, the dipping angle must be around 105$^{\circ}$, while the glycol subphase is necessary to obtain nanoparticle monolayers. Read More

Novel applications in nanotechnology rely on the design of tailored nano-architectures. For this purpose, carbon nanotubes and nanoparticles are intensively investigated. In this work we study the influence of non-functionalized carbon nanotubes on the synthesis of CdSe nanoparticles by means of organometallic colloidal routes. Read More

Targets consisting of 3,4He implanted into thin aluminum foils (approximately 100, 200 or 600 ug/cm^2) were prepared using intense (a few uA) helium beams at low energy (approximately 20, 40 or 100 keV). Uniformity of the implantation was achieved by a beam raster across a 12 mm diameter tantalum collimator at the rates of 0.1 Hz in the vertical direction and 1 Hz in the horizontal direction. Read More

2006Feb
Affiliations: 1LNS at Avery Point, UConn, 2UConn and Yale, 3Weizmann, 4Weizmann, 5PTB, Braunschweig, 6PTB, Braunschweig, 7TUNL, Duke

We studied electron amplification and light emission from avalanches in oxygen-containing gas mixtures. The mixtures investigated in this work included, among others, CO2 and N2O mixed with Triethylamine (TEA) or N2. Double-Step Parallel Gap (DSPG) multipliers and THick Gas Electron Multipliers (THGEM) were investigated. Read More

We are developing an Optical Readout Time Projection Chamber (O-TPC) detector for the study of the 12C(a,g)16O reaction that determines the ratio of carbon to oxygen in helium burning. This ratio is crucial for understanding the final fate of a progenitor star and the nucleosynthesis of elements prior to a Type II supernova; an oxygen rich star is predicted to collapse to a black hole, and a carbon rich star to a neutron star. Type Ia supernovae (SNeIa) are used as standard candles for measuring cosmological distances with the use of an empirical light curve-luminosity stretching factor. Read More

The 100 % polarized photon beam at the High Intensity gamma-ray Source (HIgS) at Duke University has been used to determine the parity of six dipole excitations between 2.9 and 3.6 MeV in the deformed nuclei 172,174 Yb in photon scattering (g,g') experiments. Read More

Based on the high-temperature organometallic route (Sun et al. Science 287, 1989 (2000)), we have synthesized powders containing CoPt_3 single crystals with mean diameters of 3.3(2) nm and 6. Read More