First demonstration of a Compton gamma imager based on silicon photomultipliers

We are developing a rugged and person-transportable Compton gamma imager for use in security investigations of radioactive materials, and for radiological incident remediation. The imager is composed of layers of scintillator with light collection for the forward layers provided by silicon photomultipliers and for the rear layer by photomultiplier tubes. As a first step, we have developed a 1/5th-scale demonstration unit of the final imager. We present the imaging performance of this demonstration unit for Cs-137 at angles of up to 30 degrees off-axis. Results are also presented for Sn-113 and Na-22. This represents the first demonstration of the use of silicon photomultipliers as an embedded component for light collection in a Compton gamma imager.Comment: 19 pages, 6 figure

Adsorption of Line Segments on a Square Lattice

We study the deposition of line segments on a two-dimensional square lattice. The estimates for the coverage at jamming obtained by Monte-Carlo simulations and by $7^{th}$-order time-series expansion are successfully compared. The non-trivial limit of adsorption of infinitely long segments is studied, and the lattice coverage is consistently obtained using these two approaches.Comment: 19 pages in Latex+5 postscript files sent upon request ; PTB93_

Mean Field Fluid Behavior of the Gaussian Core Model

We show that the Gaussian core model of particles interacting via a penetrable repulsive Gaussian potential, first considered by Stillinger (J. Chem. Phys. 65, 3968 (1976)), behaves like a weakly correlated ``mean field fluid'' over a surprisingly wide density and temperature range. In the bulk the structure of the fluid phase is accurately described by the random phase approximation for the direct correlation function, and by the more sophisticated HNC integral equation. The resulting pressure deviates very little from a simple, mean-field like, quadratic form in the density, while the low density virial expansion turns out to have an extremely small radius of convergence. Density profiles near a hard wall are also very accurately described by the corresponding mean-field free-energy functional. The binary version of the model exhibits a spinodal instability against de-mixing at high densities. Possible implications for semi-dilute polymer solutions are discussed.Comment: 13 pages, 2 columns, ReVTeX epsfig,multicol,amssym, 15 figures; submitted to Phys. Rev. E (change: important reference added

Effect of stress-triaxiality on void growth in dynamic fracture of metals: a molecular dynamics study

The effect of stress-triaxiality on growth of a void in a three dimensional single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular dynamics (MD) simulations using an embedded-atom (EAM) potential for copper have been performed at room temperature and using strain controlling with high strain rates ranging from 10^7/sec to 10^10/sec. Strain-rates of these magnitudes can be studied experimentally, e.g. using shock waves induced by laser ablation. Void growth has been simulated in three different conditions, namely uniaxial, biaxial, and triaxial expansion. The response of the system in the three cases have been compared in terms of the void growth rate, the detailed void shape evolution, and the stress-strain behavior including the development of plastic strain. Also macroscopic observables as plastic work and porosity have been computed from the atomistic level. The stress thresholds for void growth are found to be comparable with spall strength values determined by dynamic fracture experiments. The conventional macroscopic assumption that the mean plastic strain results from the growth of the void is validated. The evolution of the system in the uniaxial case is found to exhibit four different regimes: elastic expansion; plastic yielding, when the mean stress is nearly constant, but the stress-triaxiality increases rapidly together with exponential growth of the void; saturation of the stress-triaxiality; and finally the failure.Comment: 35 figures, which are small (and blurry) due to the space limitations; submitted (with original figures) to Physical Review B. Final versio

