X-Ray Measurement and Porosity in Graphite/Polyimide Composites

One of the persistent problems in the control of quality in materials used in structures is that of porosity. Whether the material is a casting, an epoxy composite, or a complex ceramic or metal matrix composite, the presence of porosity, depending upon the severity, causes a degradation of the parts performance characteristics and their longevity. The detection of porosity in many industries is important in two ways, namely, in material quality control and in monitoring process control (1). One class of materials where both of these problems arise is the graphite composites (2–4). To date, the inspection of these composites using an X-ray method has been unsatisfactory in that the film methods frequently tried did not have the sensitivity resolution to yield a quantitative measure of the porosity volume percent. Such a measure becomes even more critical in ceramic composites in that the difference between a good material and a poor one is a level of porosity rather than the presence of porosity, as is the case in graphite epoxy composites. We wish to report results of an X-ray method to measure porosity in materials using the graphite polyimide composites as an example. The method is certainly not restricted to this type of graphite composite or indeed to graphite composites

Constraints on the Lyman continuum radiation from galaxies: first results with FUSE on Mrk 54

We present Far Ultraviolet Spectroscopic Explorer observations of the star-forming galaxy Mrk 54 at z = 0.0448. The Lyman continuum radiation is not detected above the HI absorption edge in our Galaxy. An upper limit is evaluated by comparison with the background measured in regions of the detector adjacent to the observed spectrum. A spectral window of 16 A, reasonably free of additional HI Lyman series line absorption is used. No correction is needed for molecular hydrogen absorption in our Galaxy but a foreground extinction of 0.29 mag is accounted for. An upper limit of 6.15 10^{-16} erg/cm^2/s/A is obtained for the flux at ~ 900 A in the rest frame of Mrk 54. By comparison with the number of ionizing photons derived from the H-alpha flux, this limit translates into an upper limit of f_esc < 0.062 for the fraction of Lyman continuum photons that escape the galaxy without being absorbed by interstellar material. This limit compares with the limits obtained in three other nearby galaxies and is compatible with the escape fractions predicted by models. The upper limits obtained in nearby galaxies contrasts with the detection of Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z ~ 3.4. The difficulties and implications of a comparison are discussed.Comment: 9 pages, 3 figures, accepted for publication in A&A include aa.cls v5.0

The Radiative Transport of Dust in Primordial Galaxies and Second-Generation Star Formation

We investigate the radiativetransport of dust inprimordial galaxies in thepresence of the UVradiation field from thefirst metal-free stars. Wefind that dust createdin the first supernova(SN) explosions can bedriven through the interiorof the SN remnantto accumulate in theSN shells, where second-generationstars may form fromcompressed cooling gas. Thisscenario requires metal-free starsto form continuously overtimescales of up to10 Myr, consistent withrecent estimates. Silicate andgraphite grains, as wellas iron-bearing magnetites, aretransported to the shellsfor reasonable parameter assumptions,but their relative yieldsfrom primordial SNe isan important factor inthe resulting abundance ratios.We compare the resultsof segregated grain transportwith the current nucleosyntheticdata on extremely metal-poorGalactic halo stars. Fossilsignatures of this processmay already have beendetected in those iron-poorstars with enhanced carbonand silicate elements suchas magnesium, silicon andoxygen. We discuss theimplications of our resultsfor the transition fromfirst- to second-generation starformation in primordial galaxies,and the role playedby the radiative transportof dust in thisprocess

Murder on the VR Express: Studying the Impact of Thought Experiments at a Distance in Virtual Reality

Hypothetical thought experiments allow researchers to gain insights into widespread moral intuitions and provide opportunities for individuals to explore their moral commitments. Previous thought experiment studies in virtual reality (VR) required participants to come to an on-site laboratory, which possibly restricted the study population, introduced an observer effect, and made internal reflection on the participants’ part more difficult. These shortcomings are particularly crucial today, as results from such studies are increasingly impacting the development of artificial intelligence systems, self-driving cars, and other technologies. This paper explores the viability of deploying thought experiments in commercially available in-home VR headsets. We conducted a study that presented the trolley problem, a life-and-death moral dilemma, through SideQuestVR, a third-party website and community that facilitates loading applications onto Oculus headsets. Thirty-three individuals were presented with one of two dilemmas: (1) a decision to save five lives at the cost of one life by pulling a switch and (2) a decision to save five lives at the cost of one life by pushing a person onto train tracks. The results were consistent with those of previous VR studies, suggesting that a “VR-at-a-distance” approach to thought experiments has a promising future while indicating lessons for future research

OVI, NV and CIV in the Galactic Halo: II. Velocity-Resolved Observations with Hubble and FUSE

We present a survey of NV and OVI (and where available CIV) in the Galactic halo, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Hubble Space Telescope (HST) along 34 sightlines. These ions are usually produced in nonequilibrium processes such as shocks, evaporative interfaces, or rapidly cooling gas, and thus trace the dynamics of the interstellar medium. Searching for global trends in integrated and velocity-resolved column density ratios, we find large variations in most measures, with some evidence for a systematic trend of higher ionization (lower NV/OVI column density ratio) at larger positive line-of-sight velocities. The slopes of log[N(NV)/N(OVI)] per unit velocity range from -0.015 to +0.005, with a mean of -0.0032+/-0.0022(r)+/-0.0014(sys) dex/(km/s). We compare this dataset with models of velocity-resolved high-ion signatures of several common physical structures. The dispersion of the ratios, OVI/NV/CIV, supports the growing belief that no single model can account for hot halo gas, and in fact some models predict much stronger trends than are observed. It is important to understand the signatures of different physical structures to interpret specific lines of sight and future global surveys.Comment: ApJ in press 43 pages, 22 fig