Dustbuster: a compact impact-ionization time-of-flight mass spectrometer for in situ analysis of cosmic dust

We report on the design and testing of a compact impact-ionization time-of-flight mass spectrometer for analysis of cosmic dust, suitable for use on deep space missions. The instrument, Dustbuster, incorporates a large target area with a reflectron, simultaneously optimizing mass resolution, particle detection, and ion collection. Dust particles hit the 65-cm2 target plate and are partially ionized by the impact. The resulting ions, with broad energy and angular distributions, are accelerated through a modified reflectron, focusing ions of specific m/z in space and time to produce high-resolution mass spectra. The cylindrically symmetric instrument is 10 cm in diameter and 20 cm in length, considerably smaller than previous in situ dust analyzers, and can be easily scaled as needed for specific mission requirements. Laser desorption ionization of metal and mineral samples embedded in the impact plate simulated particle impacts for evaluations of instrument performance. Mass resolution in these experiments ranged from 60–180, permitting resolution of isotopes. The mass spectrometer can be combined with other instrument components to determine dust particle trajectories and sizes

2006 Housing in the Nation's Capital

Explores the interdependent relationship between public school systems and housing markets, and examines the ability of coordinated investment in affordable housing and quality education to revitalize Washington, D.C., metropolitan area neighborhoods

A safe method of extracting DNA from Coccidioides immitis

Human-pathogenic fungi such as Coccidioides immitis and Histoplasma capsulatum must be handled in Biosafety level 3 containment facilities which make for a very awkward working environment (J.Y. Richmond and R.W. McKinney, eds. Biosafety in Microbiological and Biomedical Laboratories, 3rd ed. NIH, Washington). In this paper we describe a safe and convenient method of extracting DNA from such fungi in which the culture is killed by steaming, allowing removal from the containment facilities, as soon as possible. The method was first developed with the non-pathogen Neurospora crassa, has worked well for both C. immitis and H. capsulatum, and should be useful for extracting DNA from any pathogenic fungus

The Role of the Radial Orbit Instability in Dark Matter Halo Formation and Structure

For a decade, N-body simulations have revealed a nearly universal dark matter density profile, which appears to be robust to changes in the overall density of the universe and the underlying power spectrum. Despite its universality, the physical origin of this profile has not yet been well understood. Semi--analytic models by Barnes et al. (2005) have suggested that the density structure of dark matter halos is determined by the onset of the radial orbit instability (ROI). We have tested this hypothesis using N-body simulations of collapsing dark matter halos with a variety of initial conditions. For dynamically cold initial conditions, the resulting halo structures are triaxial in shape, due to the mild aspect of the instability. We examine how variations in initial velocity dispersion affect the onset of the instability, and find that an isotropic velocity dispersion can suppress the ROI entirely, while a purely radial dispersion does not. The quantity sigma^2/vc^2 is a criterion for instability, where regions with sigma^2/vc^2 <~1 become triaxial due to the ROI or other perturbations. We also find that the radial orbit instability sets a scale length at which the velocity dispersion changes rapidly from isotropic to radially anisotropic. This scale length is proportional to the radius at which the density profile changes shape, as is the case in the semi--analytic models; however, the coefficient of proportionality is different by a factor of ~2.5. We conclude that the radial orbit instability is likely to be a key physical mechanism responsible for the nearly universal profiles of simulated dark matter halos.Comment: 13 pages, 12 figures, accepted to Ap

Seshat: The Global History Databank

The vast amount of knowledge about past human societies has not been systematically organized and, therefore, remains inaccessible for empirically testing theories about cultural evolution and historical dynamics. For example, what evolutionary mechanisms were involved in the transition from the small-scale, uncentralized societies, in which humans lived 10,000 years ago, to the large-scale societies with an extensive division of labor, great differentials in wealth and power, and elaborate governance structures of today? Why do modern states sometimes fail to meet the basic needs of their populations? Why do economies decline, or fail to grow? In this article, we describe the structure and uses of a massive databank of historical and archaeological information, Seshat: The Global History Databank. The data that we are currently entering in Seshat will allow us and others to test theories explaining how modern societies evolved from ancestral ones, and why modern societies vary so much in their capacity to satisfy their members’ basic human needsPeer reviewedFinal Published versio