Galvanic oxidation of bimetallic Zn-Fe nanoparticles for oxygen scavenging

Bimetallic nanoparticles (NP) have demonstrated outstanding multifunctional characteristics, which depend on their size, distribution and composition. In this study, we show the possibility of tailoring the oxidation behavior of Zn-Fe bimetallic nanoparticles produced by magnetron sputtering and gas agglomeration system. Zn and Fe metals were coupled to promote faster oxidation of Zn, stimulating a galvanic mechanism due to the dissimilar corrosion potential in the nanoparticles. The results revealed Zn dissolution occurring at high humidity environments for bimetallic Zn-Fe nanoparticles where no intermix exists between Zn and Fe; however, such dissolution is excluded for ZnFe alloys. The effect of the galvanic couple on the Zn dissolution was confirmed by molecular dynamic simulations. This bimetallic system can be exploited as moisture-activated oxygen scavenger materials due to the acceleration in the oxidation mechanism.This research is sponsored by FEDER funds through the program COMPETE – Programa Operacional Factores de Competitividade and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2019, and UID/ EMS/00285/2013 and in the framework of ERA-SIINN/0004/2013, PTDC/CTM-NAN/4242/2014 and PTDC/NAN-MAT/30789/2017 projects, and through IDMEC, under LAETA, project UIDB/50022/2020. This research was supported by Norte Regional Operational Program 2014-2020 (Norte2020) through the European Regional Development Fund (ERDF) Nanotechnology based functional solutions (NORTE-01- 0145-FEDER-000019) and through European Social Fund (FSE), under the National Doctoral Program in “Surfaces Engineering and Protection”, NORTE-08-5369-FSE-000047. The authors would like to acknowledge that this project received funding from the EU Framework Programme for Research and Innovation H2020, scheme COFUND – Cofunding of Regional, National and International Programmes, under Grant Agreement 713640. The authors also thank the financial support by Portuguese Foundation for Science and Technology (FCT) in the framework of the HEALTHYDENT (co-financed via FEDER (PT2020) POCI-01-0145-FEDER-030708 and FCT (PIDDAC)), in the framework of the ATRITO-0 (co-financed via FEDER (PT2020) POCI-01-0145-FEDER030446 and FCT (PIDDAC)) and in the framework of the project NANOXYPACK co-financed via FEDER (PT2020) POCI-01-0145-FEDER030789

A Stochastic Algorithm for 3D Scene Segmentation and Reconstruction

Abstract. In this paper, we present a stochastic algorithm by effective Markov chain Monte Carlo (MCMC) for segmenting and reconstructing 3D scenes. The objective is to segment a range image and its associated reflectance map into a number of surfaces which fit to various 3D surface models and have homogeneous reflectance (material) properties. In comparison to previous work on range image segmentation, the paper makes the following contributions. Firstly, it is aimed at generic natural scenes, indoor and outdoor, which are often much complexer than most of the existing experiments in the “polyhedra world”. Natural scenes require the algorithm to automatically deal with multiple types (families) of surface models which compete to explain the data. Secondly, it integrates the range image with the reflectance map. The latter provides material properties and is especially useful for surface of high specularity, such as glass, metal, ceramics. Thirdly, the algorithm is designed by reversible jump and diffusion Markov chain dynamics and thus achieves globally optimal solutions under the Bayesian statistical framework. Thus it realizes the cue integration and multiple model switching. Fourthly, it adopts two techniques to improve the speed of the Markov chain search: One is a coarse-to-fine strategy and the other are data driven techniques such as edge detection and clustering. The data driven methods provide important information for narrowing the search spaces in a probabilistic fashion. We apply the algorithm to two data sets and the experiments demonstrate robust and satisfactory results on both. Based on the segmentation results, we extend the reconstruction of surfaces behind occlusions to fill in the occluded parts.

Proximity effect at superconducting Sn-Bi2Se3 interface

We have investigated the conductance spectra of Sn-Bi2Se3 interface junctions down to 250 mK and in different magnetic fields. A number of conductance anomalies were observed below the superconducting transition temperature of Sn, including a small gap different from that of Sn, and a zero-bias conductance peak growing up at lower temperatures. We discussed the possible origins of the smaller gap and the zero-bias conductance peak. These phenomena support that a proximity-effect-induced chiral superconducting phase is formed at the interface between the superconducting Sn and the strong spin-orbit coupling material Bi2Se3.Comment: 7 pages, 8 figure

Heavy Quarks and Heavy Quarkonia as Tests of Thermalization

We present here a brief summary of new results on heavy quarks and heavy quarkonia from the PHENIX experiment as presented at the "Quark Gluon Plasma Thermalization" Workshop in Vienna, Austria in August 2005, directly following the International Quark Matter Conference in Hungary.Comment: 8 pages, 5 figures, Quark Gluon Plasma Thermalization Workshop (Vienna August 2005) Proceeding