arXiv subjects

K. Watanabe

Publications and source records attributed to K. Watanabe.

At least 433 records · Page 24Linked to original sources

arXiv2006

A Comprehensive Search for Gamma-Ray Lines in the First Year of Data from the INTEGRAL Spectrometer

We have carried out an extensive search for gamma-ray lines in the first year of public data from the Spectrometer (SPI) on the INTEGRAL mission. INTEGRAL has spent a large fraction of its observing time in the Galactic Plane with particular concentration in the Galactic Center (GC) region (~ 3 Msec in the first year). Hence the most sensitive search regions are in the Galactic Plane and Center. The phase space of the search spans the energy range 20-8000 keV and line widths from 0-1000 keV (FWHM). It includes both diffuse and point-like emission. We have searched for variable emission on time scales down to ~ 1000 sec. Diffuse emission has been searched for on a range of different spatial scales from ~ 20 deg (the approximate field-of-view of the spectrometer) up to the entire Galactic Plane. Our search procedures were verified by the recovery of the known gamma-ray lines at 511 keV and 1809 keV at the appropriate intensities and significances. We find no evidence for any previously unknown gamma-ray lines. The upper limits range from a few x 10^-5 cm^-2 s^-1 to a few x 10^-2 cm^-2 s^-1 depending on line width, energy and exposure; regions of strong instrumental background lines were excluded from the search. Comparison is made between our results and various prior predictions of astrophysical lines.

astro-ph↗

arXiv2006

Enhanced critical current density of MgB2 superconductor synthesized in high magnetic fields

The effect of high magnetic fields on the current carrying properties of both MgB2 bulks and Fe-sheathed tapes was investigated following different thermal sequences. It is found that application of a large magnetic field during processing results in the quite uniform microstructure and the better connectivity between the MgB2 grains. As a result, the Jc of these samples has shown much higher value than that of the MgB2 samples in the absence of magnetic field. The possible mechanism of the Jc enhancement under an external magnetic field is also discussed.

cond-mat.supr-con↗

arXiv2006

Enhancement of JC-B Properties in MoSi2 doped MgB2 tapes

MoSi2 doped MgB2 tapes with different doping levels were prepared through the in-situ powder-in-tube method using Fe as the sheath material. Effect of MoSi2 doping on the MgB2/Fe tapes was investigated. It is found that the highest JC value was achieved in the 2.5 at.% doped samples, more than a factor of 4 higher compared to the undoped tapes at 4.2 K, 10 T, then further increasing the doping ratio caused a reduction of JC. Moreover, all doped tapes exhibited improved magnetic field dependence of Jc. The enhancement of JC-B properties in MoSi2 doped MgB2 tapes is attributed to good grain linkage and the introduction of effective flux pining centers with the doping.

cond-mat.supr-con↗

arXiv2006

Significantly enhanced critical current densities in MgB2 tapes made by a scaleable, nano-carbon addition route

Nanocarbon-doped Fe-sheathed MgB2 tapes with different doping levels were prepared by the in situ powder-in-tube method. Compared to the undoped tapes, Jc for all the C-doped samples was enhanced by more than an order of magnitude in magnetic fields above 9 T. At 4.2 K, the transport Jc for the 5 at% doped tapes reached 1.85x104 A/cm2 at 10 T and 2.8x103 A/cm2 at 14 T, respectively. Moreover, the critical temperature for the doped tapes decreased slightly. Transmission electron microscopy showed a number of intra-granular dislocations and the dispersed nanoparticles embedded within MgB2 grains induced by the C doping. The mechanism for the enhancement of flux pinning is also discussed. These results indicate that powder-in-tube-processed MgB2 tape is very promising for high-field applications.

