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.     

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.     

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.     

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.     

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.     

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.     

A combined interpretation of cosmic ray light nuclei and antiproton measurements

Recently several experiments improved significantly our knowledge of cosmic ray (CR) spectra at high energy. In particular CREAM measured B/C, C/O and N/O ratios up to and PAMELA observed the antiproton-to-proton ratio up to with high accuracy. These results permit to put more stringent constraints on the free parameters involved in the diffusion-loss equation that describes the propagation of CRs in the Galaxy. To this purpose, we perform a new statistical analysis comparing updated experimental data with the predictions of an improved version of our numerical code DRAGON. We obtain well defined ranges of values for the diffusion coefficient normalization and index of the power-law dependence on rigidity as well as for the Alfvén velocity.     

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 .     

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.     

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.     

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%.     

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.     

Spectral analysis of young gamma-ray pulsars with Fermi

The Large Area Telescope (LAT) is the main instrument on board the Fermi Gamma-ray Space Telescope, launched on June 11, 2008. Thanks to its large effective area, good resolution and excellent sensitivity, Fermi has opened a new era for gamma-ray astronomy, in particular it has identified many new gamma-ray pulsars, that will help to study in detail the emission mechanism of these sources. We present a study on the spectrum emission and common features of J0007+7303, J0205+6449, J1833-1034 and J1124-5916 that, together with the Crab, are among the youngest pulsars in the galaxy, with an estimated age  yr.     

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.     

Response of the CALICE Si-W electromagnetic calorimeter physics prototype to electrons

A prototype silicon–tungsten electromagnetic calorimeter (ECAL) for an international linear collider (ILC) detector was installed and tested during summer and autumn 2006 at CERN. The detector had 6480 silicon pads of dimension . Data were collected with electron beams in the energy range 6–45 GeV. The analysis described in this paper focuses on electromagnetic shower reconstruction and characterises the ECAL response to electrons in terms of energy resolution and linearity. The detector is linear to within approximately the 1% level and has a relative energy resolution of . The spatial uniformity and the time stability of the ECAL are also addressed.     

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.