Measurements of high-energy -rays with detectors

The full-energy peak efficiency calibration and the energy resolution measurements of the LaBr₃ γ-ray detector are presented for γ-ray energies in the 700 keV–17.6 MeV range. Measurements were done using a combination of proton-capture nuclear reactions on , , , and for high-energy γ-rays, and radioactive sources such as and for the lowest energies. At high energies, two γ-rays in a cascade from proton resonance capture were employed using Al, Na₂WO₄, K₂SO₄ and LiBO₂ targets. The obtained results were compared to the simulations performed using a GEANT4 code.     

Depth of interaction detection for -ray imaging

A novel design for an inexpensive depth of interaction capable detector for γ-ray imaging has been developed. The design takes advantage of the strong correlation between the width of the scintillation light distribution in monolithic crystals and the interaction depth of γ-rays. We present in this work an inexpensive modification of the commonly used charge dividing circuits which enables the instantaneous and simultaneous computation of the second order moment of light distribution. This measure provides a good estimate for the depth of interaction and does not affect the determination of the position centroids and the energy release of γ-ray impact. The method has been tested with a detector consisting of a monolithic LSO block sized and a position-sensitive photomultiplier tube H8500 from Hamamatsu. The mean spatial resolution of the detector was found to be for the position centroids and for the DOI. The best spatial resolutions were observed at the center of the detector and yielded for the position centroids and for the DOI.     

Substitutional solution of silicon in cementite: A first-principles study

Cementite precipitation from austenite in steels can be suppressed by alloying with silicon. There are, however, no validated thermodynamic data to enable phase equilibria to be estimated when silicon is present in cementite. The formation energies of Fe₃C, and have therefore been estimated using first-principles calculations based on the total energy all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density functional theory. The ground state properties such as lattice constants and bulk moduli have also been calculated. The calculations show that and have about 52.06 kJ mol⁻¹ and 37.17 kJ mol⁻¹ greater formation energy, respectively, than Fe₃C. The formation energy for hypothetical cementite Si₃C has also been calculated to be about 256 kJ mol⁻¹. Silicon substitution significantly reduces the magnetic moments at the Fe(4c) site for both and , irrespective of the Si substitution sites. The calculated electronic structures indicate that the magnetic moment reduction at the Fe(4c) site by the Si substitution at 4c site is indirect through the neighboring carbon atom, whereas at the 8d site it is direct.     

Diffusion and formation energies of adatoms and vacancies on magnesium surfaces

This paper reports classical molecular statics calculations of magnesium {0 0 0 1}, , , and surfaces, specifically formation energies of defects (adatoms and surface vacancies) and flat surfaces and diffusion energy barriers of the defects. The formation energies show that the surface is thermodynamically more favorable than , and surfaces; in contrast, literature reports have often ignored the surface. The diffusion energy barriers of both adatoms and surface vacancies show strong diffusion anisotropy on , , and surfaces. Based on this anisotropy, the ratio of diffusion distances (of either adatoms or surface vacancies) along two orthogonal directions on is 37–55 at room temperature. Using the results of formation energies and diffusion energy barriers we develop a more complete understanding of surface orientations in Mg nanoblades synthesized by physical vapor deposition [F. Tang, T. Parker, H.-F. Li, G.-C. Wang, T.-M. Lu, J. Nanosci. Nanotechnol. 7 (2007) 3239]. In contrast to previous reports, we postulate that the side surfaces of Mg nanoblades are because (a) they have the second lowest surface formation energy and (b) the ratio of diffusion distances on them agrees with the experimental value of approximately 50.     

First-principles study on electronic structure and absorption spectra for the CaWO4 crystal with oxygen vacancy

The electronic structure and absorption spectra for the perfect CaWO₄ and the CaWO₄ containing oxygen vacancy have been calculated using density functional theory code CASTEP with the lattice structure being optimized. The results indicate that the CaWO₄ crystal containing , an additional absorption spectrum in the region of visible light. And the absorption spectrum can be fit into two Gaussian-shape absorption spectra with peaks at 360 and 415 nm. These peaks are located at the experimentally observed position. It is predicted that the 340 and the 420 nm absorption spectra are related to the existence of in the CaWO₄ crystal.     

