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^′.     

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.     

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.     

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.     

Influence of alloying elements on as-cast microstructure and strength of gray iron

Casting experiments were carried out to produce gray cast irons with compositions in the range (wt.%): Fe–3.2C–wCu–xMo–yMn–zSi, where w = 0.78–1.79, x = 0.11–1.17, y = 0.68–2.34 and z = 1.41–2.32. These key elements were varied systematically during sand casting into 30-mm diameter bars to assess their influence on the development of microstructure and mechanical properties. It was found that microstructures ranging from fully pearlitic to an intimate mixture of retained austenite and bainitic ferrite, termed ausferrite, were produced and a reasonable linear correlation was observed between ausferrite volume fraction and strength. The optimum combination of mechanical properties was achieved in an alloy of approximate composition Fe–3.2C–1.0Cu–0.7Mo–0.55Mn–2.0Si, which generated 100% ausferrite without alloy carbides. This alloy has a microstructure and mechanical properties comparable to austempered gray iron without many of the problems associated with austempering.     

The effect of precipitates on the mechanical properties of ZK60-Y alloy

This paper investigated the influence of the microstructure on the mechanical properties of the forged ZK60-Y alloy under different heat treatments (T₄, T₅ and T₆). The results show that the alloy at the direct artificial ageing treatment has superior strength and plasticity. X-ray analysis, SEM fracture observation and EDAX spectrum analysis reveal that bulk w-Mg₃Y₂Zn₃ and coarsened rod-like MgZn′(β^′₁) phases have great influence on the tensile properties of the alloy. Further investigations of the alloy aged at 180 °C, 200 °C and 225 °C for 0.5–100 h, respectively, suggest that the mechanical properties and the fracture characteristics are closely related with the variation of the rod-like β^′₁ precipitation. The fracture observation of the samples aged at 200 °C indicates that with the increase of the density and the size of the rod-like β^′₁ precipitation, more semi-cleavage planes will appear on the fracture surfaces.     

Reliability models based on bivariate exponential distributions

Motivated by reliability applications, we derive the exact distributions of R=X+Y and W=X/(X+Y) and the corresponding moment properties when X and Y follow five flexible bivariate exponential distributions. The expressions turn out to involve several special functions.     

A NaI activation method for the measurement of the weak thermal neutron field around the MEG experiment

A precise measurement of the thermal neutron flux (and an approximate measurement of the non-thermal neutron flux), associated with the high intensity stopping μ⁺ beam (≈) of the MEG experiment, was studied by measuring the delayed radioactivity of an activated NaI detector. This passive method provides a high sensitivity and allows the neutron flux determination at all locations, even in regions of high magnetic field. We present and discuss the results.     

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.     

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.     

Performance optimisation of the micro pixel chamber

A thorough performance analysis of large-area μ-PIC detectors has been made. Through systematic simulations and comparison with test results of the two detector versions, μ-PIC1 and μ-PIC3, we have identified that low primary electron collection was responsible for the degradation of measured gas gain of the μ-PIC3 device. The test of a new detector, μ-PIC5, fabricated with a narrower inter-cathode gap and anodes raised above the insulator surface, shows a threefold increase in gas gain reaching 10⁴ in argon 80%–ethane 20% gas mixture, which agrees with simulations. The new detector demonstrates a stable long-term operation at high gains, above 6000 measured during more than 70 h with an X-ray source, with a moderate 6% gain increase due to dielectric polarisation.     

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.     

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 .     

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.     

GRAPES-3—A high-density air shower array for studies on the structure in the cosmic-ray energy spectrum near the knee

The change in the spectral index from about -2.7 to -3.1 at 3×10¹⁵ eV in the all-particle energy spectrum of primary cosmic rays is very significant for learning about the nature of cosmic sources of ultra-high energy particles and their acceleration and propagation in the galactic disk. Any observation of a fine structure in the spectrum would be important for improving our understanding of these physical processes. The GRAPES-3 air shower array has been designed to achieve higher precision in determination of various shower parameters to enable observation of any fine structure in the energy spectrum, if it exists. The details of the shower detectors, shower trigger and the data acquisition system are presented here along with estimates of trigger efficiencies from Monte Carlo simulations for primary photons (γ-rays) and several nuclei.     

Gamma (γ) phase transformation in pulsed GTAW weld metal of duplex stainless steel

The cooling rate and large undercooling significantly affect the fusion zone microstructure in pulsed GTAW weldment under the same heat input condition. The weld pool solidified at fast cooling rate about 139 °C/s superimposed a relative amount of undercooling has a desired higher γ content of about 37 vol.% without tradition nitrogen addition or post-weld heat treatment. The final structure of the pulsed weld metal at 7 °C plate consists of a great amount of desirable intra-granular austenite γ^′₂ (IGA) inside the grain matrix, besides Widmannstätten austenite γ^′₂ (W) and grain boundary austenite γ₂ (GBA). It results in the weldment with an uniform microhardness distribution and a homogeneous mechanical property.     

GABRIELA: A new detector array for -ray and conversion electron spectroscopy of transfermium elements

With the aid of the Geant4 Monte Carlo simulation package a new detection system has been designed for the focal plane of the recoil separator VASSILISSA situated at the Flerov Laboratory of Nuclear Reactions, JINR, Dubna. GABRIELA (Gamma Alpha Beta Recoil Investigations with the ELectromagnetic Analyser VASSILISSA) has been optimised to detect the arrival of reaction products and their subsequent radioactive decays involving the emission of - and β-particles, fission fragments, γ- and X-rays and conversion electrons. The new detector system is described and the results of the first commissioning experiments are presented.     

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.     

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 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.