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.     

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.     

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.     

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.     

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.     

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.     

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.     

Development of thick-foil and fine-pitch GEMs with a laser etching technique

We have produced thick-foil and fine-pitch gas electron multipliers (GEMs) using a laser etching technique. To improve production yield we have employed a new material, liquid crystal polymer, instead of polyimide as an insulator layer. The effective gain of the thick-foil GEM with a hole pitch of , a hole diameter of , and a thickness of reached a value of 10⁴ at an applied voltage of 720 V. The measured effective gain of the thick-foil and fine-pitch GEM ( pitch, diameter, and thick) was similar to that of the thick-foil GEM. The gain stability was measured for the thick-foil and fine-pitch GEM, showing no significant increase or decrease as a function of elapsed time from applying the high voltage. The gain stability over 3 h of operation was about 0.5%. Gain mapping across the GEM showed a good uniformity with a standard deviation of about 4%. The distribution of hole diameters across the GEM was homogeneous with a standard deviation of about 3%. There was no clear correlation between the gain and hole diameter maps.     

Role of microstructures on the growth of long fatigue cracks

An analysis is presented to understand the role of microstructures on the two crack growth driving force parameters, and , without invoking the extrinsic crack closure concepts. Microstructural variables considered are: grain size, precipitates and stacking fault energy. It is shown that is strongly affected by the scale of the microstructure, such as grain size or precipitate spacing. For each case, the mode of slip deformation and environment affects the fatigue resistance as represented by . However, the microstructures seem to have a smaller effect on . Also, the enhanced planarity of slip from the reduction in stacking fault energy has a pronounced effect on when compared with the materials deforming under homogeneous slip.     

Electronic band-edge properties of rock salt PbY and SnY (Y= S, Se, and Te)

The electronic band-edges of lead chalcogenides PbY and tin chalcogenides SnY (where Y = S, Se, and Te) are investigated by the means of a full-potential linearized augmented plane wave (FPLAPW) method and the local density approximation (LDA). All six chalcogenide binaries have similar electronic structures and density-of-states, but there are differences in the symmetry of the band-edge states at and near the Brillouin zone L-point. These differences give the characteristic composition, pressure, and temperature dependences of the energy gap in Pb_1−xSn_xY alloys.We find that: (1) SnY are zero-gap semiconductors E_g = 0 if the spin–orbit (SO) interaction is excluded. The reason for this is that the conduction band (CB) and the valence band (VB) cross along the Q ≡ LW line. (2) Including the SO interaction splits this crossing and creates a direct gap along the Q-line, thus away from the L symmetry point. Hence, the fundamental band gap E_g in SnY is induced by the SO interaction and the energy gap is rather small E_g ≈ 0.2–0.3 eV. At the L-point, the CB state has symmetric and the VB state is antisymmetric thereby the L-point pressure coefficient ∂E_g(L)/∂p is a positive quantity. (3) PbY have a direct band gap at the L-point both when SO coupling is excluded and included. In contrast to SnY, the SO interaction decreases the gap energy in PbY. (4) Including the SO interaction, the LDA yields incorrect symmetries of the band-edge states at the L-point; the CB state has and the VB state has symmetry. However, a small increase of the cell volume corrects this LDA failure, producing an antisymmetric CB state and a symmetric VB state, and thereby also yields the characteristic negative pressure coefficient ∂E_g(L)/∂p in agreement with experimental findings. (5) Although PbY and SnY have different band-edge physics at their respective equilibrium lattice constants, the change of the band-edges with respect to cell volume is qualitatively the same for all six chalcogenides. (6) Finally, in the discussion of the symmetry of the band edges, it is important to clearly state the chosen unit cell origin; a shift by (a/2,0,0) changes the labeling of the irreducible representations.     

A device to measure the effects of strong magnetic fields on the image resolution of PET scanners

Very high resolution images can be achieved in small animal PET systems utilizing solid state silicon pad detectors. As these systems approach sub-millimeter resolutions, the range of the positron is becoming the dominant contribution to image blur. The size of the positron range effect depends on the initial positron energy and hence the radioactive tracer used. For higher energy positron emitters, such as and , which are gaining importance in small animal studies, the width of the annihilation point distribution dominates the spatial resolution. This positron range effect can be reduced by embedding the field of view of the PET scanner in a strong magnetic field. In order to confirm this effect experimentally, we developed a high resolution PET instrument based on silicon pad detectors that can operate in a 7 T magnetic field. In this paper, we describe the instrument and present initial results of a study of the effects of magnetic fields up to 7 T on PET image resolution for and point sources.     

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.     

Microstructured semiconductor neutron detectors

Perforated semiconductor neutron detectors are compact diode detectors that operate at low power and can be fashioned to have high thermal neutron detection efficiency. Fabricated from high-purity Si wafers, the perforations are etched into the diode surface with ICP-RIE and backfilled with ⁶LiF neutron reactive material. The intrinsic thermal neutron detection efficiency depends upon many factors, including the perforation geometry, size, and depth. Devices were fabricated from high resistivity  cm n-type Si with conformal p-type shallow junction diffusions into the perforations, which demonstrate improved neutron detection performance over previous selectively diffused designs. A comparison was made to previous selectively diffused designs, and pulse height spectra show improved signal-to-noise ratio, higher neutron counting efficiency, and excellent gamma-ray discrimination. Devices with wide deep sinusoidal trenches yielded intrinsic thermal neutron detection efficiencies of 11.94±0.078%.     

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.     

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.     

Metallization and metallicity: Universal conductivity limits

The Mott–Ioffe–Regel minimum metallic conductivity based on a minimum mean free path of inter-atomic spacing has been used as a robust criterion for signalling Fermi liquid behaviour. Instead we examine the conductivity of a system in universal terms of the excitation energy, E_exc, of a charge carrier bound to its hole. The expression for conductivity depends simply on probability of charge transfer expressed as E_exc/. The minimum conductivity for Fermi liquid state is obtained as  S cm⁻¹ (C = 1;  eV is the maximum excitonic binding energy for a mobile exciton). From simple considerations in the t–J model of the Larmor precession time, , due to an internal exchange magnetic field and the residence time, τ_W, for a charge carrier with energy E_W, we express condition for Fermi liquid () and non-Fermi liquid behaviour (). For such NFL liquids, we find that one requires C_NFL = (1/π²π^2/3)  1/21.2 to account for the reduced probability of charge transfer with conservation of spin in NFL systems. The maximum value of the conductivity, σ^±, at which the temperature coefficient of resistance (TCR) changes sign at an insulator metal transition is given by  S cm⁻¹. This value is close to that observed in several systems. We discuss these universal values of the conductivity in the context of the Herzfeld criterion, Mooij criterion, exciton transfer rates, chemical reaction rate theory, universal sheet resistance at insulator–superconductor transition, as well as the changes in resistivity during the metallization of molecular hydrogen, oxygen and iodine.