Tomohisa Mikado

Slow Positron Pulsing System for Variable Energy Positron Lifetime Spectroscopy

A positron pulsing system has been constructed for variable energy positron lifetime spectroscopy. The system consists of a reflection type chopper, a sub-harmonic prebuncher, and a double harmonic buncher. By operating the system with an intense slow positron beam generated by an electron linac, positron lifetime spectra have been successfully measured in an extended time range of more than 45 ns with a good time resolution (250 ps). Furthermore, Doppler broadening profiles of annihilation radiations can be obtained simultaneously with the lifetime measurements.

Study of defects in GaN grown by the two-flow metalorganic chemical vapor deposition technique using monoenergetic positron beams

Defects in GaN grown using metalorganic chemical vapor deposition were studied through the use of monoenergetic positron beams. For Mg-doped GaN, no large change in the diffusion length of positrons was observed before and after activation of Mg. This was attributed to the scattering of positrons by potentials caused by electric dipoles of Mg–hydrogen pairs. For Si-doped GaN, the line-shape parameter S increased as carrier density increased, suggesting an introduction of Ga vacancy due to the Fermi level effect. Based on these results, we discuss the effects of the growth polar direction of GaN on optical properties in this article. Although the optical properties of a GaN film grown toward the Ga face direction exhibited excitonic features, a film grown toward the N face (−c) direction exhibited broadened photoluminescence and transmittance spectra, and a Stokes shift of about 20 meV was observed. This difference was attributed to extended band-tail states introduced by high concentrations of donors and ...

A positron lifetime spectroscopy apparatus for surface and near-surface positronium experiments

Abstract A positron lifetime spectroscopy apparatus, which utilizes an intense pulsed positron beam generated by an electron linear accelerator, is presented. Using this apparatus we can measure positron lifetime spectra with variable energy, high count rate, wide measurable time range, and high peak-to-background ratio. These features are suitable for surface and near-surface positronium experiments.

First lasing of the NIJI-IV storage-ring free-electron laser

Abstract The first lasing at around 590 nm of a free-electron laser (FEL) in the storage ring NIJI-IV was achieved on August 18, 1992, though just above threshold. The NIJI-IV is a racetrack-type ring dedicated to FEL, which was completed in December 1990. Though the ring is compact, it has two 7.25-m straight sections. The electron beam was stacked for the first time in February 1991. After a 6.3-m optical klystron was installed in one of the straight sections, FEL related experiments were started at the end of April this year, and oscillation experiments have been carried out since the beginning of August. The electron-beam energy during the lasing was about 240 MeV, and the current was 1.1−0.2 mA/bunch. The laser wavelength ranged from 594.5 to 588.7 nm, and the bandwidth was below 0.4 nm.

A 600-MeV ETL Electron Storage Ring

A 600-MeV electron storage ring has been constructed in only ten months at a cost of 200 million yen at the Electrotechnical Laboratory (ETL) in Tsukuba. The 500-MeV high efficiency-high current electron linac being operated for high energy-dosimetric experiments serves as an injector. The ring consists of eight bending magnets (r = 2m) and four triplets of quadrupole magnet. The circumference is 31.45 m and ¿p is 22 Å. The lattice order is O/2 BdQfQdQfBd O/2. The harmonic number is 17. The maximum stored current is 160 mA and l/e lifetime is 1.5 hours at present. The radiation is used for calibrating photometry and soft X-ray standards, photoelectric analysis of electronic materials, and LSI lithography.

Measurement of laser-induced Compton backscattered photons with anti-Compton spectrometer

Quasi-monochromatic photons of energies of 1.6-8.7 MeV have been generated by the Compton backscattering of laser light on relativistic electrons in the TERAS storage ring. The spectra of the backscattered photons have been measured with an anti-Compton spectrometer system. The present system has a 155-cm/sup 3/ a coaxial-type pure-Ge detector as the central detector and a well-type NaI(Tl) scintillator of 8-in phi *8-in as the outer one. With a usual anticoincidence technique, clear photopeak spectra were obtained with a Compton suppression ratio of 2-4. The maximum energy and the energy spread of photons show reasonable agreement with numerical calculations. The divergence and energy spread of the electron beam in the storage ring are estimated from the data. >

Generation of Quasi-Monochromatic Photon Beams from Compton Backscattered Laser Light at ETL Electron Storage Ring

Generation of quasi-monochromatic photon beams of energy variable between 1.7 - 6.5 MeV has been observed by inducing the Compton backscattering of laser light quanta on electrons of energy 305 - 606 MeV at the injection straight section of the 600-MeV ETL storage ring. A Nd-YAG laser of wavelength 1.0641¿m has been used with output power 0.7 - 1.2 W. The resultant photon spectra are measured with a pure Ge detector, some of which are presented in the present report together with some description on the experimental methods. The absolute yields are being measured by the use of an 8 inch Ø × 8 inch long NaI scintillation counter.

Linearly polarized photons from Compton backscattering of laser light for nuclear resonance fluorescence experiments

Elastically scattered photons from 208Pb(γpol, γ) have been measured with completely polarized photons. The polarized photons (laser Compton photons) can be generated by Compton backscattering of laser light. The energy of the polarized LCPs ranges from 1 to 10 MeV by using a Q-switched Nd:YAG laser and the storage ring TERAS at Electrotechnical Laboratory. We can rotate the polarization axis of the LCPs by using the laser polarization controller to diminish the systematic error in the measurement system. Parities of J = 1 levels in 208Pb, Ex = 5.514 and 4.841 MeV, were clearly determined to be negative.

Characterization of Hydrogenated Amorphous Silicon Films by a Pulsed Positron Beam

A low-energy pulsed positron beam was, for the first time, applied to the characterization of hydrogenated amorphous silicon films (~1 µm in thickness) deposited by means of plasma-enhanced chemical-vapor deposition. By the use of a pulsed positron beam, positron lifetime spectra were measured on four films deposited under different rf-power densities. In two of the films prepared at the intermediate power densities, a long-lived component (τ9 ns) was observed, which indicates that a high concentration of voids exists. A component with a lifetime of 319 ps, which is due to trapped positrons at divacancies or small vacancy clusters, was observed in the film prepared at the lowest power density, while a component with a lifetime of 390 ps-440 ps, which is due to trapped positrons at large vacancy-type defects, was observed in the other films. The results clearly indicate that the defect properties are strongly influenced by the rf-power density.

Nanoporous structure of methyl-silsesquioxane films using monoenergetic positron beams

The size and depth distributions of pores in silica-based intermetal-dielectric materials were studied using monoenergetic positron beams. Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons were measured for methyl-silsesquioxane (MSSQ) spin-on-glass films. The size distribution of pores in the MSSQ films fabricated with 8% porogen load was found to be bimodal, with the major peaks located at 3 and 8 nm3. Increasing the porogen load from 8% to 40% caused the smaller pores (3 nm3) to disappear and 30-nm3 ones to appear; these pores were considered to be interconnected, and this structure makes it possible for positronium (Ps) atoms to find paths towards the surface and to escape into vacuum. The 8%-porogen MSSQ films had low porosity near the Si substrate. From measurements of the temperature dependence of the self-annihilation rate of ortho-Ps, we discuss the relationship between o-Ps emission into vacuum and the pore structure.