Solar flares monitor with Fermi-LAT

Fermi-LAT is performing an all-sky γ-ray survey from 20 MeV to with unprecedented sensitivity and angular resolution. Fermi is the only mission able to detect high energy () emission from the Sun during the new solar cycle 24. Fermi was launched in June 2008, and since then the high energy emission from the Sun was continuously monitored searching for flare events. Upper limits were derived for all the solar flares detected by other missions and experiments (RHESSI, Fermi-GBM, GOES). We present the analysis techniques used for this study and the preliminary results obtained in the first months of this search.     

Selective adsorption of cations on single-walled carbon nanotubes: A density functional theory study

The selective adsorption of cation on single-walled carbon nanotubes (SWNTs) is systemically studied by using density functional theory calculations. It is found that the adsorption energy of cations on SWNTs depends on the concentration of cations and the diameter and the electronic structure of SWNTs. The binding strength of on each SWNT increases monotonically as the concentration of decreases, undergoing a change from endothermic to exothermic reaction. Generally speaking, the binding of on SWNTs becomes weaker as the diameter increases. In the medium-diameter region (9 < d < 11 Å), prefers to interact with metallic SWNTs (m-SWNTs) rather than semiconducting SWNTs (s-SWNTs) at the same concentration of . In the small-diameter region (d < 9 Å), the binding of is nearly independent of metallicity, but it is stronger than that of on the medium-diameter s-SWNTs. In the large-diameter region (d > 11 Å), the dependence of adsorption on the electronic structure is complicated, but the binding of is weaker than that on the medium-diameter s-SWNTs. Our results are in agreement with the experimental report that the small-diameter m- and s-SWNTs and the medium-diameter m-SWNTs are etched away by while the medium-diameter s-SWNTs and the large-diameter m- and s-SWNTs are intact.     

Electrical properties of ITO/benzylated cyclodextrins (-CDs (Bz))/Al diode structures

Investigations of the electrical characteristics of benzylated cyclodextrins (β-CDs (Bz)) diodes are reported. We present current–voltage characteristics and impedance spectroscopy measurements performed on partially benzylated cyclodextrins β-CDs (Bz) thin films in sandwich structures ITO/β-CDs (Bz)/Al. The static electrical characterizations show a space charge limited conduction (SCLC) and a conductivity with power low frequency behavior characteristic of a hopping transport in disordered materials. The impedance spectra can be discussed in terms of an equivalent circuit model designed as a parallel resistor R_P and capacitor C_P network in series with resistor R_S. We extract numerical values of these parameters by fitting experimental data. Their evolution with bias voltages has shown that the SCLC mechanism is characterized by an exponential trap distribution. We estimated from the capacitance voltage characteristic an acceptor concentration of about due to trap states.     

Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation

Dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy during hot compression at 300 °C and the strain rate of 0.2 s⁻¹ was systematically investigated by electron backscattering diffraction (EBSD) analysis. Twin DRX and continuous DRX (CDRX) are observed in grains and near grain boundaries, respectively. Original coarse grains are firstly divided by primary {} tensile twins and {} compression twins, and then {}–{} double twins are rapidly propagated within these primary compression twins with increasing compressive strain. Some twin-walled grains are formed by the mutual crossing of twins or by the formation of the {}–{} double twins and furthermore, subgrains divided by low-grain boundaries in the double twins are also formed. Finally, DRXed grains are formed by the in situ evolution of the subgrains with the growth of low-angle boundaries to high-angle grain boundaries in twins. CDRX around the eutectic Mg₁₇Al₁₂ phases at grain boundaries occurs together with the precipitation of discontinuous Mg₁₇Al₁₂ phase and the fragmentation of the precipitates during compression. The discontinuous fragmented precipitates distribute at the newly formed CDRXed grain boundaries and have remarkable pinning effect on the CDRXed grain growth, resulting in the average grain size of about 1.5 μm.     