cond-mat.supr-con↗

arXiv2005

Effect of nano-C doping on the in-situ processed MgB2 tapes

The effect of nano-C doping on the microstructure and superconducting properties of Fe-sheathed MgB2 tapes prepared through the in-situ powder-in-tube method was studied. Heat treatment was performed at a low temperature of 650C for 1 h. Scanning electron microscopy investigation revealed that the smaller grain size of MgB2 in the samples with the C-doping. Further, the a-axis lattice parameter and transition temperature decreased monotonically with increasing doping level, which is due to the C substitution for B. High critical current density Jc values in magnetic fields were achieved in the doped samples because of the very fine-grained microstructure of the superconducting phase obtained with C doping.

cond-mat.supr-con↗

arXiv2005

Competition between hidden order and antiferromagnetism in URu_2Si_2 under uniaxial stress studied by neutron scattering

We have performed elastic neutron scattering experiments under uniaxial stress sigma applied along the tetragonal [100], [110] and [001] directions for the heavy electron compound URu2Si2. We found that antiferromagnetic (AF) order with large moment is developed with sigma along the [100] and [110] directions. If the order is assumed to be homogeneous, the staggered ordered moment mu_o continuously increases from 0.02 mu_B (sigma=0) to 0.22 mu_B (0.25 GPa). The rate of increase partial mu_o/partial sigma is ~ 1.0 mu_B/GPa, which is four times larger than that for the hydrostatic pressure (partial mu_o/partial P sim 0.25 mu_B/GPa). Above 0.25 GPa, mu_o shows a tendency to saturate, similar to the hydrostatic pressure behavior. For sigma||[001], mu_o shows only a slight increase to 0.028 mu_B (sigma = 0.46 GPa) with a rate of ~ 0.02 mu_B/GPa, indicating that the development of the AF state highly depends on the direction of sigma. We have also found a clear hysteresis loop in the isothermal mu_o(sigma) curve obtained for sigma||[110] under the zero-stress-cooled condition at 1.4 K. This strongly suggests that the sigma-induced AF phase is metastable, and separated from the "hidden order" phase by a first-order phase transition. We discuss these experimental results on the basis of crystalline strain effects and elastic energy calculations, and show that the c/a ratio plays a key role in the competition between these two phases.

cond-mat.str-el↗

arXiv2005

Solar Neutron Events of October-November 2003

During the period when the Sun was intensely active on October-November 2003, two remarkable solar neutron events were observed by the ground-based neutron monitors. On October 28, 2003, in association with an X17.2 large flare, solar neutrons were detected with high statistical significance (6.4 sigma) by the neutron monitor at Tsumeb, Namibia. On November 4, 2003, in association with an X28 class flare, relativistic solar neutrons were observed by the neutron monitors at Haleakala in Hawaii and Mexico City, and by the solar neutron telescope at Mauna Kea in Hawaii simultaneously. Clear excesses were observed at the same time by these detectors, with the significance calculated as 7.5 sigma for Haleakala, and 5.2 sigma for Mexico City. The detector onboard the INTEGRAL satellite observed a high flux of hard X-rays and gamma-rays at the same time in these events. By using the time profiles of the gamma-ray lines, we can explain the time profile of the neutron monitor. It appears that neutrons were produced at the same time as the gamma-ray emission.

astro-ph↗

arXiv2004

Amplification of Quantum Meson Modes in the Late Time of Chiral Phase Transition

It is shown that there exists a possibility of amplification of amplitudes of quantum pion modes with low momenta in the late time of chiral phase transition by using the Gaussian wave functional approximation in the O(4) linear sigma model. It is also shown that the amplification occurs in the mechanism of the resonance by forced oscillation as well as the parametric resonance induced by the small oscillation of the chiral condensate. These mechanisms are investigated in both the case of spatially homogeneous system and the spatially expanded system described by the Bjorken coordinate.

hep-ph↗

arXiv2004

INTEGRAL/SPI Limits on Electron-Positron Annihilation Radiation from the Galactic Plane

The center of our Galaxy is a known strong source of electron-positron 511-keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (INTErnational Gamma-RAy Astrophysics Laboratory) mission, launched in Oct. 2002, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high resolution coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic Plane. No positive annihilation flux was detected outside of the central region (|l| > 40 deg) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511-keV flux.

