Study on the Raman scattering measurements of Mn ion implanted GaN

We investigate the temperature dependent Raman spectra of Mn implanted (Ga,Mn)N samples with five Mn implantation doses. A small shoulder at 572.4 cm⁻¹ on the high energy side of the main Raman peak has been attributed to the Mn-related local vibrational mode (LVM). It is found that with the increase of Mn implantation dose the intensity ratio of LVM to that of the increases at first and tends to saturate at high implantation dose. In addition, at high temperature or after rapid thermal anneal treatment, the value of decreases significantly, explaining the reason why it is difficult to observe Mn-related LVM reported in the literature.     

Development of gas electron multiplier foils with a laser etching technique

Fine-pitch gas electron multiplier (GEM) foils have been produced for cosmic X-ray polarimeters using a carbon dioxide laser etching technique. The finest hole pitch of the foil which can be produced repeatedly is and the smallest hole diameter is . The electron amplification factor was measured as a function of applied voltage. The behavior of the factor is almost the same as the -pitch standard foil fabricated by CERN. Our GEMs had no rate-dependent gain instability, which is expected of the GEMs having holes of good cylindrical geometry. The amplification factor of the foil in a mixture of 70% argon and 30% carbon dioxide reaches about 5000 without any micro-discharge at a voltage of 570 V between foil electrodes.     

The time-of-flight technique for the HERMES experiment

This paper describes the use of the time-of-flight (TOF) technique as a particle identification method for the HERMES experiment. The TOF is measured by two scintillation hodoscopes that initially were designed only for the first-level trigger. However, the suitable time structure of the HERA electron beam allows an extension of their functions to also measure the TOF for low momentum hadron identification. Using only these conventional hodoscopes, good particle identification was achieved for protons and pions in the momentum range up to and for kaons up to .     

A parametric study of a pressurized blister test for an elastic–plastic film-rigid substrate system

A finite element simulation of a blister test of an elastic–plastic film, bonded to a substrate and subject to plane strain conditions, is performed. A traction-separation law models the fracture process ahead of the crack tip at the interface between the thin film and the substrate. Only two parameters are significant in describing the traction-separation law: adhesion energy, Γ₀ and interface strength, . The dependences of the pressure, P, and the product of the pressure with the central deflection, PH, on the adhesion properties (Γ₀ and ), the geometry and material properties of the film are studied. The latter quantity (PH) has the same unit as the adhesion energy, Γ₀, and is “conceptually” appropriate for the analysis. We suggest a method to extract the adhesion energy, Γ₀ and the interface strength, , independently from the total energy dissipated.     

Decrease of dielectric loss in giant dielectric constant CaCu3 Ti4 O12 ceramics by adding CaTiO3

CaCu₃ Ti₄ O₁₂–x CaTiO₃ ceramics (x=0,0.1,0.2,0.3,0.4 and 0.5) was studied by X-ray diffraction, scanning electron microscope and dielectric measurements. It was indicated that some CaTiO₃ entered the boundaries of CaCu₃ Ti₄ O₁₂ grains and/or subgrains. Dielectric measurement showed that the addition of CaTiO₃ lowered the dielectric loss remarkably, especially at low frequencies, while the giant dielectric constant still remained. At room temperature, the dissipation factor of the x=0.5 sample was decreased to 0.02 over the frequency range from 50 to 2000 Hz, and the dielectric constant was kept to be 4000. We explain this phenomenon in terms of internal barrier layer capacitance model by using the impedance spectroscopy analysis.     

Background reduction in neutrinoless double beta decay experiments using segmented detectors—A Monte Carlo study for the GERDA setup

The identification of gamma radiation is essential for a new generation of double beta decay experiments. The GERmanium Detector Array, GERDA, located at the INFN Gran Sasso National Laboratory (LNGS) in Italy, uses germanium, enriched in , as source and detector, and aims at a background level of less than in the region of the Q_ββ-value. For the first time highly segmented detectors will be installed in a double beta decay experiment. A detailed GEANT4 Monte Carlo study was performed to evaluate the background reduction achievable by anti-coincidence cuts between crystals and segments.

