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Journal Articles

Quasifree neutron knockout reaction reveals a small $$s$$-Orbital component in the Borromean nucleus $$^{17}$$B

Yang, Z. H.*; Kubota, Yuki*; Corsi, A.*; Yoshida, Kazuki; Sun, X.-X.*; Li, J. G.*; Kimura, Masaaki*; Michel, N.*; Ogata, Kazuyuki*; Yuan, C. X.*; et al.

Physical Review Letters, 126(8), p.082501_1 - 082501_8, 2021/02

 Times Cited Count:10 Percentile:98.7(Physics, Multidisciplinary)

A quasifree ($$p$$,$$pn$$) experiment was performed to study the structure of the Borromean nucleus $$^{17}$$B, which had long been considered to have a neutron halo. By analyzing the momentum distributions and exclusive cross sections, we obtained the spectroscopic factors for $$1s_{1/2}$$ and $$0d_{5/2}$$ orbitals, and a surprisingly small percentage of 9(2)% was determined for $$1s_{1/2}$$. Our finding of such a small $$1s_{1/2}$$ component and the halo features reported in prior experiments can be explained by the deformed relativistic Hartree-Bogoliubov theory in continuum, revealing a definite but not dominant neutron halo in $$^{17}$$B. The present work gives the smallest $$s$$- or $$p$$-orbital component among known nuclei exhibiting halo features and implies that the dominant occupation of $$s$$ or $$p$$ orbitals is not a prerequisite for the occurrence of a neutron halo.

Journal Articles

Energetic ion transport by microturbulence is insignificant in tokamaks

Pace, D. C.*; Austin, M. E.*; Bass, E. M.*; Budny, R.*; Heidbrink, W. W.*; Hillesheim, J. C.*; Holcomb, C. T.*; Gorelenkova, M.*; Grierson, B. A.*; McCune, D. C.*; et al.

Physics of Plasmas, 20(5), p.056108_1 - 056108_18, 2013/05

 Times Cited Count:29 Percentile:84.43(Physics, Fluids & Plasmas)

Energetic ion transport due to microturbulence is investigated in MHD-quiescent plasmas by way of neutral beam injection in the DIII-D tokamak. A range of on-axis and off-axis beam injection scenarios are employed to vary relevant parameters such as the character of the background microturbulence and the value of Eb/Te, where Eb is the energetic ion energy and Te the electron temperature. In all cases it is found that any transport enhancement due to microturbulence is too small to observe experimentally. These transport effects are modeled using numerical and analytic expectations that calculate the energetic ion diffusivity due to microturbulence. It is determined that energetic ion transport due to coherent modes, including possible reductions in neutral beam current drive, is a considerably larger effect and should therefore be considered more important for ITER.

Journal Articles

Study of X-ray emission enhancement via a high-contrast femtosecond laser interacting with a solid foil

Chen, L.-M.; Kando, Masaki; Xu, M. H.*; Li, Y.-T.*; Koga, J. K.; Chen, M.*; Xu, H.*; Yuan, X.-H.*; Dong, Q. L.*; Sheng, Z. M.*; et al.

Physical Review Letters, 100(4), p.045004_1 - 045004_4, 2008/02

 Times Cited Count:83 Percentile:92.7(Physics, Multidisciplinary)

We observed the increase of the K-$$alpha$$ X-ray conversion efficiency (hK) produced by a 60 fs frequency doubled high contrast laser pulse focused on Cu foil, compared to the case of the fundamental laser pulse. hK shows a strong dependence on the nonlinearly skewed shape of the laser pulse. It reaches a maximum for a 100 fs negatively skewed pulse. The electron spectrum shaping contributes to the enhancement of hK. Simulations demonstrate that high contrast lasers are an effective tool for optimizing the X-ray emission, via the enhanced Vacuum Heating mechanism.

Journal Articles

Thermal evolution of defects in as-grown and electron-irradiated ZnO studied by positron annihilation

Chen, Z. Q.*; Wang, S. J.*; Maekawa, Masaki; Kawasuso, Atsuo; Naramoto, Hiroshi*; Yuan, X. L.*; Sekiguchi, Takashi*

Physical Review B, 75(24), p.245206_1 - 24520_9, 2007/06

 Times Cited Count:61 Percentile:88.76(Materials Science, Multidisciplinary)

