2,498 research outputs found
The nature and development of modern physics : inaugural lecture delivered at Rhodes University
Inaugural lecture delivered at Rhodes University.Rhodes University Libraries (Digitisation
The high-lying Li levels at excitation energy around 21 MeV
The H+He cluster structure in Li was investigated by the
H(,H He)n kinematically complete experiment at the incident
energy = 67.2 MeV. We have observed two resonances at =
21.30 and 21.90 MeV which are consistent with the He(H, )Li
analysis in the Ajzenberg-Selove compilation. Our data are compared with the
previous experimental data and the RGM and CSRGM calculations.Comment: 12 pages, 6 figures. Accepted for publication in J. Phys. Soc. Jp
Channeling Effect and Improvement of the Efficiency of Charged Particle Registration with Crystal Scintillators
The importance is emphasized of taking into account the channeling along the
low index crystallographic axes and planes of a part of low-energy (1-10 keV)
recoil ions in measurements of their parameters with crystal scintillators of
the type of NaI(Tl) etc. Because the nucleus stopping power in channels is low
as compared with electronic stopping power, the light yield of the scintillator
must be, accordingly, higher in the given case than that for ions having higher
energy (tens keV and more), which lose most part of their energy via nuclear
collisions outside channels. Hence, in particular, it follows that the DAMA/NaI
observations in Gran Sasso of the annual modulation of the signal frequency in
a narrow range of scintillations with an amplitude of 2-6 keV electron
equivalent may be due to incidence onto the Earth of exceedingly massive
particles (of the type of Planckian objects) from elongated Earth-crossing
heliocentric orbits at a velocity of 30-50 km/s. In NaI(Tl), these particles
create the iodine recoil ions with just the energy of 2-6 keV.Comment: 10 page
A Bayesian palaeoenvironmental transfer function model for acidified lakes
A Bayesian approach to palaeoecological environmental reconstruction deriving from the unimodal responses generally exhibited by organisms to an environmental gradient is described. The approach uses Bayesian model selection to calculate a collection of probability-weighted, species-specific response curves (SRCs) for each taxon within a training set, with an explicit treatment for zero abundances. These SRCs are used to reconstruct the environmental variable from sub-fossilised assemblages. The approach enables a substantial increase in computational efficiency (several orders of magnitude) over existing Bayesian methodologies. The model is developed from the Surface Water Acidification Programme (SWAP) training set and is demonstrated to exhibit comparable predictive power to existing Weighted Averaging and Maximum Likelihood methodologies, though with improvements in bias; the additional explanatory power of the Bayesian approach lies in an explicit calculation of uncertainty for each individual reconstruction. The model is applied to reconstruct the Holocene acidification history of the Round Loch of Glenhead, including a reconstruction of recent recovery derived from sediment trap data.The Bayesian reconstructions display similar trends to conventional (Weighted Averaging Partial Least Squares) reconstructions but provide a better reconstruction of extreme pH and are more sensitive to small changes in diatom assemblages. The validity of the posteriors as an apparently meaningful representation of assemblage-specific uncertainty and the high computational efficiency of the approach open up the possibility of highly constrained multiproxy reconstructions
Segmented scintillation detectors with silicon photomultiplier readout for measuring antiproton annihilations
The Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA)
experiment at the Antiproton Decelerator (AD) facility of CERN constructed
segmented scintillators to detect and track the charged pions which emerge from
antiproton annihilations in a future superconducting radiofrequency Paul trap
for antiprotons. A system of 541 cast and extruded scintillator bars were
arranged in 11 detector modules which provided a spatial resolution of 17 mm.
Green wavelength-shifting fibers were embedded in the scintillators, and read
out by silicon photomultipliers which had a sensitive area of 1 x 1 mm^2. The
photoelectron yields of various scintillator configurations were measured using
a negative pion beam of momentum p ~ 1 GeV/c. Various fibers and silicon
photomultipliers, fiber end terminations, and couplings between the fibers and
scintillators were compared. The detectors were also tested using the
antiproton beam of the AD. Nonlinear effects due to the saturation of the
silicon photomultiplier were seen at high annihilation rates of the
antiprotons.Comment: Copyright 2014 American Institute of Physics. This article may be
downloaded for personal use only. Any other use requires prior permission of
the author and the American Institute of Physics. The following article
appeared in Review of Scientific Instruments, Vol.85, Issue 2, 2014 and may
be found at http://dx.doi.org/10.1063/1.486364
The radical character of the acenes: A density matrix renormalization group study
We present a detailed investigation of the acene series using high-level
wavefunction theory. Our ab-initio Density Matrix Renormalization Group
algorithm has enabled us to carry out Complete Active Space calculations on the
acenes from napthalene to dodecacene correlating the full pi-valence space.
