915 research outputs found
Usability Evaluation in Virtual Environments: Classification and Comparison of Methods
Virtual environments (VEs) are a relatively new type of human-computer interface in which users perceive and act in a three-dimensional world. The designers of such systems cannot rely solely on design guidelines for traditional two-dimensional interfaces, so usability evaluation is crucial for VEs. We present an overview of VE usability evaluation. First, we discuss some of the issues that differentiate VE usability evaluation from evaluation of traditional user interfaces such as GUIs. We also present a review of VE evaluation methods currently in use, and discuss a simple classification space for VE usability evaluation methods. This classification space provides a structured means for comparing evaluation methods according to three key characteristics: involvement of representative users, context of evaluation, and types of results produced. To illustrate these concepts, we compare two existing evaluation approaches: testbed evaluation [Bowman, Johnson, & Hodges, 1999], and sequential evaluation [Gabbard, Hix, & Swan, 1999]. We conclude by presenting novel ways to effectively link these two approaches to VE usability evaluation
Neutrinos, Fission Cycling, and the r-process
It has long been suggested that fission cycling may play an important role in
the r-process. Fission cycling can only occur in a very neutron rich
environment. In traditional calculations of the neutrino driven wind of the
core-collapse supernova, the environment is not sufficiently neutron rich to
produce the r-process elements. However, we show that with a reduction of the
electron neutrino flux coming from the supernova, fission cycling does occur
and furthermore it produces an abundance pattern which is consistent with
observed r-process abundance pattern in halo stars. Such a reduction can be
caused by active-sterile neutrino oscillations or other new physics.Comment: Typos corrected. Presented at NIC-IX, International Symposium on
Nuclear Astrophysics - Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland,
25-30 June, 200
Improved estimate of electron capture rates on nuclei during stellar core collapse
Electron captures on nuclei play an important role in the dynamics of the
collapsing core of a massive star that leads to a supernova explosion. Recent
calculations of these capture rates were based on microscopic models which
account for relevant degrees of freedom. Due to computational restrictions such
calculations were limited to a modest number of nuclei, mainly in the mass
range A=45-110. Recent supernova simulations show that this pool of nuclei,
however, omits the very neutron-rich and heavy nuclei which dominate the
nuclear composition during the last phase of the collapse before neutrino
trapping. Assuming that the composition is given by Nuclear Statistical
Equilibrium we present here electron capture rates for collapse conditions
derived from individual rates for roughly 2700 individual nuclei. For those
nuclei which dominate in the early stage of the collapse, the individual rates
are derived within the framework of microscopic models, while for the nuclei
which dominate at high densities we have derived the rates based on the Random
Phase Approximation with a global parametrization of the single particle
occupation numbers. In addition, we have improved previous rate evaluations by
properly including screening corrections to the reaction rates into account.Comment: 32 pages, 13 figures, 1 table; elsart; to appear in Nuclear Physics
Silicon Burning II: Quasi-Equilibrium and Explosive Burning
Having examined the application of quasi-equilibrium to hydrostatic silicon
burning in Paper I of this series, Hix & Thielemann (1996), we now turn our
attention to explosive silicon burning. Previous authors have shown that for
material which is heated to high temperature by a passing shock and then cooled
by adiabatic expansion, the results can be divided into three broad categories;
\emph{incomplete burning}, \emph{normal freezeout} and \emph{-rich
freezeout}, with the outcome depending on the temperature, density and cooling
timescale. In all three cases, we find that the important abundances obey
quasi-equilibrium for temperatures greater than approximately 3 GK, with
relatively little nucleosynthesis occurring following the breakdown of
quasi-equilibrium. We will show that quasi-equilibrium provides better
abundance estimates than global nuclear statistical equilibrium, even for
normal freezeout and particularly for -rich freezeout. We will also
examine the accuracy with which the final nuclear abundances can be estimated
from quasi-equilibrium.Comment: 27 pages, including 15 inline figures. LaTeX 2e with aaspp4 and
graphicx packages. Accepted to Ap
Ascertaining the Core Collapse Supernova Mechanism: An Emerging Picture?
Here we present the results from two sets of simulations, in two and three
spatial dimensions. In two dimensions, the simulations include multifrequency
flux-limited diffusion neutrino transport in the "ray-by-ray-plus"
approximation, two-dimensional self gravity in the Newtonian limit, and nuclear
burning through a 14-isotope alpha network. The three-dimensional simulations
are model simulations constructed to reflect the post stellar core bounce
conditions during neutrino shock reheating at the onset of explosion. They are
hydrodynamics-only models that focus on critical aspects of the shock stability
and dynamics and their impact on the supernova mechanism and explosion. In two
dimensions, we obtain explosions (although in one case weak) for two
progenitors (11 and 15 Solar mass models). Moreover, in both cases the
explosion is initiated when the inner edge of the oxygen layer accretes through
the shock. Thus, the shock is not revived while in the iron core, as previously
discussed in the literature. The three-dimensional studies of the development
of the stationary accretion shock instability (SASI) demonstrate the
fundamentally new dynamics allowed when simulations are performed in three
spatial dimensions. The predominant l=1 SASI mode gives way to a stable m=1
mode, which in turn has significant ramifications for the distribution of
angular momentum in the region between the shock and proto-neutron star and,
ultimately, for the spin of the remnant neutron star. Moreover, the
three-dimensional simulations make clear, given the increased number of degrees
of freedom, that two-dimensional models are severely limited by artificially
imposed symmetries.Comment: 9 pages, 3 figure
UK voters, including Leavers, care more about reducing non-EU than EU migration
Despite the argument that Brexit was about sovereignty and only secondarily about immigration, new data suggest otherwise. Simon Hix, Eric Kaufmann, and Thomas J. Leeper show the importance of reducing immigration levels â especially from outside the EU â to British voters
Pricing immigration
Immigration is highly salient for voters in Europe and the United States and has generated considerable academic debate about the causes of preferences over immigration. This debate centers around the relative influences of sociotropic or personal economic considerations, as well as non-economic threats. We provide a test of the competing egocentric, sociotropic, and non-economic paradigms using a novel constrained preference experiment in which respondents are asked to trade-off preferred reductions in immigration levels with realistic estimates of the personal or societal costs associated with those reductions. This survey experiment, per- formed on a national sample of British YouGov panelists, allows us to measure the price-elasticity of the publicâs preferences with regard to levels of European and non-European immigration. Respondents were willing to admit more immigrants when restriction carries economic costs, with egocentric considerations as important as sociotropic ones. People who voted for the UK to Leave the European Union in the 2016 referendum are less price-elastic than those voting Remain, indicating that non-economic concerns are also important
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