Equivalent hyperon-nucleon interactions in low-momentum space

Equivalent interactions in a low-momentum space for the $\Lambda N$, $\Sigma N$ and $\Xi N$ interactions are calculated, using the SU$_6$ quark model potential as well as the Nijmegen OBEP model as the input bare interaction. Because the two-body scattering data has not been accumulated sufficiently to determine the hyperon-nucleon interactions unambiguously, the construction of the potential even in low-energy regions has to rely on a theoretical model. The equivalent interaction after removing high-momentum components is still model dependent. Because this model dependence reflects the character of the underlying potential model, it is instructive for better understanding of baryon-baryon interactions in the strangeness sector to study the low-momentum space $YN$ interactions.Comment: 9 pages, 13 figures, accepted for publication in Phys. Rev.

Numerical simulation of the surface flow of a companion star in a close binary system

We simulate numerically the surface flow of a gas-supplying companion star in a semi-detached binary system. Calculations are carried out for a region including only the mass-losing star, thus not the mass accreting star. The equation of state is that of an ideal gas characterized by a specific heat ratio gamma, and the case with gamma=5/3 is mainly studied. A system of eddies appears on the surface of the companion star: an eddy in the low pressure region near the L1 point, one around the high pressure at the north pole, and one or two eddies around the low pressure at the opposite side of the L1 point. Gas elements starting near the pole region rotate clockwise around the north pole (here the binary system rotates counter-clockwise as seen from the north pole). Because of viscosity, the gas drifts to the equatorial region, switches to the counter-clockwise eddy near the L1 point and flows through the L1 point to finally form the L1 stream. The flow field in the L1 region and the structure of the L1 stream are also considered.Comment: 10 pages, 22 figures, Accepted for publication in A&

Simulation and Design of a Simple and Easy-to-use Small-scale Neutron Source at Kyoto University

AbstractA simple and easy-to-use compact neutron source based on a low power level proton accelerator (proton energy 3.5 MeV and 0.35kW beam power) at Kyoto University was designed with the conception of low cost, compact size, high safety and intensive thermal neutron flux via Monte Carlo method with PHITS code. By utilizing (p, n) reactions in a beryllium target coupled to a polyethylene moderator and graphite reflector with a wing configuration, this facility is expected to produce time-averaged thermal neutron fluxes suitable for neutron scattering and development of instrumentation, and play a role in educating students in neutron science and performing research with neutrons. Borated polyethylene (BPE) and ordinary concrete were combined to shield the neutron and photon. By using niobium as target backing and water as cooler, it is promising to cope with the problem of thermal damage and hydrogen embrittlement damage. The sizes of moderator and reflector are optimized to have thermal neutron flux as high as possible, while keeping the low ratio of fast neutron flux to thermal neutron flux. The neutron and gamma dose equivalent rates were evaluated and the current shielding configuration is acceptable

Ground-state electric quadrupole moment of 31Al

Ground-state electric quadrupole moment of 31Al (I =5/2+, T_1/2 = 644(25) ms) has been measured by means of the beta-NMR spectroscopy using a spin-polarized 31Al beam produced in the projectile fragmentation reaction. The obtained Q moment, |Q_exp(31Al)| = 112(32)emb, are in agreement with conventional shell model calculations within the sd valence space. Previous result on the magnetic moment also supports the validity of the sd model in this isotope, and thus it is concluded that 31Al is located outside of the island of inversion.Comment: 5 page

Inclusive pion double charge exchange on O-16 above the delta resonance

The forward inclusive pion double charge exchange reaction, ^{16}O(pi^-,pi^+)X, at T_0 = 0.50 and 0.75 GeV has been studied in the kinematical region where an additional pion production is forbidden by energy-momentum conservation. The experiment was performed with the SKS spectrometer at KEK PS. The measured ratio of double charge exchange cross-section for these energies dsigma(0.50 GeV)/dOmega / dsigma(0.75 GeV)/dOmega = 1.7 +/- 0.2, disagrees with the value of 7.2 predicted within the conventional sequential single charge exchange mechanism. Possible reasons for the disagreement are discussed in connection with the Glauber inelastic rescatterings.Comment: 4 pages, 5 figure

Spectropolarimetry of R Coronae Borealis in 1998--2003: Discovery of Transient Polarization at Maximum Brightness

We present an extended optical spectropolarimetry of R CrB from 1998 January to 2003 September. The polarization was almost constant in the phase of maximum brightness, being consistent with past observations. We detected, however, temporal changes of polarization ($\sim 0.5$ %) in 2001 March and August, which were the first detection of large polarization variability in R CrB near maximum brightness. The amplitude and the position angle of the `transient polarization' were almost constant with wavelength in both two events. There was a difference by about 20 degrees in the position angle between the two events. Each event could be explained by light scattering due to short-lived dust puff occasionally ejected off the line of sight. The flatness of the polarization against the wavelength suggests that the scatterer is a mixture of dust grains having various sizes. The rapid growth and fading of the transient polarization favors the phenomenological model of dust formation near the stellar photosphere (e.g., within two stellar radii) proposed for the time evolution of brightness and chromospheric emission lines during deeply declining periods, although the fading timescale can hardly be explained by a simple dispersal of expanding dust puff with a velocity of $\sim 200-350$ km s $^{-1}$. Higher expansion velocity or some mechanism to destroy the dust grains should be needed.Comment: 22 pages, 10 figures, accepted for publication in A