Molybdate inhibition of environmental fatigue crack propagation in Al–Zn–Mg–Cu

effectively inhibits environmentally assisted fatigue crack propagation in 7075-T651 stressed during full immersion in low-chloride solution, as understood by hydrogen environment embrittlement and film rupture where -enhanced passivity reduces H production and uptake due to reduced crack hydrolysis, buffered pH, and a diffusion-barrier film. Inhibition is governed by the balance between crack tip strain rate and repassivation kinetics which establish the stability of the passive film. Inhibition is promoted by reduced loading frequency, reduced stress intensity range, increased crack tip concentration, and potentials at or anodic to free corrosion. The inhibiting effect of parallels that of , but molybdate effectiveness is shifted to a lower frequency regime suggesting the Al_xMo_yO_z passive film is less stable against crack tip deformation. For high R loading at sufficiently low frequencies fully inhibits EFCP, quantified by reduced crack growth rate to that typical of ultra-high vacuum, reduction in crack surface facets typical of hydrogen embrittlement, and crack arrest. Chromate did not produce such complete inhibition. Methods exist to incorporate molybdate or Mo in self-healing coating systems, but the complex effects of mechanical and electrochemical variables must be understood for reliable-quantitative fatigue performance enhancement.     

Semi-empirical atomistic study of point defect properties in BCC transition metals

We have constructed a set of embedded atom method (EAM) potentials for Fe, Ta, W and V and used them in order to study point defect properties. The parametrizations of the potentials ensure that the third order elastic constants are continuous and they have been fitted to the cohesive energies, the lattice constants, the unrelaxed vacancy formation energies and the second order elastic constants. Formation energies for different self-interstitials reveal that the split dumbbell is the most stable configuration for Fe while for Ta, W and V we find that the split dumbbell is preferred. Self-interstitial migration energies are simulated using the nudged elastic band method and for Fe and W the migration energies are found to be in good agreement with experimental and ab initio data. Migration energies for Ta and V self-interstitials are found to be quite low. The calculated formation, activation and migration energies for monovacancies are in good agreement with experimental data. Formation energies for divacancies reveal that the second nearest neighbor divacancy is more energetically favorable than nearest neighbor divacancies and the migration energies indicate that nearest neighbor migration paths are more likely to occur than second nearest neighbor migration jumps. For Fe, we have also studied the influence of the pair potential behavior between the second and third nearest neighbor on the stability of the split dumbbell, which revealed that the higher the energy level of the pair potential is in that region, the more stable the split dumbbell becomes.     

Energy transfer phenomena of keV fullerenes during grazing scattering from an Al(0 0 1) surface

Angular distributions for grazing scattering of fullerenes with energies of up to some 10 keV from an atomically clean and flat Al(0 0 1) surface are studied. Scattering proceeds in the regime of surface channeling where the motions of projectiles parallel and normal to the surface are widely decoupled. At low energies for the motion with respect to the surface normal, the clusters are scattered nearly elastically, whereas for larger energies a substantial amount of normal energy is lost. The results are compared to trajectory simulations using the Tersoff potential for the cluster and 3D- as well as 1D-rigid-wall representations for the surface. We find that, despite the large mass of C₆₀, the surface can be considered as a 1D- rigid wall and that the exact form of the interaction potential with the surface does not influence the normal energy loss. Therefore, properties of the fullerene and its interaction with the surface can be studied under well-defined conditions. The energy loss is transferred to internal excitations of the fullerenes.     

Structural stabilities, electronic and optical properties of CaF2 under high pressure: A first-principles study

The structural stabilities, electronic and optical properties, the pressure-induced metallization for CaF₂ have been studied by using the density functional theory calculations. The ground phase is predicted to transform into Pnma structure at 8.1 GPa, which is well consistent with the experimental findings. Above 278 GPa, Pnma-CaF₂ transform into P6₃/mmc phase. The calculated structural data for and pnma phases are in very good agreement with experimental values. The electronic band structures show that Pnma and P6₃/mmc phases of CaF₂ are insulators at the transition pressure. Upon further compression, the band gap of P6₃/mmc decreases with pressure, and CaF₂ is predicted to undergo metallization around 2250 GPa. The possible reason for the metallization was discussed. All CaF₂ polymorphs have ionic character between Ca–F bond with the analysis of the charge–density distribution and density of states.     

