Potential-Density Basis Sets for Galactic Disks

A class of complete potential-density basis sets in cylindrical (R,phi,z) coordinates is presented. This class is suitable for stability studies of galactic disks in three dimensions and includes basis sets tailored for disks with vertical density profiles that are exponential (exp(-|z|/\zn)), Gaussian (exp(-(z/\zn)^2) or locally isothermal (sech^2(z/\zn)). The basis sets are non-discrete and non-biorthogonal; however, the extra numerical computations required (compared with discrete biorthogonal sets) are explained and constitute a small overhead. The method of construction (and proof of completeness) is simple and can be used to construct basis sets for other density distributions that are best described in circular or elliptic cylindrical coordinates. When combined with a basis set designed for spheroidal systems, the basis sets presented here can be used to study the stability of realistic disks embedded in massive halos.Comment: Accepted for publication in The Astrophysical Journal, 13 pages, plain TeX, uses mtexsis.tex, no figure

The Time Required for Nuclear Fission

The present work is concerned with the mean lifetime of compound nuclei formed in the fast-neutron-induced-fission process. A new method for the measurement of these lifetimes is proposed. The present method is based on the recoil of the compound nucleus following neutron capture and the secondary electrons emitted by fission fragments. Previous experimental and theoretical work concerning the secondary electron yields for fission fragments and other charged particles are reviewed. The importance of the higher energy electrons, the so-called delta rays, formed in the primary ionization process is considered. The subsequent secondary ionization produced by these delta rays provirles a natural explanatior of a previous experimental fact that the number of electrons emitted by a fission fragment is larger when the fragment emerges than when it enters the same surface. This viewpoint is extended t0 a consideration of the dependence of the electron yield on the depth beneath the surface at which a fission event takes place. On the basis of a qualitative theory, this yield is expected to increase with depth and approach asymptotically a limiting value which corresponds to the case where the fragment emerges from depths larger than the maximum range of the delta rays. Measurements of this dependence of the yield on the depth were performed with layers of uranium tetrafluoride. In these measurements iv the electrons were detected by accelerating and focusing them on a scintillation detector. A measure of the number of electrons emitted by each fragment was obtained from the height of the pulse from this detector. The general dependence predicted by the theory was observed. These results were used in an attempt to measure the average distance traversed by the recoiling compound nuclei during the time interval between neutron capture and fission. In one arrangement of the apparatus the compound nuclei with a given recoil velocity were expected to move away from the foil surface a distance proportional to their mean lifetime. Later, the arrangement of the apparatus was changed so that the nuclei were expected to move close to the surface. Measurements of the average electron yield were ma.de in each position. From a comparison of these results and the previously determined dependence of the yield on depth, an estimate of the average recoil distance and, therefore, a measure of the mean lifetime could be obtained. Target nuclei of uranium-238 and neptunium-237 were used in this experiment. Neutrons with an assumed fission spectrum were obtained from a reactor. The results of these measurements showed that the mean lifetimes of the compound nuclei of U-239 and Np-238 were, within the sensitivity of the present method, consistent with zero. Upper limits of the lifetimes have been obtained, however, which are considerably lower than those previously obtained. The total mean lifetimes for target nuclei of U-238 and Np-237 were found to be T(U-238)\u3c6 x 10-14 sec and T(Np-237)\u3c4 x 10-14 sec. The partial mean lifetimes for fission and neutron emission were determined from the relation r = rf + rn and the V previously measured ratio of the fission and neutron emission widths. The partial mean lifetimes are as follows: Tf(U-238) \u3c 4 x 10-13 sec; Tn(U-238) \u3c 7 x 10-14 sec; Tf(Np-237) \u3c 9 x 10-14 sec; and Tn(Np--237) \u3c 7 x 10-14 sec

Automated Accident Location Detection System (Extended)

Today, the human rely more on automobile road transportation compare to the other transportation medium. Therefore, the more road users on road will causes increasing number of accidents happened. This problem has increase the fatal rate of accidents. Although the Malaysian Emergency Response Services team (MERS 999) has implemented in Malaysia to provide support to accidents, but there are several inefficiency usage of resources in the organization. The scope of study will be based on the Malaysian drivers only. The author is extending the project from the previous final year student. He will be continuing to do the server gateway, website system and implementation of algorithm in Automated Accident Location Detection System. The project is rapidly tested and verify to ensure it is on the right track and to meet the deliverable deadline. Surveys were done to 120 respondents who are car drivers. The results show a positive result towards the objective of this project. The overall result recorded a 2.66 of ratings out of 5 on the efficiency level of MERS 999. Qualitative research is carried out on 3 of Malaysian driver and 1 staff from MERS 999. The functionalities of this system will enhance the current emergency response service by providing a platform for reporting an accident. The accident locator and website system will work together to redirect the real-time message to the nearest hospital using the modified Haversine algorithm. This will help to reduce the fatal rate as stated in the objective

