arXiv subjects

S. Ishikawa

Publications and source records attributed to S. Ishikawa.

At least 19 recordsLinked to original sources

arXiv2021

The nylon balloon for xenon loaded liquid scintillator in KamLAND-Zen 800 neutrinoless double-beta decay search experiment

The KamLAND-Zen 800 experiment is searching for the neutrinoless double-beta decay of $^{136}$Xe by using $^{136}$Xe-loaded liquid scintillator. The liquid scintillator is enclosed inside a balloon made of thin, transparent, low-radioactivity film that we call Inner Balloon (IB). The IB, apart from guaranteeing the liquid containment, also allows to minimize the background from cosmogenic muon-spallation products and $^{8}$B solar neutrinos. Indeed these events could contribute to the total counts in the region of interest around the Q-value of the double-beta decay of $^{136}$Xe. In this paper, we present an overview of the IB and describe the various steps of its commissioning minimizing the radioactive contaminations, from the material selection, to the fabrication of the balloon and its installation inside the KamLAND detector. Finally, we show the impact of the IB on the KamLAND background as measured by the KamLAND detector itself.

physics.ins-det↗

arXiv2021

Proton-$\rm ^3He$ elastic scattering at intermediate energies

We present a precise measurement of the cross section, proton and $\rm ^3He$ analyzing powers, and spin correlation coefficient $C_{y,y}$ for $p$-$\rm ^3He$ elastic scattering near 65 MeV, and a comparison with rigorous four-nucleon scattering calculations based on realistic nuclear potentials and a model with $Δ$-isobar excitation. Clear discrepancies are seen in some of the measured observables in the regime around the cross section minimum. Theoretical predictions using scaling relations between the calculated cross section and the $\rm ^3 He$ binding energy are not successful in reproducing the data. Large sensitivity to the $NN$ potentials and rather small $Δ$-isobar effects in the calculated cross section are noticed as different features from those in the deuteron-proton elastic scattering. The results obtained above indicate that $p$-$\rm ^3He$ scattering at intermediate energies is an excellent tool to explore nuclear interactions not accessible by three-nucleon scattering.

nucl-ex↗

arXiv2020

Use of a ray-tracing simulation to characterize ghost rays in the FOXSI rocket experiment

Imaging X-rays by direct focusing offers greater sensitivity and a higher dynamic range compared to techniques based on indirect imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload that uses seven sets of nested Wolter-I figured mirrors to observe the Sun in hard X-rays through direct focusing. Characterizing the performance of these optics is critical to optimize their performance and to understand their resulting data. In this paper, we present a ray-tracing simulation we created and developed to study Wolter-I X-ray mirrors. We validated the accuracy of the ray-tracing simulation by modeling the FOXSI rocket optics. We found satisfactory agreements between the simulation predictions and laboratory data measured on the optics. We used the ray-tracing simulation to characterize a background pattern of singly reflected rays (i.e., ghost rays) generated by photons at certain incident angles reflecting on only one of a two-segment Wolter-I figure and still reaching the focal plane. We used the results of the ray-tracing simulation to understand, and to formulate a set of strategies that can be used to mitigate, the impact of ghost rays on the FOXSI optical modules. These strategies include the optimization of aperture plates placed at the entrance and exit of the smallest Wolter-I mirror used in FOXSI, a honeycomb type collimator, and a wedge absorber placed at the telescope aperture. The ray-tracing simulation proved to be a reliable set of tools to study Wolter-I X-ray optics. It can be used in many applications, including astrophysics, material sciences, and medical imaging.

astro-ph.IM↗

arXiv2018

Suppression of Low-mass Galaxy Formation around Quasars at z~2-3

We have carried out deep and wide field imaging observations with narrow bands targeting 11 quasar fields to systematically study the possible photoevaporation effect of quasar radiation on surrounding low mass galaxies at $z\sim2-3$. We focused on Lyman alpha emitters (LAEs) at the same redshifts as the quasars that lie within the quasar proximity zones, where the ultra-violet radiation from the quasars is higher than the average background at that epoch. We found that LAEs with high rest-frame equivalent width of Ly$α$ emission ($EW_0$) of $\gtrsim 150$Å$~$ with low stellar mass ($\lesssim 10^8 M_{\odot}$), are predominantly scarce in the quasar proximity zones, suggesting that quasar photoevaporation effects may be taking place. The halo mass of LAEs with $EW_0>150$Å$~$ is estimated to be $3.6^{+12.7}_{-2.3}\times10^9 M_{\odot}$ either from the Spectral Energy Distribution (SED) fitting or the main sequence. Based on a hydrodynamical simulation, the predicted delay in star formation under a local UV background intensity with $J (ν_L)\gtrsim10^{-21}$ erg s$^{-1}$ cm$^{-2}$ Hz$^{-1}$ sr$^{-1}$ for galaxies having less than this halo mass is about $>20$ Myr, which is longer than the expected age of LAEs with $EW_0>150$Å. On the other hand, the photoevaporation seems to be less effective around very luminous quasars, which is consistent with the idea that these quasars are still in an early stage of quasar activity.

