The effect of the Coriolis force on Kelvin-Helmholtz-driven mixing in protoplanetary disks

We study the stability of proto-planetary disks with vertical velocity gradients in their equilibrium rotation rates; such gradients are expected to develop when dust settles into the midplane. Using a linear stability analysis of a simple three-layer model, we show that the onset of instability occurs at a larger value of the Richardson number, and therefore for a thicker layer, when the effects of Coriolis forces are included. This analysis also shows that even-symmetry (midplane-crossing) modes develop faster than odd-symmetry ones. These conclusions are corroborated by a large number of nonlinear numerical simulations with two different parameterized prescriptions for the initial (continuous) dust distributions. Based on these numerical experiments, the Richardson number required for marginal stability is more than an order of magnitude larger than the traditional 1/4 value. The dominant modes that grow have horizontal wavelengths of several initial dust scale heights, and in nonlinear stages mix solids fairly homogeneously over a comparable vertical range. We conclude that gravitational instability may be more difficult to achieve than previously thought, and that the vertical distribution of matter within the dust layer is likely globally, rather than locally, determined.Comment: Accepted for publication in Ap

Development of Large area Gamma-ray Camera with GSO(Ce) Scintillator Arrays and PSPMTs

We have developed a position-sensitive scintillation camera with a large area absorber for use as an advanced Compton gamma-ray camera. At first we tested GSO(Ce) crystals. We compared light output from the GSO(Ce) crystals under various conditions: the method of surface polishing, the concentration of Ce, and co-doping Zr. As a result, we chose the GSO(Ce) crystals doped with only 0.5 mol% Ce, and its surface polished by chemical etching as the scintillator of our camera. We also made a 16$\times$16 cm$^2$ scintillation camera which consisted of 9 position-sensitive PMTs (PSPMTs Hamamatsu flat-panel H8500), the each of which had 8$\times$8 anodes with a pitch of 6 mm and coupled to 8$\times$8 arrays of pixelated 6$\times6\times$13 mm$^3$ GSO(Ce) scintillators. For the readout system of the 576 anodes of the PMTs, we used chained resistors to reduce the number of readout channels down to 48 to reduce power consumption. The camera has a position resolution of less than 6mm and a typical energy resolution of 10.5% (FWHM) at 662 keV at each pixel in a large area of 16$\times$16 cm$^2$. %to choose the best scintillator for our project. Furthermore we constructed a 16$\times$16 array of 3$\times3\times$13 mm$^3$ pixelated GSO(Ce) scintillators, and glued it to a PMT H8500. This camera had the position resolution of less than 3mm, over an area of 5$\times$5 cm$^2$, except for some of the edge pixels; the energy resolution was typically 13% (FWHM) at 662 keV.Comment: Proceedings of PSD7 appear in NIM

Strength functions, entropies and duality in weakly to strongly interacting fermionic systems

We revisit statistical wavefunction properties of finite systems of interacting fermions in the light of strength functions and their participation ratio and information entropy. For weakly interacting fermions in a mean-field with random two-body interactions of increasing strength $\lambda$, the strength functions $F_k(E)$ are well known to change, in the regime where level fluctuations follow Wigner's surmise, from Breit-Wigner to Gaussian form. We propose an ansatz for the function describing this transition which we use to investigate the participation ratio $\xi_2$ and the information entropy $S^{\rm info}$ during this crossover, thereby extending the known behavior valid in the Gaussian domain into much of the Breit-Wigner domain. Our method also allows us to derive the scaling law for the duality point $\lambda = \lambda_d$, where $F_k(E)$, $\xi_2$ and $S^{\rm info}$ in both the weak ($\lambda=0$) and strong mixing ($\lambda = \infty$) basis coincide as $\lambda_d \sim 1/\sqrt{m}$, where $m$ is the number of fermions. As an application, the ansatz function for strength functions is used in describing the Breit-Wigner to Gaussian transition seen in neutral atoms CeI to SmI with valence electrons changing from 4 to 8

Development of an advanced Compton camera with gaseous TPC and scintillator

A prototype of the MeV gamma-ray imaging camera based on the full reconstruction of the Compton process has been developed. This camera consists of a micro-TPC that is a gaseous Time Projection Chamber (TPC) and scintillation cameras. With the information of the recoil electrons and the scattered gamma-rays, this camera detects the energy and incident direction of each incident gamma-ray. We developed a prototype of the MeV gamma-ray camera with a micro-TPC and a NaI(Tl) scintillator, and succeeded in reconstructing the gamma-rays from 0.3 MeV to 1.3 MeV. Measured angular resolutions of ARM (Angular Resolution Measure) and SPD (Scatter Plane Deviation) for 356 keV gamma-rays were $18^\circ$ and $35^\circ$, respectively.Comment: 4 pages, 5 figures. Proceedings of the 6th International Workshop On Radiation Imaging Detector