Equation-free modeling of evolving diseases: Coarse-grained computations with individual-based models

We demonstrate how direct simulation of stochastic, individual-based models can be combined with continuum numerical analysis techniques to study the dynamics of evolving diseases. % Sidestepping the necessity of obtaining explicit population-level models, the approach analyzes the (unavailable in closed form) `coarse' macroscopic equations, estimating the necessary quantities through appropriately initialized, short `bursts' of individual-based dynamic simulation. % We illustrate this approach by analyzing a stochastic and discrete model for the evolution of disease agents caused by point mutations within individual hosts. % Building up from classical SIR and SIRS models, our example uses a one-dimensional lattice for variant space, and assumes a finite number of individuals. % Macroscopic computational tasks enabled through this approach include stationary state computation, coarse projective integration, parametric continuation and stability analysis.Comment: 16 pages, 8 figure

Age-specific mortality during the 1918 influenza pandemic: unravelling the mystery of high young adult mortality.

The worldwide spread of a novel influenza A (H1N1) virus in 2009 showed that influenza remains a significant health threat, even for individuals in the prime of life. This paper focuses on the unusually high young adult mortality observed during the Spanish flu pandemic of 1918. Using historical records from Canada and the U.S., we report a peak of mortality at the exact age of 28 during the pandemic and argue that this increased mortality resulted from an early life exposure to influenza during the previous Russian flu pandemic of 1889-90. We posit that in specific instances, development of immunological memory to an influenza virus strain in early life may lead to a dysregulated immune response to antigenically novel strains encountered in later life, thereby increasing the risk of death. Exposure during critical periods of development could also create holes in the T cell repertoire and impair fetal maturation in general, thereby increasing mortality from infectious diseases later in life. Knowledge of the age-pattern of susceptibility to mortality from influenza could improve crisis management during future influenza pandemics

THE OPTIMAL N-BODY METHOD FOR STABILITY STUDIES OF GALAXIES

The stability of a galaxy model is most easily assessed through N-body simulation. Particle-mesh codes have been widely used for this purpose, since they enable the largest numbers of particles to be employed. We show that the functional expansion technique, originally proposed by Clutton-Brock for other simulation problems, is in fact superior for stability work. For simulations of linear evolution it is not much slower than grid methods using the same number of particles, and reproduces analytical results with much greater accuracy. This success rests on its ability to represent global modes with a modest number of basis functions; grid methods may be more effective for other applications, however. Our conclusions are based on implementations of functional expansion and grid algorithms for disk galaxies.Comment: Accepted for publication in The Astrophysical Journal, to appear October 1, 1995; 16 pages including 4 figures, self-unpacking uuencoded gzipped postscript, also available by email from [email protected]

Generation of potential/surface density pairs in flat disks Power law distributions

We report a simple method to generate potential/surface density pairs in flat axially symmetric finite size disks. Potential/surface density pairs consist of a ``homogeneous'' pair (a closed form expression) corresponding to a uniform disk, and a ``residual'' pair. This residual component is converted into an infinite series of integrals over the radial extent of the disk. For a certain class of surface density distributions (like power laws of the radius), this series is fully analytical. The extraction of the homogeneous pair is equivalent to a convergence acceleration technique, in a matematical sense. In the case of power law distributions, the convergence rate of the residual series is shown to be cubic inside the source. As a consequence, very accurate potential values are obtained by low order truncation of the series. At zero order, relative errors on potential values do not exceed a few percent typically, and scale with the order N of truncation as 1/N**3. This method is superior to the classical multipole expansion whose very slow convergence is often critical for most practical applications.Comment: Accepted for publication in Astronomy & Astrophysics 7 pages, 8 figures, F90-code available at http://www.obs.u-bordeaux1.fr/radio/JMHure/intro2applawd.htm

Dynamical epidemic suppression using stochastic prediction and control

We consider the effects of noise on a model of epidemic outbreaks, where the outbreaks appear. randomly. Using a constructive transition approach that predicts large outbreaks, prior to their occurrence, we derive an adaptive control. scheme that prevents large outbreaks from occurring. The theory inapplicable to a wide range of stochastic processes with underlying deterministic structure.Comment: 14 pages, 6 figure