astro-ph.GA↗

arXiv2018

CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region

The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 3rd September 2015 measured the linear polarization produced by scattering processes in the hydrogen Ly-$α$ line of the solar disk radiation, whose line-center photons stem from the chromosphere-corona transition region (TR). These unprecedented spectropolarimetric observations revealed an interesting surprise, namely that there is practically no center-to-limb variation (CLV) in the $Q/I$ line-center signals. Using an analytical model, we first show that the geometrical complexity of the corrugated surface that delineates the TR has a crucial impact on the CLV of the $Q/I$ and $U/I$ line-center signals. Secondly, we introduce a statistical description of the solar atmosphere based on a three-dimensional (3D) model derived from a state-of-the-art radiation magneto-hydrodynamic simulation. Each realization of the statistical ensemble is a 3D model characterized by a given degree of magnetization and corrugation of the TR, and for each such realization we solve the full 3D radiative transfer problem taking into account the impact of the CLASP instrument degradation on the calculated polarization signals. Finally, we apply the statistical inference method presented in a previous paper to show that the TR of the 3D model that produces the best agreement with the CLASP observations has a relatively weak magnetic field and a relatively high degree of corrugation. We emphasize that a suitable way to validate or refute numerical models of the upper solar chromosphere is by confronting calculations and observations of the scattering polarization in ultraviolet lines sensitive to the Hanle effect.

astro-ph.SR↗

arXiv2018

A Statistical Inference Method for Interpreting the CLASP Observations

On 3rd September 2015, the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) successfully measured the linear polarization produced by scattering processes in the hydrogen Lyman-$α$ line of the solar disk radiation, revealing conspicuous spatial variations in the $Q/I$ and $U/I$ signals. Via the Hanle effect the line-center $Q/I$ and $U/I$ amplitudes encode information on the magnetic field of the chromosphere-corona transition region (TR), but they are also sensitive to the three-dimensional structure of this corrugated interface region. With the help of a simple line formation model, here we propose a statistical inference method for interpreting the Lyman-$α$ line-center polarization observed by CLASP.

astro-ph.SR↗

arXiv2017

Discovery of Scattering Polarization in the Hydrogen Lyman-$α$ Line of the Solar Disk Radiation

There is a thin transition region (TR) in the solar atmosphere where the temperature rises from 10,000 K in the chromosphere to millions of degrees in the corona. Little is known about the mechanisms that dominate this enigmatic region other than the magnetic field plays a key role. The magnetism of the TR can only be detected by polarimetric measurements of a few ultraviolet (UV) spectral lines, the Lyman-$α$ line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV spectrum) being of particular interest given its sensitivity to the Hanle effect (the magnetic-field-induced modification of the scattering line polarization). We report the discovery of linear polarization produced by scattering processes in the Lyman-$α$ line, obtained with the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket experiment. The Stokes profiles observed by CLASP in quiet regions of the solar disk show that the $Q/I$ and $U/I$ linear polarization signals are of the order of 0.1 % in the line core and up to a few percent in the nearby wings, and that both have conspicuous spatial variations with scales of $\sim 10$ arcsec. These observations help constrain theoretical models of the chromosphere-corona TR and extrapolations of the magnetic field from photospheric magnetograms. In fact, the observed spatial variation from disk to limb of polarization at the line core and wings already challenge the predictions from three-dimensional magnetohydrodynamical models of the upper solar chromosphere.

astro-ph.SR↗

arXiv2016

Geometry-dependent constitutive law for granular slow frictional drag

Frictional constitutive law for very slow vertical withdrawing of a thin rod from a granular bed is experimentally studied. Using a very precise creep meter, geometry-dependent granular frictional constitutive law is particularly examined. In some previous works, a dimensionless number $I=\dotγD_g/\sqrt{p/ρ_g}$ has been used to characterize granular frictional constitutive laws, where $\dotγ$, $D_g$, $p$, and $ρ_g$ are the shear strain rate, grain diameter, confining pressure, and bulk density of granular bed, respectively. It has been considered that granular frictional constitutive law expressed by $I$ is universal (almost geometry-independent) in dense flow regime. In this study, however, we find that the geometry of the system is much more crucial to characterize granular friction in a very slow withdrawing regime. Specifically, the ratio between rod and grain diameters must be an essential parameter to describe the granular frictional constitutive law. Physical meaning of the geometry-dependent constitutive law is discussed on the basis of grains-contact-number dependence of granular behavior.