Joint interval reliability for Markov systems with an application in transmission line reliability

We consider Markov reliability models whose finite state space is partitioned into the set of up states and the set of down states . Given a collection of k disjoint time intervals I_ℓ=[t_ℓ,t_ℓ+x_ℓ], ℓ=1,…,k, the joint interval reliability is defined as the probability of the system being in for all time instances in I₁I_k. A closed form expression is derived here for the joint interval reliability for this class of models. The result is applied to power transmission lines in a two-state fluctuating environment. We use the Linux versions of the free packages Maxima and Scilab in our implementation for symbolic and numerical work, respectively.     

Use of the nuclear reaction for diagnostics of energetic particles in burning plasmas

Application of reaction-produced γ-rays to diagnostics of energetic particles in burning plasmas is analyzed. Particularly, we focus on 0.981 MeV γ-rays emitted in the nuclear reaction solely governed in the plasmas by energetic tritons. It is shown that these γ quanta can serve as a promising tool to diagnose α knock-on tritons and α-particles confined in burning DT plasmas. Key parameters of the α knock-on triton population and the α-particle confinement property can be obtained by comparing the experimental γ-ray yield and spectrum with theoretical slowing-down calculations. Even if the γ-ray spectral shape cannot be acquired, one can monitor densities of these tritons and α-particles at energies of 0.6–1.8 MeV and 2.0–3.5 MeV, respectively, in a nearly steady-state plasma. The 0.981-MeV photons also would help to display time evolution of the α-particle population in experiments of deuterium plasmas with pulsed tritium beam shots.     

Structural evolution of clusters using genetic algorithm and density functional theory method

Structural evolution of clusters have been studied using an extensive, unbiased search based on genetic algorithm and density functional theory (DFT) methods. Cationic, neutral, and anionic silver clusters have planar shapes for their lowest-energy structures up to n = 7, 6, and 6, respectively. Most of the competitive candidates for are found to adopt close-flat configurations. The present results obtained by employing the Perdew–Wang 91 (PW91) exchange-correlation functional are significantly different from those predicted in earlier work using empirical and semi-empirical potentials, and partly in line with the previous first-principles calculations. The dependences of the lowest-energy structures of on second finite differences of total energy, binding energies per atom, highest occupied and lowest unoccupied molecular orbital energy gaps, ionization potentials, and electron affinities are studied in detail. The calculated ionization potentials and electron affinities of the optimal clusters display distinct even–odd oscillations. The neutral Ag clusters with 6-, 8-, and 14-atoms are suggested to be “magic” clusters by an analysis of their geometric and electronic properties.     

Scintillation fiber array detector for measurement of neutron beam profile

We built and tested a detector to measure the profile of fast-neutron beams delivered by the MC50 cyclotron at the Korea Institute of Radiological and Medical Science (KIRAMS). The core component of the detector is a 2×46 array of scintillation fibers. The light output of the scintillation fibers is transformed into a current signal by a 46-channel silicon photodiode and digitized by a current-mode signal processor. This scanning device was designed to cover a neutron beam area of . The detector was tested in a neutron beam delivered by the MC50 cyclotron at KIRAMS. We demonstrate that the detector can successfully measure the neutron beam profile at various beam currents from 10 to . The proposed neutron beam profile detector will be useful, for example, in radiotherapy applications with neutron intensities above .     

Impurity-defect induced noncentrosymmetricity in nonlinear optical processes

Noncentrosymmetric nanosize-material processes in cadmium iodide are formed by doping it with the impurity copper. The noncentrosymmetricity in the processes are probed by the observation of the second-order optical susceptibility . The value of is found to depend fashionably on the impurity content of the nanomaterials. The results also show that a significant enhancement in the noncentrosymmetric response is achieved in nanomaterials with reduced sizes and at low temperatures.