93,064 research outputs found

    Positron cooling and annihilation in noble gases

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    Understanding the dynamics of positron cooling in gases, including the fraction of positrons surviving to thermalisation, is critical for accurate interpretation of positron lifetime spectra, for the development of efficient positron cooling in traps and accumulators, and for a cryogenically cooled, ultra-high-energy-resolution, trap-based positron beam. Here, positron cooling and annihilation in noble gases is simulated using accurate scattering and annihilation cross sections calculated \emph{ab initio} with many-body theory. It is shown that a strikingly small fraction of positrons survive to thermalisation: \sim0.1 in He, \sim0 in Ne (due to cooling effectively stalling in the relatively deep momentum-transfer cross-section minimum), \sim0.15 in Ar, \sim0.05 in Kr and \sim0.01 in Xe. For Xe, the time-varying annihilation rate Zˉeff(τ)\bar{Z}_{\rm eff}(\tau) is shown to be highly sensitive to the depletion of the distribution due to annihilation, conclusively explaining the long-standing discrepancy between gas-cell and trap-based measurements in Xe. The \emph{ab initio} calculations enable the first simultaneous probing of the energy dependence of the the scattering cross section and annihilation rate. Overall, the use of the accurate atomic data gives Zˉeff(τ)\bar{Z}_{\rm eff}(\tau) in close agreement with experiment for all noble gases except Ne, the experiment for which is proffered to have suffered from incomplete knowledge of the fraction of positrons surviving to thermalisation and/or the presence of impurities.Comment: Supplementary videos of positron cooling in He, Ne, Ar, Kr and Xe at http://www.am.qub.ac.uk/users/dgreen09/coolingvideos.html ; This version contains additional References, is significantly reduced in size and has improved tex

    Defending the Time Culture : The Public and Private Interests of Media Corporations

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    Part I of this essay discusses the “public interest” standard under the Federal Communications Act and describes parallels in corporation doctrine. Part II considers whether broadcasters satisfy their public interest obligations by addressing audience interest. Part III discusses the prerogatives of the management of the corporate broadcaster to consider non-financial factors in selecting programming. Part IV describes the non-traditional philosophy of the corporation\u27s legitimate object, which led to the subject case. Part V discusses the central legal issues of the cognizable business interests of corporations. Finally, the Conclusion offers a view on desirable public interest objectives of media corporations

    Comments on the radio spectrum of HB 3

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    It has recently been suggested that the radio spectrum of the Galactic supernova remnant HB 3 shows flattening at higher frequencies (above about 1 GHz). Here I review the radio spectrum of HB 3, noting the difficulties in deriving accurate flux densities for this remnant, particularly at high frequencies, due to the proximity of bright, thermal emission from W3 and its surroundings. A flux density for HB 3 at 2695 MHz is derived from Effelsberg survey data. The spectrum of HB 3 is well represented by a simple power-law spectrum from 22 to 2695 MHz, with a spectral index of 0.56 +/- 0.03. It is concluded that contamination with thermal emission from adjacent regions is the cause for the reported spectral flattening of HB 3.Comment: Accepted for publication by the Bulletin of the Astronomical Society of Indi

    [Review of] Francis Paul Prucha, Indian-White Relations in the United States: A Bibliography of Works Published, 1975-1980

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    This volume is the long-awaited supplement to Francis Paul Prucha\u27s Bibliographical Guide to the History of Indian-White Relations in the United States, published by the University of Chicago Press in 1977. That work, which contained 9705 items, was complete to 1975. The supplement, with 3400 titles, covers the historical literature made available between 1975 and 1980. Organized into fifteen subject divisions and excellently cross-referenced with a thirty-six page index, the supplement continues the same high quality of Prucha\u27s previous efforts to bring some useable order to the bewildering complexity of American Indian historiography

    Efficient circuit triggers high-current, high-voltage pulses

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    Modified circuit uses diodes to effectively disconnect the charging resistors from the circuit during the discharge cycle. Result is an efficient parallel charging, high voltage pulse modulator with low voltage rating of components

    The Passage of the Fair Housing Act of 1968: Stories to Be Told

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    The enactment of the Fair Housing Act of 1968 ( FHA ) is a story filled with intrigue - coercion, duplicity, and back-room deals. In The Secret History of the Fair Housing Act, Professor Jonathan Zasloff provides a riveting account of the maneuvers by the various protagonists in that story. Review of Jonathan Zasloff’s The Secret History of the Fair Housing Act, 53 Hary. J. on Legis. 247 (2016), http://property.jotwell.com/the-passage-of-the-fair-housing-act-of-1968-stories-to-be-told/

    Recapturing Water for Sustainability Through Redefinitions of Navigability and Ownership

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    In Defining Navigability : Balancing State Court Flexibility and Private Rights in Waterways, 36 Cardozo L. Rev. 1415 (2015), Maureen Brady explains that over the last two centuries, state courts have broadened the concept of navigability, and applied the new definitions to alter existing land titles. As a consequence, many non-navigable waterways have become navigable waterways, increasing public ownership and extinguishing private rights