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Nishimura N.,Keele University | Sawai H.,Research Organization for Information Science and Technology | Sawai H.,Waseda University | Takiwaki T.,Japan National Astronomical Observatory | And 2 more authors.
Astrophysical Journal Letters | Year: 2017

We investigated r-process nucleosynthesis in magneto-rotational supernovae, based on a new explosion mechanism induced by the magneto-rotational instability (MRI). A series of axisymmetric magnetohydrodynamical simulations with detailed microphysics including neutrino heating is performed, numerically resolving the MRI. Neutrino-heating dominated explosions, enhanced by magnetic fields, showed mildly neutronrich ejecta producing nuclei up to A ∼ 130 (i.e., the weak r-process), while explosion models with stronger magnetic fields reproduce a solar-like r-process pattern. More commonly seen abundance patterns in our models are in between the weak and regular r-process, producing lighter and intermediate-mass nuclei. These intermediate r-processes exhibit a variety of abundance distributions, compatible with several abundance patterns in r-processenhanced metal-poor stars. The amount of Eu ejecta ∼ 10-5 M. in magnetically driven jets agrees with predicted values in the chemical evolution of early galaxies. In contrast, neutrino-heating dominated explosions have a significant amount of Fe (56Ni) and Zn, comparable to regular supernovae and hypernovae, respectively. These results indicate magneto-rotational supernovae can produce a wide range of heavy nuclei from iron-group to r-process elements, depending on the explosion dynamics. © 2017. The American Astronomical Society. All rights reserved.

Sato T.,Japan Atomic Energy Agency | Endo A.,Japan Atomic Energy Agency | Niita K.,Research Organization for Information Science and Technology
Advances in Space Research | Year: 2013

For the estimation of the radiation risk for astronauts, not only the organ absorbed doses but also their mean quality factors must be evaluated. Three functions have been proposed by different organizations for expressing the radiation quality, including the Q(L), Q(y), and QNASA(Z, E) relationships as defined in International Committee of Radiological Protection (ICRP) Publication 60, International Commission on Radiation Units and Measurements (ICRU) Report 40, and National Aeronautics and Space Administration (NASA) TP-2011-216155, respectively. The Q(L) relationship is the most simple and widely used for space dosimetry, but the use of the latter two functions enables consideration of the difference in the track structure of various charged particles during the risk estimation. Therefore, we calculated the mean quality factors in organs and tissues in ICRP/ICRU reference voxel phantoms for the isotropic exposure to various mono-energetic particles using the three Q-functions. The Particle and Heavy Ion Transport code System PHITS was employed to simulate the particle motions inside the phantoms. The effective dose equivalents and the phantom-averaged effective quality factors for the astronauts were then estimated from the calculated mean quality factors multiplied by the fluence-to-dose conversion coefficients and cosmic-ray fluxes inside a spacecraft. It was found from the calculations that QNASA generally gives the largest values for the phantom-averaged effective quality factors among the three Q-functions for neutron, proton, and lighter-ion irradiation, whereas Q(L) provides the largest values for heavier-ion irradiation. Overall, the introduction of QNASA instead of Q(L) or Q(y) in astronaut dosimetry results in the increase the effective dose equivalents because the majority of the doses are composed of the contributions from protons and neutrons, although this tendency may change by the calculation conditions. © 2013 COSPAR. Published by Elsevier Ltd. All rights reserved.

Hirota M.,Japan Atomic Energy Agency | Tokuda S.,Research Organization for Information Science and Technology
Physics of Plasmas | Year: 2010

Invariance of wave-action for eigenmodes and continuum modes around quasistationary equilibrium state is investigated in a general framework that allows for the ideal magnetohydrodynamic system and the Vlasov-Maxwell system. By utilizing the averaging method for the variational principle, the wave-action of each mode is shown to be conserved if its frequency (spectrum) is sufficiently separated from other ones, whereas some conservative exchange of the wave-action may occur among the modes with close frequencies. This general conservation law is, as an example, demonstrated for a situation where the Landau damping (or growth) occurs due to a resonance between an eigenmode and a continuum mode. The damping (or growth) rate is closely related to the spectral linewidth (equal to phase mixing rate) of the continuum mode, which can be estimated by the invariance of wave-action without invoking the conventional analytic continuation of the dispersion relation. © 2010 American Institute of Physics.