cond-mat.soft↗

arXiv2016

Fine-pitch CdTe detector for hard X-ray imaging and spectroscopy of the Sun with the FOXSI rocket experiment

We have developed a fine-pitch hard X-ray (HXR) detector using a cadmium telluride (CdTe) semiconductor for imaging and spectroscopy for the second launch of the Focusing Optics Solar X-ray Imager (FOXSI). FOXSI is a rocket experiment to perform high sensitivity HXR observations from 4-15 keV using the new technique of HXR focusing optics. The focal plane detector requires < 100 um position resolution (to take advantage of the angular resolution of the optics) and about 1 keV energy resolution (FWHM) for spectroscopy down to 4 keV, with moderate cooling (> -30 C). Double-sided silicon strip detectors were used for the first FOXSI flight in 2012 to meet these criteria. To improve the detectors' efficiency (66 at 15 keV for the silicon detectors) and position resolution of 75 um for the second launch, we fabricated double-sided CdTe strip detectors with a position resolution of 60 um and almost 100 % efficiency for the FOXSI energy range. The sensitive area is 7.67 mm x 7.67 mm, corresponding to the field of view of 791'' x 791''. An energy resolution of about 1 keV (FWHM) and low energy threshold of 4 keV were achieved in laboratory calibrations. The second launch of FOXSI was performed on December 11, 2014, and images from the Sun were successfully obtained with the CdTe detector. Therefore we successfully demonstrated the detector concept and the usefulness of this technique for future HXR observations of the Sun.

astro-ph.IM↗

arXiv2013

Three-body calculations of triple-alpha reaction

Recently, the triple-α(3α) process, by which three 4He nuclei are fused into a 12C nucleus in stars, was studied by different methods in solving the quantum mechanical three-body problem. Their results of the thermonuclear reaction rate for the process differ by several orders at low stellar temperatures of 10^7 - 10^8 K. In this paper, we will present calculations of the 3αprocess by a modified Faddeev three-body formalism in which the long-range effects of Coulomb interactions are accommodated. The reaction rate of the process is calculated via an inverse process, three-alpha (3-α) photodisintegration of a 12C nucleus. Calculated reaction rate is about 10 times larger than that of the Nuclear Astrophysics Compilation of Reaction Rates (NACRE) at 10^7 K, and is remarkably smaller than the results of the recent three-body calculations. We will discuss a possible reason of the difference.

nucl-th↗

arXiv2012

Kappa Distribution Model for Hard X-Ray Coronal Sources of Solar Flares

Solar flares produce hard X-ray emission of which the photon spectrum is often represented by a combination of thermal and power-law distributions. However, the estimates of the number and total energy of non-thermal electrons are sensitive to the determination of the power-law cutoff energy. Here we revisit an `above-the-loop' coronal source observed by RHESSI on 2007 December 31 and show that a kappa distribution model can also be used to fit its spectrum. Because the kappa distribution has a Maxwellian-like core in addition to the high-energy power-law tail, the emission measure and temperature of the instantaneous electrons can be derived without assuming the cutoff energy. Moreover, the non-thermal fractions of electron number/energy densities can be uniquely estimated because they are functions of the power-law index only. With the kappa distribution model, we estimated that the total electron density of the coronal source region was ~2.4x10^10 cm^-3. We also estimated without assuming the source volume that a moderate fraction (~20%) of electrons in the source region was non-thermal and carried ~52% of the total electron energy. The temperature was 28 MK, and the power-law index d of the electron density distribution was -4.3. These results are compared to the conventional power-law models with and without a thermal core component.

astro-ph.SR↗

arXiv2012

Initial Results from the Nobeyama Molecular Gas Observations of Distant Bright Galaxies