Vacancy-type defects in as-grown ZnO single crystals have been identified using positron annihilation spectroscopy. The grown-in defects are supposed to be zinc vacancy V$$_{Zn}$$-related defects, and can be easily removed by annealing above 600$$^{circ}$$C. V$$_{Zn}$$-related defects are also introduced in ZnO when subjected to 3 MeV electron irradiation with a dose of 5.5$$times$$10$$^{18}$$ cm$$^{-2}$$. Most of these irradiation-induced V$$_{Zn}$$ are annealed at temperatures below 200$$^{circ}$$C through recombination with the close interstitials. However, after annealing at around 400$$^{circ}$$C, secondary defects are generated. A detailed analysis of the Doppler broadening measurements indicates that the irradiation introduced defects and the annealing induced secondary defects belong to different species. It is also found that positron trapping by these two defects has different temperature dependences. The probable candidates for the secondary defects are tentatively discussed in combination with Raman scattering studies. After annealing at 700$$^{circ}$$C, all the vacancy defects are annealed out. Cathodoluminescence measurements show that V$$_{Zn}$$ is not related to the visible emission at 2.3 eV in ZnO, but would rather act as nonradiative recombination centers.

Journal Articles

Experimental study on $$K$$$$_{alpha}$$ X-ray emission from intense femtosecond laser-solid interactions

Xu, M. H.*; Chen, L.-M.; Li, Y.-T.*; Yuan, X.-H.*; Liu, Y.-Q.*; Nakajima, Kazuhisa; Tajima, Toshiki; Wang, Z.-H.*; Wei, Z.-Y.*; Zhao, W.*; et al.

Acta Physica Sinica, 56(1), p.353 - 358, 2007/01

The characteristics of $$K$$$$_{alpha}$$ X-ray sources generated by p-polarized femtosecond laser-solid interactions are experimentally studied in the relativistic regime. By use of knife-edge image technique and a single-photon-counting X-ray CCD camera, we obtain the source size, the spectrum and the conversion efficiency of the $$K$$$$_{alpha}$$ X-ray sources. The experimental results show that the conversion efficiency of $$K$$$$_{alpha}$$ photons reaches an optimum value 7.08$$times$$10$$^{-6}$$/sr at the laser intensity of 1.6$$times$$10$$^{18}$$W/cm$$^{2}$$, which is different from the Reich's simulation results. We find that about 10% of laser energy is converted into the forward hot electrons at the laser intensity of 1.6$$times$$10$$^{18}$$W/cm$$^{2}$$.

Journal Articles

Microvoid formation in hydrogen-implanted ZnO probed by a slow positron beam

Chen, Z. Q.; Kawasuso, Atsuo; Xu, Y.; Naramoto, Hiroshi*; Yuan, X. L.*; Sekiguchi, Takashi*; Suzuki, Ryoichi*; Odaira, Toshiyuki*

Physical Review B, 71(11), p.115213_1 - 115213_8, 2005/03

 Times Cited Count:101 Percentile:94.02(Materials Science, Multidisciplinary)

ZnO crystals were implanted with 20-80 keV hydrogen ions up to a total dose of 4.4$$times$$10$$^{15}$$ cm$$^{-2}$$. Positron annihilation measurements show introduction of zinc vacancies, which are filled with hydrogen atoms. After isochronal annealing at 200-500 $$^{circ}$$C, the vacancies agglomerate into hydrogen bubbles. Further annealing at 600-700 $$^{circ}$$C causes release of hydrogen out of the bubbles, leaving large amount of microvoids. These microvoids are annealed out at high temperature of 1000 $$^{circ}$$C. Cathodoluminescence measurements reveal that hydrogen ions also passivate deep level emission centers before their release from the sample, and lead to the improvement of the UV emission.

Journal Articles

Production and recovery of defects in phosphorus-implanted ZnO

Chen, Z. Q.; Kawasuso, Atsuo; Xu, Y.; Naramoto, Hiroshi; Yuan, X. L.*; Sekiguchi, Takashi*; Suzuki, Ryoichi*; Odaira, Toshiyuki*

Journal of Applied Physics, 97(1), p.013528_1 - 013528_6, 2005/01

 Times Cited Count:135 Percentile:96.29(Physics, Applied)

Phosphorus ions were implanted into ZnO crystals with energies of 50-380 keV to a dose of 10$$^{13}$$-10$$^{15}$$ cm$$^{-2}$$. Positron annihilation measurements show the introduction of vacancy clusters after implantation. These vacancy clusters evolve to microvoids after annealing at a temperature of 600$$^{circ}$$C, and disappear gradually up to 1100$$^{circ}$$C. Raman scattering measurements show the production of oxygen vacancies (V$$_{O}$$). They are annealed up to 700$$^{circ}$$C accompanying the agglomeration of vacancy clusters. The light emissions of ZnO are suppressed due to the competing nonradiative recombination centers introduced by implantation. Recovery of the light emission occurs above 600$$^{circ}$$C. The vacancy-type defects detected by positrons might be part of the nonradiative recombination centers. Hall measurement shows n-type conductivity for the P$$^+$$-implanted ZnO layer, which suggests that phosphorus is an amphoteric dopant.