Ogawa T.,Japan Atomic Energy Agency | Sato T.,Japan Atomic Energy Agency | Hashimoto S.,Japan Atomic Energy Agency | Satoh D.,Japan Atomic Energy Agency | And 2 more authors.
Physical Review C - Nuclear Physics | Year: 2015

The fragment production excitation functions of C12 interacting with carbon nuclei were measured in the energy range of 100-400 MeV/u. To measure cross sections as excitation functions, fragments produced from a thick carbon target bombarded with a monoenergetic C12 ion beam were detected by a telescope detector placed behind the target. By this measurement, eight isotopes - B11,B10,Be10,Be9,Be7,Li8,Li7, and Li6 - were identified, and the energy dependence of their production cross sections in the few 100 MeV/u range was obtained. Based on the measured cross sections, the JAERI quantum molecular dynamics model was updated to incorporate the mechanisms particularly important for peripheral collisions. The improved version of the model coupled with a statistical decay model reproduces the measured data typically within a few 10% of difference. © 2015 American Physical Society.

Ueki T.,University of New Mexico | Ueki T.,Research Organization for Information Science and Technology
Journal of Nuclear Science and Technology | Year: 2010

Time series methodology has been studied to assess the feasibility of statistical error estimation in the continuous space and energy Monte Carlo calculation of the three-dimensional whole reactor core. The noise propagation was examined and the fluctuation of track length tallies for local fission rate and power has been formally shown to be represented by the autoregressive moving average process of orders p and p - 1 [ARMA(p,p - 1)], where p is an integer larger than or equal to two. Therefore, ARMA(p,p - 1) fitting was applied to the real standard deviation estimation of the power of fuel assemblies at particular heights. Numerical results indicate that straightforward ARMA(3,2) fitting is promising, but a stability issue must be resolved toward the incorporation in the distributed version of production Monte Carlo codes. The same numerical results reveal that the average performance of ARMA(3,2) fitting is equivalent to that of the batch method with a batch size larger than 100 and smaller than 200 cycles for a 1,100MWe pressurized water reactor. ©Atomic Energy Society of Japan.

Sawai H.,Research Organization for Information Science and Technology | Sawai H.,Waseda University | Yamada S.,Waseda University
Astrophysical Journal | Year: 2016

We carried out two-dimensional axisymmetric MHD simulations of core-collapse supernovae for rapidly rotating magnetized progenitors. By changing both the strength of the magnetic field and the spatial resolution, the evolution of the magnetorotational instability (MRI) and its impacts upon the dynamics are investigated. We found that the MRI greatly amplifies the seed magnetic fields in the regime where the buoyant mode, not the Alfvé n mode, plays a primary role in the exponential growth phase. The MRI indeed has a powerful impact on the supernova dynamics. It makes the shock expansion faster and the explosion more energetic, with some models being accompanied by the collimated jet formations. These effects, however, are not made by the magnetic pressure except for the collimated jet formations. The angular momentum transfer induced by the MRI causes the expansion of the heating region, by which the accreting matter gain additional time to be heated by neutrinos. The MRI also drifts low-Yp matter from deep inside of the core to the heating region, which makes the net neutrino heating rate larger by the reduction of the cooling due to the electron capture. These two effects enhance the efficiency of the neutrino heating, which is found to be the key to boosting the explosion. Indeed, we found that our models explode far more weakly when the net neutrino heating is switched off. The contribution of the neutrino heating to the explosion energy could reach 60% even in the case of strongest magnetic field in the current simulations. © 2016. The American Astronomical Society. All rights reserved..

