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Search for sub-GeV dark matter by annual modulation using XMASS-I detector
Kobayashi, M.,Abe, K.,Hiraide, K.,Ichimura, K.,Kishimoto, Y.,Kobayashi, K.,Moriyama, S.,Nakahata, M.,Ogawa, H.,Sato, K.,Sekiya, H.,Suzuki, T.,Takeda, A.,Tasaka, S.,Yamashita, M.,Yang, B.S.,Kim, N.Y.,K North-Holland Pub. Co 2019 Physics letters. Section B Vol.795 No.-
<P><B>Abstract</B></P> <P>A search for dark matter (DM) with mass in the sub-GeV region (0.32–1 GeV) was conducted by looking for an annual modulation signal in XMASS, a single-phase liquid xenon detector. Inelastic nuclear scattering accompanied by bremsstrahlung emission was used to search down to an electron equivalent energy of 1 keV. The data used had a live time of 2.8 years (3.5 years in calendar time), resulting in a total exposure of 2.38 ton-years. No significant modulation signal was observed and 90% confidence level upper limits of 1.6 × <SUP> 10 − 33 </SUP> cm<SUP>2</SUP> at 0.5 GeV was set for the DM-nucleon cross section. This is the first experimental result of a search for DM mediated by the bremsstrahlung effect. In addition, a search for DM with mass in the multi-GeV region (4–20 GeV) was conducted with a lower energy threshold than previous analysis of XMASS. Elastic nuclear scattering was used to search down to a nuclear recoil equivalent energy of 2.3 keV, and upper limits of 2.9 × <SUP> 10 − 42 </SUP> cm<SUP>2</SUP> at 8 GeV was obtained.</P>
T. Ogawa,K. Seto,D. Hasegawa,H. T. Yang,H. Kura,M. Doi,M. Takahashi 한국자기학회 2011 Journal of Magnetics Vol.16 No.3
In order to obtain mono-dispersed Fe NPs with high saturation magnetization, quantitative analysis method to investigate the growth dynamics of the Fe NPs synthesized by a conventional thermal decomposition method has been developed. As a result, fast nucleation process promotes formation of ~4 ㎚ of initial nucleus with a non-equilibrium phase, resulting in low saturation magnetization. And slow particle growth with atomic-scaled surface precipitation mode (< 100 atoms/(minㆍ㎚²)) can form the growth layer on the surface of initial nucleus with high saturation magnetization (~190 emu/gFe) as an equilibrium a phase of Fe. Therefore, higher stabilization of small initial nucleus generated just after the injection of Fe(CO)? should be one of the key issues to achieve much higher Ms of Fe NPs.
One-pot synthesis of high magnetization air-stable FeCo nanoparticles by modified polyol method
Abbas, M.,Nazrul Islam, Md.,Parvatheeswara Rao, B.,Ogawa, T.,Takahashi, M.,Kim, C. North-Holland 2013 Materials letters Vol.91 No.-
High magnetization FeCo nanoparticles with different Fe/Co ratios have been successfully synthesized by surfactant free simple modified polyol method. Polyethylene glycol (PEG) was used as solvent and reducing agent simultaneously in this synthesis process. All the synthesized samples of FeCo nanoparticles were annealed at 600<SUP>o</SUP>C before characterizations. X-ray diffraction (XRD) data on the samples confirm formation of a body-centered-cubic single phase structure in all the compositions. Transmission Electron Microscopy (TEM) data suggest that the annealed FeCo nanoparticles are of 50-90nm in size. The use of PEG and the annealing procedure employed ensure that the obtained nanoparticles are stable in air. This observation is well supported by both the analysis of Energy Dispersive Spectrometry (EDS) and the images of TEM which establish the formation of a thin passive oxide layer over the FeCo nanoparticles thereby resulting in the stability of the nanoparticles. The physical Property Measurement System (PPMS) reveals that the Fe<SUB>60</SUB>Co<SUB>40</SUB> composition among all the samples exhibit highest saturation magnetization of 230.14emu/g at 5K.