Within the overall geometry of GERDA, the segmentation scheme considered here provides around an order of magnitude of extra background reduction.     

A 5 ps TOF-counter with an MCP–PMT

We measured the timing resolution of TOF counters by means of Cherenkov-light detection with a Micro-Channel Plate Photo-Multiplier Tube, and attained using pion-beams with a readout electronics of 4.1 ps resolution. The optimum radiator thickness was examined and the detectable number of photo-electrons was analyzed.     

Coupled simulation of microstructural formation and deformation behavior of ferrite–pearlite steel by phase-field method and homogenization method

A coupled simulation by the phase-field (PF) method and the finite element method based on the homogenization theory (FEH) is developed to predict the microstructure formations and mechanical properties of ferrite–pearlite steels. The formation of the phase during the isothermal transformation is simulated by the PF method. Furthermore, the FEH analysis is performed to clarify the effects of the predicted microstructure on the deformation behavior of the steels. In order to link to the FEH analysis, the microstructure in the steel is described by the representative volume element (RVE) based on the results of the PF simulation. The results reveal that although the macroscopic stress–strain relationship is mainly characterized by the volume fraction of the constituent phase, the localization of plastic strain is reduced due to the fine-grained phase. This numerical model provides a systematic way of predicting the mechanical properties of steel depending on the microstructure.     

Behavior of corrosion-damaged RC columns wrapped with FRP under combined flexural and axial loading

This paper presents the results of a research program aimed at investigating the effectiveness of carbon fiber-reinforced polymers (CFRP) to upgrade corrosion-damaged eccentrically loaded reinforced concrete (RC) columns. A total of 16 square RC columns with end corbels were constructed. Test specimen had an overall length of 1200 mm whereas each end corbel had a cross section of and a length of 350 mm. The specimen in the test region was having longitudinal steel ratio of 1.9%. The damaged specimens were exposed to 30 days of accelerated corrosion that corresponded to a steel mass loss of about 4.25%. The main test parameters were the CFRP repair scheme (no wrapping, full-wrapping, and partial-wrapping) and the eccentricity-to-section height (e/h) ratio (0.3, 0.43, 0.57, and 0.86). The strength of the damaged columns fully wrapped with CFRP was up to 40% higher than that of the control undamaged columns. The strength gain was inversely proportional to the eccentricity ratio. Partial CFRP-wrapping was 8% less effective than full CFRP-wrapping at nominal e/h of 0.3. At higher e/h values, the confinement level had a negligible effect on the columns’ strength. An analytical model was then proposed to predict the columns’ strength under eccentric loading. A comparative analysis between predicted and experimental results demonstrated the model’s accuracy and reliability.     

Dissociated dislocations in Ni: a computational study

A systematic computational study of the behavior of a dissociated screw dislocation in fcc nickel is presented, in which atomic interactions are described through an embedded-atom potential. A suitable external stress is applied on the system, both for modifying the equilibrium separation distance d and moving the dislocation complex. The structure of the dislocation and its corresponding changes during the motion are studied in the framework of the two-dimensional Peierls model, for different values of the ratio d/a^′, where a^′ is the period of the Peierls potential. The distance between the edge and screw components of the partials, as well as their widths, undergo a modulation with period a^′, as the dislocation moves, and the amplitudes of such oscillations are shown to depend on d/a^′. The stress profile acting on the dislocation complex is analyzed and the effective Peierls stress is estimated for different values of d/a^′.     

Study on the electronic structures and absorption spectra for sheelite- and the sheelite-like structures lead tungstate crystal

The electronic structures and absorption spectra of perfect crystals and crystals containing lead vacancy for both sheelite(PbWO₄) and sheelite-like(Pb_7.5W₈O₃₂) structures have been calculated using density functional theory with the lattice structure optimized. The optical properties of lead tungstate exhibit anisotropy. Numerical calculation shows that the absorption spectra of the sheelite structure crystal containing exhibit additional absorption bands in the visible and near ultra-violet ranges. However, this is not the case for other structures of the PWO crystals such as the sheelite structure crystal without lead vacancy and the sheelite-like structure crystal with or without lead vacancy.     