Hirai R.,Waseda University | Sawai H.,Waseda University | Sawai H.,Research Organization for Information Science and Technology | Yamada S.,Waseda University
Astrophysical Journal | Year: 2014

The majority of massive stars are formed in binary systems. It is hence reasonable to expect that most core-collapse supernovae (CCSNe) take place in binaries and the existence of a companion star may leave some imprints in observed features. Having this in mind, we have conducted two-dimensional hydrodynamical simulations of the collisions of CCSNe ejecta with the companion star in an almost-equal-mass (∼10 M) binary to find out possible consequences of such events. In particular we pay attention to the amount of mass removed and its dependence on the binary separation. In contrast to the previous surmise, we find that the companion mass is stripped not by momentum transfer but by shock heating. Up to 25% of the original mass can be removed for the closest separations and the removed mass decreases as M uba-4.3 with the binary separation a. By performing some experimental computations with artificially modified densities of incident ejecta, we show that if the velocity of ejecta is fixed, the density of incident ejecta is the single important parameter that actually determines the removed mass as . On the other hand, another set of simulations with modified velocities of incident ejecta demonstrate that the strength of the forward shock, which heats up the stellar material and causes the mass loss of the companion star, is actually the key parameter for the removed mass. © 2014. The American Astronomical Society. All rights reserved..

Furukawa M.,University of Tokyo | Tokuda S.,Research Organization for Information Science and Technology
Physics of Plasmas | Year: 2011

A new matching method has been invented for linear stability analysis of magnetohydrodynamics (MHD) modes for plasmas marginally stable against ideal MHD. An inner region with a finite width is utilized as in our previous study M. Furukawa, S. Tokuda, and L.-J. Zheng, Phys. Plasmas 17, 052502 (2010). An ordering scheme for the outer region has been newly developed, thereby it succeeds to include effects of small plasma inertia and resistivity perturbatively in the outer region. The corresponding boundary condition requires direct, not asymptotic, matching of the outer and inner solutions, which assumes nothing special for the behavior of parallel electric field across the matching points. The union of the ordering scheme and the boundary condition enables us to apply our matching method even for plasmas marginally stable against ideal MHD. Because our matching method is not asymptotic, it is easy to implement numerically. The stability analysis of resistive MHD modes, such as internal kink and tearing modes, is satisfactory. © 2011 American Institute of Physics.

Ogawa T.,Research Group for Radiation Protection | Sato T.,Research Group for Radiation Protection | Hashimoto S.,Research Group for Radiation Protection | Niita K.,Research Organization for Information Science and Technology
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2014

A new phenomenological approach is developed to reproduce the stochastic distributions of secondary particle energy and angle with conservation of momentum and energy in reactions ejecting more than one ejectiles using inclusive cross-section data. The summation of energy and momentum in each reaction is generally not conserved in Monte-Carlo particle transport simulation based on the inclusive cross-sections because the particle correlations are lost in the inclusive cross-section data. However, the energy and angular distributions are successfully reproduced by randomly generating numerous sets of secondary particle configurations which are compliant with the conservation laws, and sampling one set considering their likelihood. This developed approach was applied to simulation of (n,xn) reactions (x≥2) of various targets and to other reactions such as (n,np) and (n,2nα). The calculated secondary particle energy and angular distributions were compared with those of the original inclusive cross-section data to validate the algorithm. The calculated distributions reproduce the trend of original cross-section data considerably well especially in case of heavy targets. The developed algorithm is beneficial to improve the accuracy of event-by-event analysis in particle transport simulation. © 2014 Elsevier B.V.

Yamagishi T.,Research Organization for Information Science and Technology | Matsumura Y.,Hokkaido University
Procedia Computer Science | Year: 2016

To meet the demand for fast and detailed calculations in numerical ocean simulations, we implemented a non-hydrostatic ocean model on a graphics processing unit (GPU). We improved the model's Poisson/Helmholtz solver by optimizing the memory access, using instruction-level parallelism, and applying a mixed precision calculation to the preconditioning of the Poisson/Helmholtz solver. The GPU-implemented model was 4.7 times faster than a comparable central processing unit execution. The output errors due to this implementation will not significantly influence oceanic studies. © The Authors. Published by Elsevier B.V.

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