Osamu Yoda

Time-and-space resolved X-ray absorption spectroscopy of laser-ablated Si particles

We have performed time-and-space resolved X-ray absorption spectroscopy with a time resolution of 10 ns to study laser-ablated Si particles in a time scale ranging from 0 ns to 120 ns. Neutral and charged particles produced by laser ablation are observed through X-ray absorption spectra. Assignments of transitions from 2s and 2p initial states to higher Rydberg states of Si atom and ions are achieved, and we experimentally determine the LI \kern-1ptI,I \kern-1ptI \kern-1ptI absorption edges of neutral Si atom ( Si0) and Si ions such as Si+, Si2+, Si3+ and Si4+. The main ablated particles are found to be Si atom and Si ions in the initial stage of 0 ns to 120 ns. The relative amounts depend strongly on times and laser energy densities. We find that the spatial distributions of particles produced by laser ablation are changed with supersonic helium gas bombardment, but no cluster formation takes place. This suggests that a higher-density region of helium gas is formed at the top of the plume of ablated particles, and free expansion of particles is restrained by this helium cloud, and that it takes more than 120 ns to form Si clusters.

Positron annihilation in electron irradiated Cz-Si

Defects in electron irradiated Cz-Si were studied by the positron annihilation technique. In order to know effects of the thermal history of crystals on the introduction of defects, the specimen was quenched from 1390°C to room temperature before irradiation. A clear correlation between results obtained by the positron annihilation experiments and those by infrared spectroscopy was established. From the isochronal annealing experiment, it was found that oxygen clusters were introduced by quenching treatments.

New evaluation beamline for soft x-ray optical elements

An evaluation system capable of measuring the wavelength and angular characteristics of the absolute reflectivity (or diffraction efficiency) of soft x-ray optical elements has been designed and constructed. The system was installed on a beamline (BL-11) of the AURORA, a superconducting compact storage ring, at the Synchrotron Radiation Center, Ritsumeikan University. To cover a wavelength range of 0.5 nm<λ<25 nm, this system incorporates two types of Monk–Gillieson monochromators. One is a conventional type equipped with three varied-line-spacing gratings, allowing a choice of two included angles. The other is a new type that employs a scanning mechanism based on surface normal rotation. The outline of the system and some preliminary experimental data obtained in the course of test runs are described.

Electron Irradiation Effects on a Ba2YCu3O7 Superconductor

A single-phased Ba2YCu3O7-y pellet was sequentially irradiated with 3 MeV electrons up to 3?1022 m-2 at about 370 K, and the electrical resistance was measured as a function of temperature. The superconducting transition temperature of 92.5 K decreased almost linearly with electron dose at a rate of 3 K/1023 m-2 concurrently with a linear increase in electrical resistivity at 95 K from 2.9 ???m with 7 ???m/1023 m-2. The offset temperature measured with 105 kA ?m-2 in current density was enhanced from 88.6 to 90.4 K by electron irradiation to 1.5?1022 m-2, and then reduced upon further irradiation.

Analysis of the Asymmetric 002 X-Ray Line Profiles of γ-Irradiated Polyethylene

Asymmetric line profiles of 002 X-ray reflection diffracted from polyethylene have been analyzed. It is found that the asymmetry of the line profile is evidence for a specific correlation between the size and the lattice spacing in the crystallites. This means that the asymmetrical profile consists of two symmetrical profiles whose centroid positions and widths differ from each other. Application to γ-irradiated polyethylene reveals that degradation of crystallites occurs from the surface and that smaller crystallites with larger lattice spacing disappear more quickly when degraded by γ-irradiation.

Varied-line-spacing laminar-type holographic grating for the standard soft x-ray flat-field spectrograph

An aspheric wave-front recording system was designed to produce a holographic grating for use in a standard soft X- ray flat field spectrograph interchangeable with a mechanically ruled varied-line-spacing (VLS) grating. The grating grooves recorded with the designed aspheric wave- front recording system were processed to form a laminar groove profile by means of reactive ion etching. Measurements done with synchrotron radiation and a laboratory X-ray source are reported for this laminar-type grating and a commercial grating replicated from a mechanically ruled VLS grating that was specifically designed and fabricated for the standard soft X-ray flat- field spectrography. The laminar-type holographic grating is found to have an absolute first-order efficiency of approximately 10% for wavelengths of approximately 4.5 - 12 nm. It is also shown that the holographic grating is effective in suppressing the higher orders and stray-light level for soft X-ray of 4.36 nm (C-K) and has a comparable spectral resolution to the replica VLS grating.

Crystallite size distribution and the lattice distortions in highly ?-irradiated linear polyethylene

Crystallite size distributions and lattice distortions have been analysed for highlyγ-irradiated linear polyethylene in the direction normal to 〈1 1 0〉 and 〈0 0 1〉 lattice planes through X-ray line profile analysis. It has been found that the lateral crystallite size is little affected irradiation greater than 1000 Mrad, whereas that in the chain direction decreases almost linearly with the dose of irradiation. A minor difference in the crystallite size between irradiation atmospheres, i.e. irradiation in air or in vacuum, has been disclosed at a very high dose of irradiation. The above degradation behaviour of crystallites and the change in lattice distortions have proved that radiation cross-links are mainly formed in the amorphous region near the lamellar surface of polyethylene.

RDF analysis for the degree of interchain ordering in amorphous solid polyethylene

The differential radial distribution function (DRDF) of solid polyethylene amorphized by γ irradiation has been derived from the measurements of x‐ray diffuse scatterings. The first halo in the intensity distribution has been found to play the most important role on the degree of interchain ordering. The existence of interchain ordering which reaches as far as ∼20 A has been proved in the amorphized polyethylene. The degree of ordering differs in accordance with the dose and atmosphere of irradiation and the heat treatment.

Dynamics of Si plume produced by laser ablation in ambient inert gas and formation of Si nanoclusters

We have performed (1) time-resolved soft X-ray absorption measurements on a time scale up to 15 μs after pulsed-laser ablation of silicon (Si) in an ambient Ar gas with a transient pressure of about 10 Torr and (2) investigation of the correlation between 1.6-eV photoluminescence (PL) from Si nanocluster-based films and the surface oxidation. No soft X-ray absorption lines corresponding to Si clusters were observed, indicating that it is likely that after 15 μs, significant clustering takes place and then Si nanoclusters can grow. From experiments involving hydrogen termination and hydrogen atom treatment in addition to natural oxidation, it is found that the 1.6-eV PL originates from the interface of a Si nanocluster core and a surface oxide layer and occurs without hydrogen atoms.

Crystallite size distribution and lattice distortions in polyethylene from analyses of Debye‐Scherrer line profiles

Distributions of crystallite sizes and lattice distortions in the direction normal to (110) of the polyethylene crystal are derived from 110 Debye‐Scherrer profiles, by means of a Fourier technique developed by one of the present authors. Samples investigated are linear polyethylene powder, single‐crystal mats grown at 85 °C and 70 °C, and branched polyethylene powder with a different methyl‐group concentration. It is found that in the crystallite size distribution each of the polyethylenes examined has a common peak at about 100, 250, 350, and 450 A. The presence of branches with lower methyl concentration does not seriously affect the distribution, but at the highest concentration of branches (29 CH3/1000 CH2) peaks at 350 and 450 A disappear. Lattice distortions in the crystallites of polyethylene cannot be always explained by paracrystalline or strained‐lattice‐type strain alone, but may be generally expressed by a superposition of them.