astro-ph↗

arXiv2003

Constraining the Cosmic Star Formation Rate with the MeV Background

The Cosmic Gamma-ray Background (CGB) in the MeV regime has been measured with COMPTEL and SMM. The origin of the CGB in this energy regime is believed to be dominated by gamma-rays from Type Ia supernovae. We calculate the CGB spectrum within the framework of FRW cosmology as a function of the cosmic star formation rate, SFR(z). Several estimates of the SFR(z) have been reported since the pioneering work of Madau et al. Here we discuss observational constraints on SFR(z) derived from models of the CGB. In particular, we consider the SFR obtained from Gamma-Ray Burst observations, which increases dramatically with redshift beyond z ~ 1 in contrast to most estimates which saturate or show a mild increase with redshift. Gamma-ray bursts may be the most powerful tracers of star formation in the early universe and thus provide signposts of the initial epoch of element synthesis. The star formation rate implied by GRB statistics results in a gamma-ray background that matches the observations more closely than that inferred from other tracers of star formation. This may provide some support for the GRB/SFR-paradigm, which in turn promises a powerful diagnostic of star formation, and thus cosmic chemical evolution, from the era of Population III stars to the present.

astro-ph↗

arXiv2003

The Data Acquisition System Based on PMC Bus

High energy physics experiments in KEK/Japan rush into over KHz trigger stage. Thus, we need a successor of the data acquisition(DAQ) system that replaces the CAMAC or FASTBUS systems. To meet these needs, we have developed a DAQ system which includes a crate, base-board modules, daughter cards for front-end A/D or T/D conversion, and back-end communication cards for data transfer and timing control. The size of the crate is for the 9U Euro-cards with the standard VME32 bus and extension connectors for power supply. The base-board comprises of a local bus with the sequencer connected to the front-end daughter cards via event buffering FIFOs, and the standard PMC (PCI mezzanine card) bus to be set a PMC processor unit to reduce data size from the front-end daughter cards. A data transfer module, which is connected to the event building system, and a trigger control unit, which communicates with the central timing controller are installed on the back-end communication card connected to the rear end of the base-board. We describe the design of this DAQ system and evaluate the performance of it.

physics.ins-det↗

arXiv2003

Development of a PCI Based Data Acquisition Platform for High Intensity Accelerator Experiments

Data logging at an upgraded KEKB accelerator or the J-PARC facility, currently under commission, requires a high density data acquisition platform with integrated data reduction CPUs. To follow market trends, we have developed a DAQ platform based on the PCI bus, a choice which permits a fast DAQ and a long expected lifetime of the system. The platform is a 9U-VME motherboard consisting of four slots for signal digitization modules, readout FIFOs for data buffering, and three PMC slots, on one of which resides a data reduction CPU. We have performed long term and thermal stability tests. The readout speed on the platform has been measured up to 125 MB/s in DMA mode.

hep-ex↗

arXiv2003

Solar Neutron Event in Association with a Large Solar Flare on November 24, 2000

Solar neutrons have been detected using the neutron monitor located at Mt. Chacaltaya, Bolivia, in association with a large solar flare on November 24, 2000. This is the first detection of solar neutrons by the neutron monitor that have been reported so far in solar cycle 23. The statistical significance of the detection is 5.5 sigma. In this flare, the intense emission of hard X-rays and gamma-rays has been observed by the Yohkoh Hard X-ray Telescope (HXT) and Gamma Ray Spectrometer (GRS), respectively. The production time of solar neutrons is better correlated with those of hard X-rays and gamma-rays than with the production time of soft X-rays. The observations of the solar neutrons on the ground have been limited to solar flares with soft X-ray class greater than X8 in former solar cycles. In this cycle, however, neutrons were detected associated with an X2.3 solar flare on November 24, 2000. This is the first report of the detection of solar neutrons on the ground associated with a solar flare with its X-ray class smaller than X8.