While we find that the ground-state is a singlet for all chain-lengths,
examination of several measures of radical character, including the natural
orbitals, effective number of unpaired electrons, and various correlation
functions, suggests that the longer acene ground-states are polyradical in
nature.Comment: 10 pages, 8 figures, supplementary material, to be published in J.
Chem. Phys. 127, 200
Optical Properties of Pentacene and Perfluoropentacene Thin Films
The optical properties of pentacene (PEN) and perfluoropentacene(PFP) thin
films on various SiO2 substrates were studied using variable angle
spectroscopic ellipsometry. Structural characterization was performed using
X-ray reflectivity and atomic force microscopy. A uniaxial model with the optic
axis normal to the sample surface was used to analyze the ellipsometry data. A
Strong optical anisotropy was observed and enabled the direction of the
transition dipole of the absorption bands to be determined. Furthermore,
comparison of the optical constants of PEN and PFP thin films with the
absorption spectra of the monomers in solution shows significant changes due to
the crystalline environment. Relative to the monomer spectrum the HOMO-LUMO
transition observed in PEN (PFP) thin film is reduced by 210 meV (280 meV).
Surprisingly, a second absorption band in the PFP thin film shows a slight
blueshift (40 meV) compared to the spectrum of the monomer with its transition
dipole perpendicular to that of the first absorption band.Comment: 6 pages, 6 figures, submitted to J. Chem. Phy
Measurement of the quenching factor of Na recoils in NaI(Tl)
Measurements of the quenching factor for sodium recoils in a 5 cm diameter
NaI(Tl) crystal at room temperature have been made at a dedicated neutron
facility at the University of Sheffield. The crystal has been exposed to 2.45
MeV mono-energetic neutrons generated by a Sodern GENIE 16 neutron generator,
yielding nuclear recoils of energies between 10 and 100 keVnr. A cylindrical
BC501A detector has been used to tag neutrons that scatter off sodium nuclei in
the crystal. Cuts on pulse shape and time of flight have been performed on
pulses recorded by an Acqiris DC265 digitiser with a 2 ns sampling time.
Measured quenching factors of Na nuclei range from 19% to 26% in good agreement
with other experiments, and a value of 25.2 \pm 6.4% has been determined for 10
keV sodium recoils. From pulse shape analysis, the mean times of pulses from
electron and nuclear recoils have been compared down to 2 keVee. The
experimental results are compared to those predicted by Lindhard theory,
simulated by the SRIM Monte Carlo code, and a preliminary curve calculated by
Prof. Akira Hitachi.Comment: 21 pages, 13 figure
'Moving life stories tell us just why politics mattersâ: personal narratives in tabloid anti-austerity campaigns
This article examines the use of personal narratives in two tabloid newspaper campaigns against a controversial welfare reform popularly known as the âbedroom taxâ. It aims firstly to evaluate whether the personal narratives operate as political testimony to challenge government accounts of welfare reform and dominant stereotypes of benefits claimants, and secondly to assess the potential for and limits to progressive advocacy in popular journalism. The study uses content analysis of 473 articles over the course of a year in the Daily Mirror and Sunday People newspapers, and qualitative analysis of a sub-set of 113 articles to analyse the extent to which the campaign articles extrapolated from the personal to the general, and the role of âvictim-witnessesâ in articulating their own subjectivity and political agency. The analysis indicates that both newspapers allowed affected individuals to express their own subjectivity to challenge stereotypes, but it was civil society organisations and opinion columnists who most explicitly extrapolated from the personal to the political. Collectively organised benefits claimants were rarely quoted, and there was some evidence of ventriloquization of the editorial voice in the political criticisms of victim-witnesses. However, a campaigning columnist in the Mirror more actively empowered some of those affected to speak directly to politicians. This indicates the value of campaigning journalism when it is truly engaged in solidarity with those affected, rather than instrumentalising victim-witnesses to further the newspapersâ campaign goals
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