10,498 research outputs found
Beyond simulation: designing for uncertainty and robust solutions
Simulation is an increasingly essential tool in the design of our environment, but any model is only as good as the initial assumptions on which it is built. This paper aims to outline some of the limits and potential dangers of reliance on simulation, and suggests how to make our models, and our buildings, more robust with respect to the uncertainty we face in design. It argues that the single analyses provided by most simulations display too precise and too narrow a result to be maximally useful in design, and instead a broader description is required, as might be provided by many differing simulations. Increased computing power now allows this in many areas. Suggestions are made for the further development of simulation tools for design, in that these increased resources should be dedicated not simply to the accuracy of single solutions, but to a bigger picture that takes account of a design’s robustness to change, multiple phenomena that cannot be predicted, and the wider range of possible solutions. Methods for doing so, including statistical methods, adaptive modelling, machine learning and pattern recognition algorithms for identifying persistent structures in models, will be identified. We propose a number of avenues for future research and how these fit into design process, particularly in the case of the design of very large buildings
Simulations of a Scintillator Compton Gamma Imager for Safety and Security
We are designing an all-scintillator Compton gamma imager for use in security
investigations and remediation actions involving radioactive threat material.
To satisfy requirements for a rugged and portable instrument, we have chosen
solid scintillator for the active volumes of both the scatter and absorber
detectors. Using the BEAMnrc/EGSnrc Monte Carlo simulation package, we have
constructed models using four different materials for the scatter detector:
LaBr_3, NaI, CaF_2 and PVT. We have compared the detector performances using
angular resolution, efficiency, and image resolution. We find that while PVT
provides worse performance than that of the detectors based entirely on
inorganic scintillators, all of the materials investigated for the scatter
detector have the potential to provide performance adequate for our purposes.Comment: Revised text and figures, Presented at SORMA West 2008, Published in
IEEE Transactions on Nuclear Scienc
Compositional nanodomain formation in hybrid formate perovskites
We report the synthesis and structural characterisation of three mixed-metal
formate perovskite families [C(NH)]MCu(HCOO) (M = Mn,
Zn, Mg). Using a combination of infrared spectroscopy, non-negative matrix
factorization, and reverse Monte Carlo refinement, we show that the Mn- and
Zn-containing compounds support compositional nanodomains resembling the polar
nanoregions of conventional relaxor ferroelectrics. The M = Mg family exhibits
a miscibility gap that we suggest reflects the limiting behaviour of nanodomain
formation.Comment: 4 pages, 3 figure
Randomness amplification against no-signaling adversaries using two devices
Recently, a physically realistic protocol amplifying the randomness of
Santha-Vazirani sources producing cryptographically secure random bits was
proposed; however for reasons of practical relevance, the crucial question
remained open whether this can be accomplished under the minimal conditions
necessary for the task. Namely, is it possible to achieve randomness
amplification using only two no-signaling components and in a situation where
the violation of a Bell inequality only guarantees that some outcomes of the
device for specific inputs exhibit randomness? Here, we solve this question and
present a device-independent protocol for randomness amplification of
Santha-Vazirani sources using a device consisting of two non-signaling
components. We show that the protocol can amplify any such source that is not
fully deterministic into a fully random source while tolerating a constant
noise rate and prove the composable security of the protocol against general
no-signaling adversaries. Our main innovation is the proof that even the
partial randomness certified by the two-party Bell test (a single input-output
pair () for which the conditional probability
is bounded away from for all no-signaling
strategies that optimally violate the Bell inequality) can be used for
amplification. We introduce the methodology of a partial tomographic procedure
on the empirical statistics obtained in the Bell test that ensures that the
outputs constitute a linear min-entropy source of randomness. As a technical
novelty that may be of independent interest, we prove that the Santha-Vazirani
source satisfies an exponential concentration property given by a recently
discovered generalized Chernoff bound.Comment: 15 pages, 3 figure
Spin Susceptibility and Gap Structure of the Fractional-Statistics Gas
This paper establishes and tests procedures which can determine the electron
energy gap of the high-temperature superconductors using the model
with spinon and holon quasiparticles obeying fractional statistics. A simpler
problem with similar physics, the spin susceptibility spectrum of the spin 1/2
fractional-statistics gas, is studied. Interactions with the density
oscillations of the system substantially decrease the spin gap to a value of
, much less than the mean-field value of
. The lower few Landau levels remain visible, though broadened
and shifted, in the spin susceptibility. As a check of the methods, the
single-particle Green's function of the non-interacting Bose gas viewed in the
fermionic representation, as computed by the same approximation scheme, agrees
well with the exact results. The same mechanism would reduce the gap of the
model without eliminating it.Comment: 35 pages, written in REVTeX, 16 figures available upon request from
[email protected]
The use of tobacco and related substances in ethnic minorities : the development of a culturally valid measure
Introduction. Meeting the needs of migrant groups in Europe requires cross-culturally valid questionnaires, a substantial challenge to researchers. The Rose Angina Questionnaire (RAQ) is an important measure of coronary heart disease prevalence. It consists of seven items that collectively yield a diagnosis of angina. It has been shown to perform inconsistently across some ethnic groups in Britain. This study aimed to assess the need for modifying the RAQ for cross-culturally valid use in the three main ethnic groups in Scotland.Methods. Interviews were carried out with Pakistani Punjabi speakers (n=26), Chinese Cantonese speakers (n=29) and European-origin English speakers (n=25). Bilingual project workers interviewed participants and provided translation and commentary to the English-speaking researcher. Participants were asked about general and cardiovascular health beliefs and behaviours, and about attitudes to pain and chest pain. They were also asked to comment on their understanding of an existing version of the RAQ in their language.Results. No dominant themes in the cultural construction of health, pain or cardiovascular knowledge emerged that may significantly influence RAQ response between language groups. Problems were encountered with the Punjabi and Cantonese translations of the RAQ. For example, the translation for “chest” was interpreted by some Pakistani and fewer Chinese women to mean “breasts”. “Walking uphill” was translated in Chinese as “walking the hill”, without stipulation of the direction, so that some Cantonese speakers interpreted the question as meaning walking downhill. In addition, many Chinese interpreted RAQ items to be referring to breathlessness rather than chest pain due to ambiguous wording.Conclusion. Existing Punjabi and Cantonese versions of the RAQ should be modified before being used in multi-ethnic surveys. Current versions are unlikely to be yielding data that is comparable across groups. Other language versions also require similar investigation to study the cardiovascular health of Europe’s migrant groups.<br /
Spin Transition in Strongly Correlated Bilayer Two Dimensional Electron Systems
Using a combination of heat pulse and nuclear magnetic resonance techniques
we demonstrate that the phase boundary separating the interlayer phase coherent
quantum Hall effect at in bilayer electron gases from the weakly
coupled compressible phase depends upon the spin polarization of the nuclei in
the host semiconductor crystal. Our results strongly suggest that, contrary to
the usual assumption, the transition is attended by a change in the electronic
spin polarization.Comment: 4 pages, 3 postscript figur
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