Position sensitivity of the TIGRESS 32-fold segmented HPGe clover detector

Sensitivity to the locations of γ-ray interactions within a 32-fold segmented clover-type HPGe detector has been investigated through 90 Compton scattering of -rays from a collimated source. A mean position sensitivity of 0.44 mm at an energy of 373 keV is deduced by comparing the average pulse shapes for net charge collecting signals, as well as transient induced signals in neighbouring contacts, from 1007 pairs of three-dimensionally localized interaction points. The reconstruction of individual event locations based on a χ² comparison with the measured set of basis waveforms is presented.     

Differential die-away analysis system response modeling and detector design

Differential die-away-analysis (DDAA) is a sensitive technique to detect presence of fissile materials such as and . DDAA uses a high-energy (14 MeV) pulsed neutron generator to interrogate a shipping container. The signature is a fast neutron signal hundreds of microseconds after the cessation of the neutron pulse. This fast neutron signal has decay time identical to the thermal neutron diffusion decay time of the inspected cargo. The theoretical aspects of a cargo inspection system based on the differential die-away technique are explored. A detailed mathematical model of the system is developed, and experimental results validating this model are presented.     

Production of a low-energy radioactive nuclear beam with high purity using JAERI-RMS

Low-energy (1–2 MeV/nucleon) radioactive nuclear beams (RNBs) of ⁸Li, ¹²B and ¹⁶N have been produced through nuclear transfer reactions using a recoil mass separator (RMS) at Japan Atomic Energy Research Institute (JAERI). The contamination of the primary beam to the RNB was investigated quantitatively and was well suppressed. The typical beam intensities of ⁸Li-, ¹²B- and ¹⁶N-RNB were 1.4×10⁴, 7.8×10³ and , respectively. Their purities were 99%, 98% and 98.5%, respectively.     

Liquid-filled ionization chamber temperature dependence

Temperature and pressure corrections of the read-out signal of ionization chambers have a crucial importance in order to perform high-precision absolute dose measurements. In the present work the temperature and pressure dependences of a sealed liquid isooctane filled ionization chamber (previously developed by the authors) for radiotherapy applications have been studied.

We have analyzed the thermal response of the liquid ionization chamber in a interval around room temperature. The temperature dependence of the signal can be considered linear, with a slope that depends on the chamber collection electric field. For example, a relative signal slope of for an operation electric field of has been measured in our detector. On the other hand, ambient pressure dependence has been found negligible, as expected for liquid-filled chambers.

The thermal dependence of the liquid ionization chamber signal can be parametrized within the Onsager theory on initial recombination. Considering that changes with temperature of the detector response are due to variations in the free ion yield, a parametrization of this dependence has been obtained. There is a good agreement between the experimental data and the theoretical model from the Onsager framework.     

Sensitivity series and friction surface analysis of non-metallic friction materials

Nine non-metallic friction material formulations contained fibers, fillers and binder without strong abrasives were designed using Golden Section sequence combined with least-square method. Seven ingredients used without strong abrasives were selected based on the combinatorial approach. Wear (w) and coefficient of friction (μ) are expressed as a function of volume fraction of the ingredients selected as and . The formulations were optimized using the sensitivity series obtained from least-square method. An optimized formulation (S-10) was obtained with a total wear loss of 6.69 wt% and an average μ of 0.375. Friction surface of both brake pad and disc was observed using SEM, EDX and XRD. The non-metallic friction materials without abrasives exhibit unique friction performance and phenomena compared with the non-metallic friction materials with abrasives and semi-metallic brake linings. The temperature measured during friction is lower and the oxidation of the cast iron disc is not rigorous.     