Studies of the performance of different front-end systems for flat-panel multi-anode PMTs with CsI(Tl) scintillator arrays

We have studied the performance of two different types of front-end systems for our gamma camera based on Hamamatsu H8500 (flat-panel 64 channels multi-anode PSPMT) with a CsI(Tl) scintillator array. The array consists of 64 pixels of $6\times6\times20{\rm mm}^3$ which corresponds to the anode pixels of H8500. One of the system is based on commercial ASIC chips in order to readout every anode. The others are based on resistive charge divider network between anodes to reduce readout channels. In both systems, each pixel (6mm) was clearly resolved by flood field irradiation of $^{137}$Cs. We also investigated the energy resolution of these systems and showed the performance of the cascade connection of resistive network between some PMTs for large area detectors.Comment: 9 pages, 6 figures, proceedings of the 7th International Workshop on Radiation Imaging Detectors (IWORID7), submitted to NIM

Antisymmetric solitons and their interactions in strongly dispersion-managed fiber-optic systems

By means of the variational approximation (VA), a system of ordinary differential equations (ODEs) is derived to describe the propagation of antisymmetric solitons in a multi-channel (WDM) optical fiber link subject to strong dispersion management. Results are reported for a prototypical model including two channels. Using the VA technique, conditions for stable propagation of the antisymmetric dispersion-managed (ASDM) solitons in one channel are found, and complete and incomplete collisions between the solitons belonging to the different channels are investigated. In particular, it is shown that formation of a bound inter-channel state of two ASDM solitons is possible under certain conditions (but may be easily avoided). The VA predictions for the single- and two-channel systems are compared with direct simulations of the underlying partial differential equations. In most cases, the agreement is very good, but in some cases (very closely spaced channels) the collision may destroy the ASDM solitons. The timing-jitter suppression factor (JSF) for the ASDM soliton in one channel, and the crosstalk timing jitter induced by collision between the solitons belonging to the different channels are also estimated analytically. In particular, the JSF for the ASDM soliton may be much larger than for its fundamental-soliton counterpart in the same system.Comment: 15 pages, 10 figures, accepted for publication in Optics Communication

Effect of a span wise flow on the laminar-turbulent transition

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.The boundary-layer transition to turbulence has been the subject of research for a long time. The transition process, however, has not yet been fully explained though the final stage of the boundary-layer transition has been explained that some small turbulent sources (turbulent spots) occur suddenly in the boundary layer. As these turbulent spots fill up the boundary layer, they induce a transition to turbulence of a laminar boundary layer. The most important process is the turbulentspot appearance in the transitional boundary layer because a progression to turbulence that is not present in the laminar state is promoted. The process of the transition from the prior state to turbulent spots, however, has not been ascertained. Thus, the mechanism of turbulent-spot appearance has only been explained as the word "breakdown". The computational simulation by Brandt [1] demonstrates that the breakdown is induced by the interaction of streaks which move laterally and slowly. Meanwhile, we investigate a downstream development of a single hair-pin-type vortex generated by an artificial small jet. From the velocity field measured in detail, this vortex grows and increases in number downstream, and finally the developed vortices constitute a spot. In the initial stage of downstream development where the vortices propagate in the streamwise direction, the velocity perturbations in a spot reiterate the in-phase wave form. In addition, the low- and the high-speed streaks in the spot are elongated straight in streamwise direction. In the transition stage, it is shown that the amplitude in the instantaneous velocity signals in the spot become irregular locally, where the low- and the high-speed streaks distort laterally. Further downstream, it is clarified that the occurrence of the momentum-transfer accompanied with local and temporary ejection movements and sweep movements become irregular in the spot, where the low-speed and the highspeed streaks cross one another and switch their positions with each other in spanwise direction. The appearance of the crossover of the streaks shows the break-up of the spot structure, i.e., the beginning of its breakdown. A crossover of the streaks produces a new crossover in a chain reaction, so that the transition to turbulence (breakdown) progress rapidly. And finally the spot enter into a turbulent region. The irregularity in the velocity field, showing the other distinct feature of the spot, is occurred owing to the distortion of streaks in spanwise direction and, therefore, the streaks cross one another. Thus, we considered that these characteristics of velocity field are induced by a spanwise flow. In this study, we pay much attention to a spanwise flow and investigate its effect on the boundary-layer transition. From the measurement of the streamwise and spanwise component of velocity using a small X-type hot-wire probe, we found that the spanwise distortion of the velocity field and the irregularity of the velocity perturbation are caused by the spanwise flow in the spot. These results show that the spanwise flow have a critical role in laminar-turbulent transition.dc201