astro-ph↗

arXiv2002

Picosecond Nonlinear Relaxation of Photoinjected Carriers in a Single GaAs/AlGaAs Quantum Dot

Photoemission from a single self-organized GaAs/AlGaAs quantum dot (QD) is temporally resolved with picosecond time resolution. The emission spectra consisting of the multiexciton structures are observed to depend on the delay time and the excitation intensity. Quantitative agreement is found between the experimental data and the calculation based on a model which characterizes the successive relaxation of multiexcitons. Through the analysis we can determine the carrier relaxation time as a function of population of photoinjected carriers. Enhancement of the intra-dot carrier relaxation is demonstrated to be due to the carrier-carrier scattering inside a single QD.

cond-mat.mtrl-sci↗

arXiv2002

Nonequilibrium Antiferromagnetic State in the Heavy Electron Compound URu_2Si_2

We have investigated the nature of the antiferromagnetic (AF) phase induced by uniaxial stress sigma in URu2Si2, by performing elastic neutron scattering measurements up to 0.4 GPa. We have found that the AF Bragg-peak intensity shows a clear hysteresis loop with sigma under the zero-stress cooling condition. The result strongly suggests that the sigma-induced AF phase is metastable and separated from the coexisting "hidden ordered" phase by a first-order phase transition. We also present the analyses of the crystalline strain effects, and suggest that the c/a ratio plays an important role in the competition between these two phases.

cond-mat.str-el↗

arXiv2002

Effects of Uniaxial Stress on Antiferromagnetic Moment in the Heavy Electron Compound URu_2Si_2

We have performed the elastic neutron scattering experiments under uniaxial stress σalong the tetragonal [100], [110] and [001] directions for URu2Si2. For σ// [100] and [110], the antiferromagnetic moment μ_o is strongly enhanced from 0.02 μ_B (σ=0) to 0.22 μ_B (σ=2.5 kbar) at 1.5 K. The rate of increase dμ_o/dσis roughly estimated to be ~ 0.1 μ_B/kbar, which is much larger than that for the hydrostatic pressure (~ 0.025 μ_B/kbar). Above 2.5 kbar, μ_o shows a tendency to saturate similar to the behavior in the hydrostatic pressure. For σ// [001], on the other hand, μ_o shows only a slight increase to 0.028 μ_B (σ= 4.6 kbar) with a rate of ~ 0.002 μ_B/kbar. The observed anisotropy suggests that the competition between the hidden order and the antiferromagnetic state in URu2Si2 is strongly coupled with the tetragonal four-fold symmetry and the c/a ratio, or both.

cond-mat.str-el↗

arXiv2002

Gamma-Ray Bursts and Cosmic Radiation Backgrounds

GRBs contribute to the evolving cosmic radiation field. We discuss the contribution of GRBs to the high-energy background, and the effect of pair creation off low-energy photons on their observable TeV spectrum.

astro-ph↗

arXiv2001

Contributions of GRBs and Cen A-like Radio Galaxies to the Cosmic Gamma-ray Background

The contribution to the cosmic diffuse gamma-ray background (CGB) from Gamma Ray Bursts (GRBs) is studied in the 40 keV - 2 MeV regime. We use High Energy Resolution (HER) data from the Burst And Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory (CGRO) to generate a GRB template spectrum. Although the GRB contribution to the CGB is generally small, in comparison to the dominant flux from Type Ia supernovae, the integrated GRB flux is in fact comparable to that from SNIa in the narrow 10-40 keV range. GRBs contribute to the CGB at the same level as Type II supernovae do. Although BATSE data are not available below ~40 keV, extrapolation of the template spectrum suggests that bursts can fill a significant part of the existing gap between Seyfert galaxies (dominating the CGB below ~ 100 keV) and SNIa (dominating at ~1 MeV). We estimate contributions from Cen A-like (FR I) radio galaxies in this energy regime, where INTEGRAL data is expected to provide major advances.

astro-ph↗