Successive wave crests in Gaussian seas

In this paper, following the theory of quasi-determinism of Boccotti [Boccotti P. Wave mechanics for ocean engineering. Oxford: Elsevier Science.], the necessary and sufficient conditions, for the occurrence of two successive wave crests of large heights in a gaussian sea, are given. It is proven that the first two-peaks part of the autocovariance function ψ(T) describes the structure of two successive-wave patterns. As a corollary, it is shown that the tail probability of the joint distribution of two successive wave crests is given by a bivariate Weibull distribution. The Weibull parameter is equal to . Here, is the abscissa of the second absolute maximum of the autocovariance function ψ(T). The analytical results are in agreement with Monte Carlo simulations. Finally, as an application, the maximum expected wave crest pressure in an undisturbed deep water waves is evaluated by taking into account the stochastic dependence of successive wave crests.     

Development of a high-efficiency pulsed slow positron beam for measurements with orthopositronium in vacuum

We have developed a high-efficiency pulsed slow positron beam for experiments with orthopositronium in vacuum. The new pulsing scheme is based on a double-gap coaxial buncher powered by an RF pulse of appropriate shape. The modulation of the positron velocity in the two gaps is used to adjust their time-of-flight to a target. This pulsing scheme allows to minimize non-linear aberrations in the bunching process and to efficiently compress positron pulses with an initial pulse duration ranging from 300 to 50 ns into bunches of 2.3 to 0.4 ns width, respectively, with a repetition period of . The compression ratio achieved is 100, which is a factor 5 better than has been previously obtained with slow positron beams based on a single buncher. Requirements on the degree, to which the moderated positrons should be mono-energetic and on the precision of the waveform generation are presented. Possible applications of the new pulsed positron beam for measurements of thin films are discussed.     

A new analogue sampling readout system for the COMPASS RICH-1 detector

A new electronic readout for CsI-coated multiwire proportional chambers (MWPC), used as photon detectors in the COMPASS ring imaging Cherenkov (RICH) detector, is described. A prototype system comprising more than 5000 channels has been built and tested in high-intensity beam conditions. It is based on the APV25-S1 analogue sampling chip, and replaces the GASSIPLEX chip readout used previously. The APV25 chip, although originally designed for Silicon microstrip detectors, is shown to perform well even with “slow” signals from an MWPC, maintaining a signal-to-noise ratio (SNR) of 9. For every trigger the system reads out three consecutive amplitudes in time, thus allowing to extract information on both the signal amplitude and its timing. This information is used to reduce pile-up events in a high-rate environment. Prototype tests of the new readout electronics on a central RICH photocathode in nominal COMPASS beam conditions showed that the effective time window is reduced from more than for the GASSIPLEX to less than for the APV25 chip. This leads to a significant improvement of the signal-to-background ratio (SBR) with respect to the original readout. A gain by a factor of 5–6 was experimentally verified in the very forward region of phase space, where pile-up due to the muon beam halo is most significant. Owing to its pipelined architecture, the new readout system also considerably reduces the dead time per event, thus allowing to make use of trigger rates exceeding .     

A weakly singular boundary integral equation in elastodynamics for heterogeneous domains mitigating fictitious eigenfrequencies

A boundary element formulation applied to dynamic soil–structure interaction problems with embedded foundations may give rise to inaccurate results at frequencies that correspond to the eigenfrequencies of the finite domain embedded in an exterior domain of semi-infinite extent. These frequencies are referred to as fictitious eigenfrequencies. This problem is illustrated and mitigated modifying the original approach proposed by Burton and Miller for acoustic problems, which combines the boundary integral equations in terms of the displacement and its normal derivative using a complex coupling parameter . Hypersingular terms in the original boundary integral equation are avoided by replacing the normal derivative by a finite difference approximation over a characteristic distance h, still leading to an exact boundary integral equation. A proof of the uniqueness of this formulation for small h and a smooth boundary is given, together with a parametric study for the case of a rigid massless cylindrical embedded foundation. General conclusions are drawn for the practical choice of the dimensionless coupling parameter and the dimensionless distance