T. Ogawa,M. Nakayama,M. Haraguchi,M. Kuwahara,M. Fukui,S. Matsuo,T. Okamoto 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.1
We have macroscopically and microscopically investigated the localized surface plasmons (LSPs)on Ag anoparticles embedded in porous TiO2 glass. We have prepared TiO2 glass containing Ag nanoparticles (Ag/TiO2) by the sol-gel process and changed the porosity of the TiO2 glass by drying under a high vacuum condition. Using a spectrometer, we have macroscopically measured the absorption spectra (ABS) on all Ag nanoparticles and using near-field scanning optical microscopy (NSOM), we have microscopically measured the scattering spectra on a single Ag nanoparticle in the Ag/TiO2 after each drying process. Fitting the spectra derived from the numerical calculation based on the Mie theory to that evaluated from the ABS and the NSOM measurements, we have evaluated the deviation of the full width at half maximum evaluated by the experimental measurements from that derived from the theoretical calculation (FWHM) and the bound (Hashin-Shtrikman bounds) of the porosity of the TiO2 glass by the use of the Hashin Shtrikman limit, similar to the literature [1]. It has been found that the FWHM of the scattering spectra on the single Ag nanoparticle can be much smaller than one of the ABS on all Ag nanoparticles in the Ag/TiO2. This result is due to that the ABS is influenced by the aggregation and the size distribution of all Ag nanoparticles. Furthermore, on extrapolating from the plot of FWHM vs porosity, the FWHM is reduced to zero when the porosity is about 15 %.
Abe, K.,Hiraide, K.,Ichimura, K.,Kishimoto, Y.,Kobayashi, K.,Kobayashi, M.,Moriyama, S.,Nakahata, M.,Norita, T.,Ogawa, H.,Sato, K.,Sekiya, H.,Takachio, O.,Takeda, A.,Tasaka, S.,Yamashita, M.,Yang, B.S Elsevier 2018 Nuclear Instruments & Methods in Physics Research. Vol.884 No.-
<P><B>Abstract</B></P> <P>We established a method to assay <SUP>210</SUP>Pb and <SUP>210</SUP>Po contaminations in the bulk of copper samples using a low-background alpha particle counter. The achieved sensitivity for the <SUP>210</SUP>Pb and <SUP>210</SUP>Po contaminations reaches a few mBq/kg. Due to this high sensitivity, the <SUP>210</SUP>Pb and <SUP>210</SUP>Po contaminations in oxygen free copper bulk were identified and measured for the first time. The <SUP>210</SUP>Pb contaminations of our oxygen free copper samples were 17–40 mBq/kg. Based on our investigation of copper samples in each production step, the <SUP>210</SUP>Pb in oxygen free copper was understood to be a small residual of an electrolysis process. This method to measure bulk contaminations of <SUP>210</SUP>Pb and <SUP>210</SUP>Po could be applied to other materials.</P>
SEARCH FOR NEUTRINOS FROM GRB 080319B AT SUPER-KAMIOKANDE
Thrane, E.,Abe, K.,Hayato, Y.,Iida, T.,Ikeda, M.,Kameda, J.,Kobayashi, K.,Koshio, Y.,Miura, M.,Moriyama, S.,Nakahata, M.,Nakayama, S.,Obayashi, Y.,Ogawa, H.,Sekiya, H.,Shiozawa, M.,Suzuki, Y.,Takeda, IOP Publishing 2009 The Astrophysical journal Vol.697 No.1
Direct dark matter search by annual modulation with 2.7 years of XMASS-I data
Abe, K.,Hiraide, K.,Ichimura, K.,Kishimoto, Y.,Kobayashi, K.,Kobayashi, M.,Moriyama, S.,Nakahata, M.,Norita, T.,Ogawa, H.,Sato, K.,Sekiya, H.,Takachio, O.,Takeda, A.,Tasaka, S.,Yamashita, M.,Yang, B. American Physical Society 2018 Physical Review D Vol.97 No.10
<P>An annual modulation signal due to the Earth orbiting around the Sun would be one of the strongest indications of the direct detection of dark matter. In 2016, we reported a search for dark matter by looking for this annual modulation with our single-phase liquid xenon XMASS-I detector. That analysis resulted in a slightly negative modulation amplitude at low energy. In this work, we included more than one year of additional data, which more than doubles the exposure to 800 live days with the same 832 kg target mass. When we assume weakly interacting massive particle (WIMP) dark matter elastically scattering on the xenon target, the exclusion upper limit for the WIMP-nucleon cross section was improved by a factor of 2 to 1.9 x 10(-41) cm(2) at 8 GeV/c(2) at 90% confidence level with our newly implemented data selection through a likelihood method. For the model-independent case, without assuming any specific dark matter model, we obtained more consistency with the null hypothesis than before with a p-value of 0.11 in the 1-20 keV energy region. This search probed this region with an exposure that was larger than that of DAMA/LIBRA. We also did not find any significant amplitude in the data for periodicity with periods between 50 and 600 days in the energy region between 1 to 6 keV.</P>