Journal Articles

Hydrogen bubble formation in H-implanted ZnO studied using a slow positron beam

Chen, Z. Q.; Maekawa, Masaki; Kawasuso, Atsuo; Yamamoto, Shunya; Yuan, X. L.*; Sekiguchi, Takashi*; Suzuki, Ryoichi*; Odaira, Toshiyuki*

JAERI-Review 2004-025, TIARA Annual Report 2003, p.193 - 195, 2004/11

20-80 keV hydrogen ions were implanted into ZnO single crystals up to a total dose of 4.4$$times$$10$$^{15}$$ cm$$^{-2}$$. Positron annihilation measurements using a slow positron beam revealed introduction of vacancies after implantation, which are filled with hydrogen impurities. After annealing, these hydrogen filled vacancies grow into large hydrogen bubbles. At annealing temperature of 500-700$$^{circ}$$C, these hydrogen impurities are released from the bubbles, and remain open microvoids. These microvoids are finally annealed out at about 1100$$^{circ}$$C. The effects of hydrogen implantation on the light luminescence in ZnO will also be discussed.

Journal Articles

N$$^+$$ ion-implantation-induced defects in ZnO studied with a slow positron beam

Chen, Z. Q.; Sekiguchi, Takashi*; Yuan, X. L.*; Maekawa, Masaki; Kawasuso, Atsuo

Journal of Physics; Condensed Matter, 16(2), p.S293 - S299, 2004/01

 Times Cited Count:25 Percentile:73(Physics, Condensed Matter)

Undoped ZnO single crystals were implanted with multiple energy N$$^+$$ ions ranging from 50 to 380 keV with dose from 10$$^{12}$$/cm$$^2$$ to 10$$^{14}$$/cm$$^2$$. Positron annihilation measurements show that vacancy defects are introduced in the implanted layers. The concentration of the vacancy defects increases with increasing ion dose. Annealing behavior of the defects can be divided into four stages, which correspond to the formation and recovery of large vacancy clusters, formation and disappearance of vacancy-impurity complexes, respectively. All the implantation induced defects are removed by annealing at 1200$$^{circ}$$C. Cathodoluminescence measurements show that the ion implantation induced defects act as nonradiative recombination centers to suppress the ultraviolet emission. After annealing, these defects disappear gradually and the ultraviolet emission reappears, which coincides with positron annihilation measurement. The Hall measurements reveal that after N$$^+$$-implantation, the ZnO layer still shows n-type conductivity.

Journal Articles

Evolution of voids in Al$$^+$$-implanted ZnO probed by a slow positron beam

Chen, Z. Q.; Maekawa, Masaki; Yamamoto, Shunya; Kawasuso, Atsuo; Yuan, X. L.*; Sekiguchi, Takashi*; Suzuki, Ryoichi*; Odaira, Toshiyuki*

Physical Review B, 69(3), p.035210_1 - 035210_10, 2004/01

 Times Cited Count:89 Percentile:93.88(Materials Science, Multidisciplinary)

Introduction and annealing behavior of defects in Al$$^+$$-implanted ZnO have been studied using energy variable slow positron beam. Vacancy clusters are produced after Al$$^+$$-implantation. With increasing ion dose above 10$$^{14}$$ Al$$^+$$/cm$$^2$$ the implanted layer is amorphized. Heat treatment up to 600 $$^{circ}$$C enhances the creation of large voids that allow the positronium formation. The large voids disappear accompanying the recrystallization process by the further heat treatment above 600 $$^{circ}$$C. Afterwards, implanted Al impurities are completely activated to contribute the n-type conduction. The ZnO crystal quality is also improved after recrystallization.

Journal Articles

Postgrowth annealing on defects in ZnO studied by positron annihilation, X-ray diffraction, rutherford backscattering, cathodoluminescence and hall measurements

Chen, Z. Q.; Yamamoto, Shunya; Maekawa, Masaki; Kawasuso, Atsuo; Yuan, X. L.*; Sekiguchi, Takashi*

Journal of Applied Physics, 94(8), p.4807 - 4812, 2003/10

 Times Cited Count:162 Percentile:96.86(Physics, Applied)

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