A novel apparatus for the investigation of material properties for the storage of ultracold neutrons

We have built a novel apparatus for the investigation of materials for the storage of ultracold neutrons. Neutrons are filled into a storage volume, confined at the bottom by a magnetic field, at the top by gravity and at the sides by the slit-less sample surface under investigation. For different beryllium and diamond-like carbon samples, storage times up to 200 s were obtained at room temperature. The corresponding loss parameters η for ultracold neutrons varied between 4.2 and 6.8×10^-4 per wall collision.     

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.     

Magnetic field effects on particle trajectories in the muon-spin relaxation experiment: Towards a high-field spectrometer

As a step towards the design of a muon-spin relaxation (μ⁺SR) spectrometer with a high magnetic field capability, we have carried out an investigation into the effects of applying a large magnetic field on the trajectories of particles in a μ⁺SR experiment. We compare the results of measurements and Geant4 simulations of the influence of applied fields on the incoming muon beam and the outgoing decay positron ensemble. Good agreement between measurements and simulations is found and the limits of this agreement are discussed.     

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.     

ILC beam energy measurement based on synchrotron radiation from a magnetic spectrometer

We propose to measure, on a bunch-to-bunch basis, the beam energy at the International Linear e⁺e^- Collider by recording synchrotron radiation (SR) light emitted in the magnets of an energy spectrometer based on beam position monitors. Measuring the width of the horizontal SR fan permits to determine the relative beam energy with a precision better than 10^-4. There are two different measuring schemes possible. The first one is based on edge measurements of the direct SR fan, while the second option includes mirrors to deflect soft SR light to detectors located sufficiently off the beamline. Three possibilities for high-spatial resolution detectors are considered: a standard silicon strip detector, a novel-type Si detector and a gas amplification detector, both with exceptional position resolution. The main issue of the first scheme is the high radiation dose expected in the direct SR fan. If mirrors are used this dose is strongly reduced and allows application of any of the three detectors proposed.     

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.     

Coincidence Doppler broadening study on hydrocarbons with pi and sigma valence electrons: positronium correction

The coincidence Doppler broadening (CDB) technique was applied to study the electron momentum distribution in anthracene, diphenyl, naphthalene, and polystyrene. A method for separation of the positron and positronium (Ps) components from the Doppler-broadened annihilation line (DBAL) was developed further to be applicable to hydrocarbons with different π and σ valence electron distributions. This method allows extraction of the electron momentum distribution (EMD) from DBAL for samples when Ps formation occurs. The annihilation on π valence electrons was detected as broadening of the EMD compared to that obtained for a polymer sample only with σ valence electrons. The broadening appeared as a significant change in the shape of the CDB ratio of the corresponding positronium-corrected curves: a slight enhancement above the unity line in the low-momentum region and a drop in the momentum region, 10–20×10⁻³m_oc.     

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.     

Composite detector for mixed radiations based on CsI(Tl) and dispersions of small ZnSe(Te) crystals

A new large area detector of high-energy X-ray and β-radiation has been designed and studied. A composite material based on small-crystalline ZnSe(Te) was applied onto the wide surface of a light guide. An experimental specimen has been prepared, which showed β-sensitivity . The spectrograms of a ⁹⁰Sr+⁹⁰Y β-source obtained with the specimen under study make it possible to evaluate the age of the source by the ratio of low- and high-energy regions of the spectrum.

The combined detector (CD) comprises a single crystalline plate of ZnSe(Te) placed onto the output window of a scintillating transparent light guide made of CsI(Tl) in the shape of a truncated pyramid. The CsI(Tl) light guide is used to create an additional channel for detection of γ-radiation, as well as for protecting the photodiode from the penetrating radiation. It is shown that introduction of the light guide does not worsen the energy resolution characteristics of ZnSe(Te). Separate detection of - and γ-radiation has been achieved under simultaneous excitation by ²³⁹Pu (ZnSe(Te), R=6%) and ²⁴¹Am (CsI(Tl), R_γ=20%). The use of selective optical filters allows separation of the peaks of total absorption (p.t.a.) in the case of their superposition.     

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.     

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.     

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^′.     

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.     

Application of large area avalanche photodiodes for alpha liquid scintillation counting

In order to improve the resolution in alpha liquid scintillation and to promote this method for the measurement of actinides in environment, a silicon Large Area Avalanche Photodiode (LAAPD) has been tested as liquid scintillation detector. The set-up consists of a LAAPD (16 mm diameter) coupled to a thin vial containing alpha-emitters and a liquid scintillation cocktail. After optimization of several parameters like bias voltage, temperatures, counting geometry and composition of scintillating cocktail, energy resolutions are found to be better than those obtained with photomultiplier tubes: 5% (200 keV FWHM) for ²³²Th and 3.9% (260 keV FWHM) for ²¹⁶Po. The improvement in energy resolution is not due to higher quantum efficiency of the silicon device but to better uniformity of the spatial photoresponse. A few energy spectra of thorium, plutonium and americium are given as example.     

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 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.     

A 600 kV 15 mA Cockcroft–Walton high-voltage power supply with high stability and low-ripple voltage

A Cockcroft–Walton high-voltage power supply with high stability and low-ripple voltage has been developed. This power supply has been operated in a ns pulse neutron generator. The maximum non-load voltage is 600 kV while the working voltage and load current are 550 kV and 15 mA, respectively. The tested results indicate that when the power supply is operated at 300 kV, 6.7 mA and the input voltage varies ±10%, the long-term stability of the output voltage is S=(0.300–1.006)×10^-3. The ripple voltage is at 300 kV, 6.8–8.3 mA and the ratio of δU_P−P to the output voltage V_H is δU_P-P/V_H=2.1×10^-5.     

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.     

Response of the GLAST LAT calorimeter to relativistic heavy ions

The CsI calorimeter of the Gamma-Ray Large-Area Space Telescope (GLAST) will be calibrated in flight with cosmic-ray heavy ions. In order to determine the response of the calorimeter to relativistic heavy ions lighter than Fe, an experiment was carried out at the GSI heavy ion facility using the Fragment Separator (FRS). The measured response exhibits an unexpected feature for light ions, opposite to that observed at low incident energy: for a given deposited energy, the observed signal is greater for these ions than for protons (or more generally Z=1 minimum ionizing particles). Pulse shapes are found to be almost identical for carbon ions and Z=1 particles, with a significant slow scintillation component, which constitutes another departure from the low-energy behavior. Data on the energy resolution for the individual CsI crystals and on the loss of ions due to nuclear reactions in the calorimeter are also presented.     

Diffraction field of tightly focused laser beams and its influence to laser accelerator

In this paper, we have studied the dynamic characteristics of relativistic electron injected into stationary intense vacuum laser fields. We found the dynamic trajectories can basically be classified as three categories, namely Inelastic Scattering (IS), Capture and Acceleration Scenario (CAS) and Penetrate into Axial Region and Move (PARM) trajectory. The physical mechanism as to the three kinds of electrons have been examined. In particular, the PARM trajectory which we presented in this paper is different from the CAS and IS trajectory which we had already found in our previous work. We will show the PARM stems from the strong diffraction effect of a tightly focused laser field. In addition, the initial condition for the three kinds of electrons to emerge were detailed investigated. It has been found that there are four factors which chiefly decide the appearance of the three kinds of dynamics trajectories, namely the laser beam width w₀ and intensity a₀, the electrons incident angle θ and initial transversal momentum p_ti. The implication of the PARM electrons to the planned vacuum laser accelerators is illustrated.