We present initial results from the CO survey toward high redshift galaxies using the Nobeyama 45m telescope. Using the new wide bandwidth spectrometer equipped with a two-beam SIS receiver, we have robust new detections of three high redshift (z=1.6-3.4) submillimeter galaxies (SXDF 1100.001, SDP9, and SDP17), one tentative detection (SDSS J160705+533558), and one non-detection (COSMOS-AzTEC1). The galaxies observed during the commissioning phase are sources with known spectroscopic redshifts from previous optical or from wide-band submm spectroscopy. The derived molecular gas mass and line widths from Gaussian fits are ~10^11 Msun and 430-530 km/s, which are consistent with previous CO observations of distant submm galaxies and quasars. The spectrometer that allows a maximum of 32 GHz instantaneous bandwidth will provide new science capabilities at the Nobeyama 45m telescope, allowing us to determine redshifts of bright submm selected galaxies without any prior redshift information.

astro-ph.CO↗

arXiv2011

Analyzing power in elastic scattering of 6He from polarized proton target at 71 MeV/nucleon

The vector analyzing power has been measured for the elastic scattering of neutron-rich 6He from polarized protons at 71 MeV/nucleon making use of a newly constructed solid polarized proton target operated in a low magnetic field and at high temperature. Two approaches based on local one-body potentials were applied to investigate the spin-orbit interaction between a proton and a 6He nucleus. An optical model analysis revealed that the spin-orbit potential for 6He is characterized by a shallow and long-ranged shape compared with the global systematics of stable nuclei. A semimicroscopic analysis with a alpha+n+n cluster folding model suggests that the interaction between a proton and the alpha core is essentially important in describing the p+6He elastic scattering. The data are also compared with fully microscopic analyses using non-local optical potentials based on nucleon-nucleon g-matrices.

nucl-ex↗

arXiv2010

Analyzing power for the proton elastic scattering from neutron-rich 6He nucleus

Vector analyzing power for the proton-6He elastic scattering at 71 MeV/nucleon has been measured for the first time, with a newly developed polarized proton solid target working at low magnetic field of 0.09 T. The results are found to be incompatible with a t-matrix folding model prediction. Comparisons of the data with g-matrix folding analyses clearly show that the vector analyzing power is sensitive to the nuclear structure model used in the reaction analysis. The alpha-core distribution in 6He is suggested to be a possible key to understand the nuclear structure sensitivity.

nucl-ex↗

arXiv2009

Coordinate space proton-deuteron scattering calculations including Coulomb force effects

We present a practical method to solve the proton-deuteron scattering problem at energies above the three-body breakup threshold, in which we treat three-body integral equations in coordinate space accommodating long-range proton-proton Coulomb interactions. The method is examined for phase shift parameters, and then applied to calculations of differential cross sections in elastic and breakup reactions, analyzing powers, etc. with a realistic nucleon-nucleon force and three-nucleon forces. Effects of the Coulomb force and the three-nucleon forces on these observables are discussed in comparing with experimental data.

nucl-th↗

arXiv2007

Spin-dependent three-nucleon force effects on nucleon-deuteron scattering

We construct a phenomenological three-nucleon force (3NF) model that gives a good description of polarization observables in elastic nucleon-deuteron (N-$d$) scattering at a low energy together with a realistic nucleon-nucleon force and a 3NF arising from the exchange of two pions. Parameters of the model, which consists of spin-independent, spin-orbit, and tensor components, are determined to reproduce the three-nucleon binding energy and polarization observables in N-d scattering at 3 MeV. Predictions of the model 3NF on N-d polarization observables at higher energies are examined, and effects of each component on the observables are investigated.

nucl-th↗

arXiv2007

Two-pion exchange three-nucleon potential: O(q^4) chiral expansion

We present the expansion of the two-pion exchange three-nucleon potential (TPE-3NP) to chiral order q^4, which corresponds to a subset of all possibilities at this order and is based on the \piN amplitude at O(q^3). Results encompass both numerical corrections to strength coefficients of previous O(q^3) terms and new structures in the profile functions. The former are typically smaller than 10% whereas the latter arise from either loop functions or non-local gradients acting on the wave function. The influence of the new TPE-3NP over static and scattering three-body observables has been assessed and found to be small, as expected from perturbative corrections.

nucl-th↗

arXiv2007

Operation of Faddeev-Kernel in Configuration Space

We present a practical method to solve Faddeev three-body equations at energies above three-body breakup threshold as integral equations in coordinate space. This is an extension of previously used method for bound states and scattering states below three-body breakup threshold energy. We show that breakup components in three-body reactions produce long-range effects on Faddeev integral kernels in coordinate space, and propose numerical procedures to treat these effects. Using these techniques, we solve Faddeev equations for neutron-deuteron scattering to compare with benchmark solutions.

nucl-th↗