1,560 research outputs found
House of Commons Library: Briefing paper: Number 7998, 16 June 2017: Key issues in fostering: capacity, working conditions, and fostering agencies
Optically stimulated luminescence dating of Ocean Drilling Program Core 658B:Complications arising from authigenic uranium uptake and lateral sediment movement
Positronium ionisation in collision with He atoms
Positronium is the lightest known atom, consisting of an electron and its antiparticle the positron. Because of its light mass (comparable with that of the electron and positron, rather than conventional atoms), recoil effects are expected to play an important role in its scattering from atomic and molecular targets. In Positronium the centres of charge and mass coincide, leading to a zero-static interaction and enhancing the relative importance of electron-exchange effects. Up until now, Positronium beam experiments have been restricted to total cross-section measurements from simple target atoms and molecules i.e. H2, He and Ar. Significant discrepancies exist among various (theoretical and experimental) determinations of the Positronium-He total cross-section. In addition to their intrinsic interest. Positronium-atom partial-cross-sections are expected to provide a more sensitive test of our understanding of this collision system. In this work, the ionisation cross-section of Positronium has been measured for the first time. A monoenergetic Positronium beam has been created through charge exchange of positrons in a gaseous target and positrons, originating from the break-up of positronium in collision with He atoms, have been detected with a time-of-flight system. Measurements are presented in the energy range 10 - 40eV and absolute break-up cross-section values have been achieved by measuring explicitly both the positron and Positronium detection efficiencies. From the measured times-of-flight, longitudinal energy spreads of the residual positrons have also been obtained. The distributions have been found to be peaked at around 50% of the residual energy, suggesting a strong correlation between the residual particles. The present work is expected to stimulate further theoretical and experimental activity in the study of Positronium-atom interactions. Possible future new directions are discussed
Testing and application of luminescence techniques using sediment from the southeast African coast
Testing the applicability of optically stimulated luminescence dating to Ocean Drilling Program cores
Magnetically driven accretion in protoplanetary discs
We characterize magnetically driven accretion at radii between 1 au and 100
au in protoplanetary discs, using a series of local non-ideal
magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass
Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of
specified strength. Confirming previous results, we find that the Hall effect
has only a modest impact on accretion at 30 au, and essentially none at 100 au.
At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion
process, with vertical magnetic fields aligned to the disc rotation supporting
a strong laminar Maxwell stress that is absent if the field is anti-aligned. In
the anti-aligned case, we instead find evidence for bursts of turbulent stress
at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear
instability. The presence or absence of these bursts depends upon the details
of the adopted chemical model, which suggests that appreciable regions of
actual protoplanetary discs might lie close to the borderline between laminar
and turbulent behaviour. Given the number of important control parameters that
have already been identified in MHD models, quantitative predictions for disc
structure in terms of only radius and accretion rate appear to be difficult.
Instead, we identify robust qualitative tests of magnetically driven accretion.
These include the presence of turbulence in the outer disc, independent of the
orientation of the vertical magnetic fields, and a Hall-mediated bi-modality in
turbulent properties extending from the region of thermal ionization to 10 au.Comment: accepted to MNRAS after very minor revision
Evidence for universality in the initial planetesimal mass function
Planetesimals may form from the gravitational collapse of dense particle
clumps initiated by the streaming instability. We use simulations of
aerodynamically coupled gas-particle mixtures to investigate whether the
properties of planetesimals formed in this way depend upon the sizes of the
particles that participate in the instability. Based on three high resolution
simulations that span a range of dimensionless stopping time no statistically significant differences in the initial
planetesimal mass function are found. The mass functions are fit by a
power-law, , with and
errors of . Comparing the particle density fields prior
to collapse, we find that the high wavenumber power spectra are similarly
indistinguishable, though the large-scale geometry of structures induced via
the streaming instability is significantly different between all three cases.
We interpret the results as evidence for a near-universal slope to the mass
function, arising from the small-scale structure of streaming-induced
turbulence.Comment: 7 pages, 4 figures, accepted to ApJ Letters after minor
modifications, including two new figures and some new text that better
clarify our result
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