Atsunari Katsuki

Collision Dynamics of Two Barchan Dunes Simulated Using a Simple Model

The collision processes of two crescentic dunes called barchans are systematically studied using a simple computer simulation model. The simulated processes, coalescence, ejection and reorganization, qualitatively correspond to those observed in a water tank experiment. Moreover we found the realized types of collision depend both on the mass ratio and on the lateral distance between barchans under initial conditions. A simple set of differential equations to describe the collision of one-dimensional (1D) dunes is introduced.

Emergence of a Barchan Belt in a Unidirectional Flow: Experiment and Numerical Simulation

We observed the time evolution of dune fields in a water tank experiment and simulated it by using a simple model without taking complex fluid dynamics into account. The initial sand bed changed its form to transverse ripples, that is, dunes with straight crest lines perpendicular to the flow direction. Then crescentic dunes, called barchans, evolved from transverse ripples.We observed the time evolution of dune fields in a water tank experiment and simulated it by using a simple model without taking complex fluid dynamics into account. The initial sand bed changed its form to transverse ripples, that is, dunes with straight crest lines perpendicular to the flow direction. Then crescentic dunes, called barchans, evolved from transverse ripples.

Neutrino-less double-β decay of 48Ca studied by CaF2(Eu) scintillators

We searched for the neutrino-less double-β decay(0νββ) of 48 Ca by using CaF 2 (Eu) scintillators. Analysis of their pulse shapes was effective to reduce backgrounds. No events are observed in the Q ββ value region for the data of 3394 kg day. It gives a lower limit (90% confidence level) of T 0νββ ½ > 2.7 x 10 22 year for the half-life of 0νββ of 48 Ca. Combined with our previous data for 1553 kg day [I. Ogawa et al., Nucl. Phys. A730, 215 (2004)], we obtained a more stringent limit of T 0νββ ½ > 5.8 × 10 22 year.

Double beta decay study of 48Ca by CaF2 scintillator

Abstract A CaF 2 scintillation detector system (ELEGANT VI) is developed to search for neutrinoless double beta decay (0νDBD) of 48 Ca. No events were observed around the Q -value energy region after the analysis of 4.23 kg yr data. To derive the lower limit for the half-life, the expected number of background events was estimated by a Monte Carlo simulation using the measured activities of 214 Bi and 220 Rn inside CaF 2 crystals. A new lower limit is obtained to be 1.8 × 10 22 yr at the 90% C.L.

Simulation of barchan dynamics with inter-dune sand streams

A group of barchans, crescent sand dunes, exhibit a characteristic flying-geese pattern in deserts on Earth and Mars. This pattern implies that an indirect interaction between barchans, mediated by an inter-dune sand stream, which is released from one barchans horns and caught by another barchan, plays an important role in the dynamics of barchan fields. We used numerical simulations of a recently proposed cell model to investigate the effects of inter- dune sand streams on barchan fields. We found that a sand stream from a point source moves a downstream barchan laterally until the head of the barchan is finally situated behind the stream. Thisfinal configuration was shown to be stable by a linear stability analysis. These results indicate that flying-geese patterns are formed by the lateral motion of barchans mediated by inter-dune sand streams. By using simulations we also found a barchan mono-corridor generation effect, which is another effect of sand streams from point sources.

Neutrino-less double-beta decay of Ca-48 studied by Ca F(2)(Eu) scintillators

We searched for the neutrino-less double-β decay(0νββ) of 48 Ca by using CaF 2 (Eu) scintillators. Analysis of their pulse shapes was effective to reduce backgrounds. No events are observed in the Q ββ value region for the data of 3394 kg day. It gives a lower limit (90% confidence level) of T 0νββ ½ > 2.7 x 10 22 year for the half-life of 0νββ of 48 Ca. Combined with our previous data for 1553 kg day [I. Ogawa et al., Nucl. Phys. A730, 215 (2004)], we obtained a more stringent limit of T 0νββ ½ > 5.8 × 10 22 year.

Dark matter search with CaF2 scintillator at Osaka

A CaF2 scintillator complex system (ELEGANT VI) is developed to search for the axial-vector coupled dark matter (WIMPs) and study the neutrino-less double beta decay of 48 Ca. Active light guides of pure CaF2 crystals which are on both sides of the central CaF2(Eu) crystal act as a 4π active shield, combined with surrounding CsI(T1) scintillators. The whole system is at the underground laboratory (Oto Cosmo Observatory) located in Nara. In this article our current status of the investigation are described, especially putting emphasis on the development of flash scaler to do a single photon counting.

Size Distribution of Barchan Dunes by a Cellular Dune Model

Barchans, which are crescent sand dunes, are observed in desert and on the surface of the Mars. They form barchan field through interaction such as collision processes. In order to investigate dynamics of barchan field, we used cellular dune model. The model includes only saltation and avalanche as the basic sand transport processes. We succeed to reproduce a few hundred of barchans in a numerical simulation. The size of barchans grows more and the number of them is less. Also the size distribution has long-time tail like log-normal distribution.

Coupled ODEs Model for the Dynamics of Dunes

A coupled ODEs model is proposed to describe the dynamics of barchans and other types of dunes under a few number of assumptions considering the geometrical characters of dunes and the surface flow over them. Using the model, the transition between coalescence and ejection of two colliding barchans depending on their initial sizes, is numerically and analytically studied. Also, the shape stability of barchans and other types of dunes is discussed through the model. This simple model supplies us with a new tool for a clear‐cut understanding of the seemingly complex dynamic of dunes.

Memories In Paste Control Crack Patterns

We experimentally find that a paste, i.e., a densely packed colloidal suspension with plasticity, has memories of external mechanical fields it suffered, such as vibration and flow. We also find that the memories in pastes can be visualized as macroscopically anisotropic crack patterns which appear when we dry the pastes after the external mechanical fields are removed. These memories are sustained as microscopically anisotropic network structures of colloidal particles inside paste. The memory effect of paste can be a very useful tool for technological applications in future, because we can imprint the direction of crack propagation into pastes and control to make various crack patterns, such as cellular, lamellar, radial, ring, spiral, and so on.